Thermostat variable fan-off delay

Abstract

A method for providing a variable fan-off delay after a thermostat call for cooling or after a heating on cycle for a Heating, Ventilation, Air Conditioning (HVAC) system. The variable fan-off delay may be ended based on comparing a measurement of a Conditioned Space Temperature (CST) to a CST threshold. The CST threshold may be based on a previous measurement of the CST monitored during the current variable fan-off delay. The CST threshold may be an inflection point where the rate of change of the CST with respect to time equals zero plus or minus a confidence interval tolerance. The CST threshold may also be a fan-off delay differential offset or a thermostat setpoint differential where the variable fan-off delay is ended when the measurement of the CST during the variable fan-off delay crosses the differential at least once after the cooling or heating on cycle.

Claims

1. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: at least one of: cooling a conditioned space with an Air Conditioning (AC) compressor and the HVAC system fan until a drybulb temperature measurement of a Conditioned Space Temperature (CST) reaches a lower cooling differential used to end a thermostat call for cooling and providing a variable fan-off delay after the thermostat call for cooling has been satisfied to decrease the CST below the lower cooling differential, and heating the conditioned space with a heater and the HVAC system fan until the CST reaches an upper heating differential used to end a heating on cycle and providing the variable fan-off delay after a heating on cycle to increase the CST above the upper heating differential; and wherein the variable fan-off delay is ended based on comparing the CST to a CST variable fan-off delay threshold.

2. The method of claim 1, wherein the CST variable fan-off delay threshold is a cooling fan-off delay differential offset from the lower cooling differential.

3. The method of claim 1, wherein the CST variable fan-off delay threshold is a heating fan-off delay differential offset from the upper heating differential.

4. The method of claim 1, wherein the CST variable fan-off delay threshold is a lower cooling differential, and the variable fan-off delay is ended when the drybulb temperature measurement of the CST during the variable fan-off delay crosses the lower cooling differential at least once after the thermostat call for cooling has been satisfied.

5. The method of claim 1, wherein the CST variable fan-off delay threshold is an upper heating differential, and the variable fan-off delay is ended when the drybulb temperature measurement of the CST during the variable fan-off delay crosses the upper heating differential at least once after the end of the heating on cycle.

6. The method of claim 1, further including at least one of: the lower cooling differential used to turn off the AC compressor is less than or equal to a cooling setpoint; and the upper heating differential used to turn off the heater is greater than or equal to a heating setpoint.

7. The method of claim 1, wherein: the lower cooling differential used to turn off the AC compressor is a lower variable cooling differential used to increase or decrease the duration of the thermostat call for cooling; and the upper heating differential used to turn off the heater is an upper variable heating differential used to increase or decrease the heating on cycle.

8. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; wherein the CST variable fan-off delay threshold is based on a previous CST measurement monitored during the current variable fan-off delay; and wherein the variable fan-off delay for cooling is ended when the CST increases above the previous CST measurement or the variable fan-off delay for heating is ended when the CST decreases below the previous CST measurement.

9. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; and wherein the CST variable fan-off delay threshold is an inflection point where the rate of change of the CST with respect to time (dT/dt) equals zero plus or minus a tolerance wherein the rate of change of the CST with respect to time is defined as a difference in temperature between at least two measurements of the CST divided by a difference in time between the at least two measurements of the CST.

10. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a variable fan-off delay CST threshold; wherein the variable fan-off delay duration is adjusted based on a duration of a cooling off cycle; and wherein the variable fan-off delay duration is increased when the cooling off cycle duration is greater than a cooling on cycle duration or the variable fan-off delay duration is decreased when the cooling off cycle duration is less than the cooling on cycle duration.

11. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a variable fan-off delay CST threshold; and wherein the variable fan-off delay duration is decreased when the cooling off cycle duration is less than the cooling on cycle duration minus a tolerance based on a first coefficient times the cooling on cycle duration wherein the first coefficient varies as a function of the cooling on cycle duration.

12. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; wherein the variable fan-off delay duration is increased when the cooling off cycle duration is greater than the cooling on cycle duration plus a tolerance based on a second coefficient times the cooling on cycle duration wherein the second coefficient varies as a function of the cooling on cycle duration.

13. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; wherein the variable fan-off delay duration is adjusted based on a duration of a heating off cycle; and wherein the variable fan-off delay duration is increased when the heating off cycle duration is greater than a heating on cycle duration or the variable fan-off delay duration is decreased when the heating off cycle duration is less than the heating on cycle duration.

14. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle has ended or providing a variable fan-off delay after a heating on cycle has ended; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; and wherein the variable fan-off delay duration after the heating on cycle is decreased when the heating off cycle duration is less than the heating on cycle duration minus a tolerance based on a third coefficient times the heating on cycle duration wherein the third coefficient varies as a function of the heating on cycle duration.

15. A method for controlling a Heating Ventilating Air Conditioning (HVAC) system fan with a thermostat, the method comprising: providing a variable fan-off delay after a cooling on cycle or providing a variable fan-off delay after a heating on cycle; wherein the variable fan-off delay is ended based on comparing a drybulb temperature measurement of a Conditioned Space Temperature (CST) to a CST variable fan-off delay threshold; and wherein the variable fan-off delay duration after the heating on cycle is increased when the previous heating off cycle duration is greater than the current heating on cycle duration plus a tolerance based on a fourth coefficient times the heating on cycle duration wherein the fourth coefficient varies as a function of the heating on cycle duration.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The above and other aspects, features and advantages of the variable fan-off delay or FDD method will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

(2) FIG. 1 shows the efficient fan controller connected to a Heating, Ventilation, Air Conditioning (HVAC) system with a gas furnace, an electric resistance, or an hydronic heating system.

(3) FIG. 2 shows the efficient fan controller connected to a Heat Pump (HP) HVAC system with reversing valve energized for cooling.

(4) FIG. 3 shows the efficient fan controller connected to a heat pump HVAC system with reversing valve energized for heating.

(5) FIG. 4 shows elements of the efficient fan controller for HVAC systems with direct-expansion Air Conditioning (AC) and gas furnace, heat pump, electric resistance, or hydronic heating.

(6) FIG. 5 shows the efficient fan controller with and without a method providing variable fan-off delays and identifying low sensible cooling capacity and correcting the final variable fan-off delay to improve sensible cooling efficiency.

(7) FIG. 6 shows a graph of the sensible cooling Energy Efficiency Ratio (EER*), cooling system power, outdoor air temperature, thermostat temperature, and rate of change of thermostat temperature with respect to time (dT/dt) for a direct-expansion cooling system with known control and the variable fan-off delay control method.

(8) FIG. 7 shows a graph of heating efficiency, outdoor air temperature, indoor thermostat temperature, and rate of change of indoor thermostat temperature versus time of operation for a gas furnace heating system with a known control and the variable fan-off delay control method.

(9) FIG. 8 shows a known control with a fan-on setting providing continuous fan operation for 60 minutes and a FDD method detecting, reporting, and overriding the fan-on setting, turning off the HVAC fan for a fraction of a fan-on setting duration, and turning on the HVAC fan during a thermostat call for cooling or heating or a fan-off delay.

(10) FIG. 9 shows a first method for determining what type of HVAC system is connected and what fan controller operating mode and what aspect of the FDD method to perform.

(11) FIG. 10 shows a method for determining a variable fan-off delay P2 after a heating cycle.

(12) FIG. 11 shows a method for determining a variable fan-off delay P2 after a cooling cycle.

(13) FIG. 12 provides a flow chart of a fan-on FDD method to diagnose and optionally report a fan-on setting without a thermostat call for cooling or heating in order to override and turn off the fan to save fan energy and cooling or heating energy.

(14) Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

(15) The following description is of the best mode presently contemplated for carrying out the Fault Detection Diagnostic (FDD) method. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the method. The scope of the method should be determined with reference to the claims.

(16) Where the terms “about” or “generally” are associated with an element of the FDD method, it is intended to describe a feature's appearance to the human eye or human perception, and not a precise measurement.

(17) FIG. 1 shows the efficient fan controller 211 connected to a Heating, Ventilating, Air Conditioning (HVAC) system with an Air Conditioning (AC) compressor or AC compressor control 203 for direct-expansion cooling and the heat source control 202 for a gas furnace, an electric resistance, or a hydronic heating system. The HVAC system 340 is shown as a dashed line with the heat source control 202, the AC compressor control 203, the fan relay 205, and the HVAC fan 206. The thermostat or equipment control terminals 201 are connected and transmitting an active thermostat control signal or an inactive thermostat control signal using analog 24 Volts Alternating Current (VAC) signals to the efficient fan controller 211 inputs: 1) the fan G terminal 204 transmits an active fan signal from the thermostat or inactive fan signal from the thermostat to the input 204A of the fan relay 205 or to the fan G input 214, 2) the AC/HP Y terminal 207 transmits an active AC/HP compressor signal from the thermostat or inactive AC/HP compressor signal from the thermostat to the input 207A of the AC compressor control 203 or to the AC/HP Y input 215 using an optional wire 276, 3) the heat W terminal 208 transmits an active heating signal from the thermostat or inactive heating signal (or gas heating signal) from the thermostat to the input 208A of the heat source control 202 or to the heat W input 216, 4) the common terminal 210a from the system transformer 210 connects to the COM B input 221 and also connects using the wire 223 to the heat source control 202 and the AC compressor control 203 and the fan relay 205, and 5) the hot R terminal 209 from the system transformer 210 connects to the hot R input 213. The Hot R terminal 209 can be optionally connected to the HP input 234 to enable fan control for a Heat Pump (HP) system. The dashed line 217 indicates where the original thermostat fan signal wire to the fan relay 205 has been disconnected to connect this signal to the efficient fan controller 211 and transfer control of the fan relay 205 and the HVAC fan 206 to the efficient fan controller 211. The efficient fan controller 211 transmits a 24 VAC control signal to the fan relay 205 through the fan signal output 212 of the efficient fan controller 211.

(18) FIG. 2 shows the efficient fan controller 211 connected to an HVAC system with an HP compressor control 203b for Direct Expansion (DX) cooling and heating and a reversing valve 263 energized for cooling. The HVAC system 340 is shown as a dashed line with the reversing valve 263 energized for cooling, the HP compressor control 203b, the fan relay 205, and the HVAC fan 206. The efficient fan controller 211 is connected to the thermostat or equipment control terminals 201 which transmit an active thermostat control signal or an inactive thermostat control signal using 24 VAC analog signals to the efficient fan controller 211 inputs: 1) the fan G terminal 204 connects to the input 204A of the fan relay 205 or to the fan G input 214, 2) the AC/HP Y terminal 207 connects to the input 207A of the HP compressor control 203b or to the AC/HP Y input 215 using an optional wire 276, 3) the REV O terminal 235 connects to the input 235A of the reversing valve 263 or to the heat W input 216, 4) the common terminal 210a from the system transformer 210 connects to the COM B input 221 and also connects using the wire 223 to the reversing valve 263 and the HP compressor control 203b and the fan relay 205, and 5) Hot R terminal 209 from the system transformer 210 connects to the hot R input 213. The hot R terminal 209 may also be connected to the HP input 234 using the wire 265. If the efficient fan controller 211 detects current flowing in both the positive cycle and negative cycle on the HP input 234, then the efficient fan controller 211 uses this signal to detect a HP system with the reversing valve 263 energized for cooling. The dashed line 217 indicates where the original thermostat fan signal wire to the fan relay 205 has been disconnected in order to route this signal to the fan G input 214 of the efficient fan controller 211. The efficient fan controller transmits a 24 VAC control signal through the fan signal output 212 to control the fan relay 205 and the HVAC fan 206.

(19) FIG. 3 shows the efficient fan controller 211 connected to an HVAC system with the HP compressor control 203b for DX cooling and heating and a heat pump reversing valve 264 energized for heating. The HVAC system 340 is shown as a dashed line with the reversing valve 264 energized for heating, the HP compressor control 203b, the fan relay 205, and the HVAC fan 206. The efficient fan controller 211 is connected to the thermostat or equipment control terminals 201 which transmit active or inactive 24 VAC analog signals to the efficient fan controller (211) inputs: 1) the fan G terminal 204 is connected to the input 204A of the fan relay 205 or to the fan G input 214, 2) the AC/HP Y terminal 207 is connected to the input 207A of the HP compressor control 203b or to the AC/HP Y input 215 using an optional wire 276, 3) the REV BR terminal 236 (reversing valve) is connected to the input 236A of the reversing valve 264 or to the heat W input 216, 4) the common terminal 210a from the system transformer 210 is connected to the COM B input 221 and also connects using the wire 223 to the reversing valve 264 and the HP compressor control 203b and the fan relay 205, and 5) the hot R terminal 209 is connected to the hot R input 213 The hot R terminal may also be connected to efficient fan controller 211 through the HP input 234 with a diode 275 to detect a HP with a reversing valve 264 energized for heating. The diode 275 only allows current to flow to the efficient fan controller 211 on positive cycles of the hot R signal from the system transformer 210. By detecting current flowing only during the positive cycle and not on the negative cycle, the efficient fan controller 211 provides control for a HP system with the reversing valve 264 energized for heating. The dashed line 217 indicates where the original thermostat fan signal wire to the fan relay 205 has been disconnected in order to route this signal to the fan G input 214. The efficient fan controller transmits an active 24 VAC analog control signal through the fan signal output 212 to control the fan relay 205 and the HVAC fan 206.

(20) FIG. 4 shows components of the efficient fan controller 211 used to control HVAC systems with DX cooling and gas furnace, electric resistance, heat pump, or hydronic heating. A Normally Closed (NC) relay or the NC relay 309 connects the active or inactive 24 VAC analog signal from the thermostat to the fan signal output 212 of the efficient fan controller 211. The NC relay 309 in the efficient fan controller 211 provides a fail-safe option for the fan G terminal 204 to always be connected to the fan relay 205 and allow the HVAC fan 206 to properly operate. Under normal operation, when the efficient fan controller 211 is controlling the fan relay 205, the microprocessor 304 provides a signal 836 to the NC relay 309 to be in a Normally Open (NO) position. When the efficient fan controller 211 receives a fan signal on the fan G input 214, the microprocessor 304 provides a fan control output signal 834 which is a non-zero Volts Direct Current (VDC) digital signal to a WIFI or switching device 301 which provides an analog fan control signal 212a to the NC relay 309 to energize the fan relay 205 and control the HVAC fan 206. The WIFI or switching device 301 may be used to send and/or receive a wired signal or a wireless signal to the fan relay or any other HVAC system device using a smart communicating thermostat with temperature sensors and WIFI technology for wireless local area networking based on the IEEE 802.11. The efficient fan controller 211 has the following 24 VAC analog signal inputs from the thermostat or equipment control terminals 201: 1) the fan G input 214, 2) the AC/HP Y input 215, 3) the heat W input 216, and 4) the HP input 234. The fan signal output 212 of the efficient fan controller 211 is used to energize the fan relay 205 and operate the HVAC fan 206. The input signals including the fan G input 214, the AC/HP Y input 215, the heat W input 216, and the HP input 234 and an output of the zero crossing detector 302 pass through a signal conditioning element 308 to provide a zero VDC digital signal or a non-zero VDC digital signal to the microprocessor 304. The signal conditioning element 308 converts active analog HVAC control signals to zero VDC digital HVAC control signals and converts inactive analog HVAC control signals to non-zero VDC digital HVAC control signals. The microprocessor 304 is used to control the WIFI or switching device 301 and the NC relay 309. The microprocessor 304 also has an input from a zero crossing detector 302. The zero crossing detector 302 monitors a COM B input 221 signal (see FIGS. 1-3) of the system transformer 210. The COM B input may be switched with a Hot R 210b input using simple electrical circuit modifications. The zero crossing detector 302 provides an analog zero crossing signal 272 to the signal conditioning element 308 which provides a zero VDC or a non-zero VDC digital to the microprocessor 304 which enables the microprocessor 304 to determine when the hot R input 213 from the system transformer passes above zero volts and below zero volts. This information is used by the microprocessor 304 to count cycles for timekeeping and to determine when to provide the fan control output signal 834. The zero crossing times are also required when the WIFI or switching device 301 is a triac. To operate the triac as a switch, the triac must be fired at all zero crossing transitions. The AC-DC converter 303 has inputs from the system transformer to the COM B input 221 and at least one 24 VAC analog signal selected from the group consisting of: an active heating signal or an inactive heating signal (or gas heating signal) from the thermostat to a heat W input 216, an active AC/HP compressor signal or an inactive AC/HP compressor signal from the thermostat to a AC/HP Y input 215, and an active fan signal or an inactive fan signal from the thermostat to a fan G input 214. Any of these signals can be rectified in the AC-DC converter 303 to provide Direct Current (DC) power to the microprocessor 304 and to keep an optional battery 306 charged. An optional super capacitor 312 can be charged from the AC-DC converter 303 and used to power the fan controller until sufficient voltage can be generated from the input signals. A DC rail voltage signal 270 from the AC-DC converter 303 or the optional battery 306 may be used to power the microprocessor and charge the optional super capacitor 312. There is also an optional user interface 305 which may be used to configure the microprocessor 304 to perform in an alternate manner. A HP input 234 to detect a Heat Pump (HP) is passed through the signal conditioning element 308 before being passed to the microprocessor 304. The zero crossing detector 302 processes the fan signal output 212 and the COM B input 221 and passes these signals to the signal conditioning element 308 which provides a zero VDC or a non-zero VDC digital for the microprocessor 304 to detect when the thermostat signals are above ground and below ground. If the HP input 234 is not connected to the system transformer 210 as shown in FIG. 1, then the microprocessor 304 detects the signal to the HP input 234 as floating and detects it is not connected to a HP system. If the HP input 234 is connected to the system transformer 210 as shown in FIG. 2, the microprocessor 304 detects the HP signal driven above and below ground and the microprocessor 304 detects it is connected to a HP system with the reversing valve energized for cooling.

(21) When a diode 275 is introduced as shown in FIG. 3, the signal to the HP input 234 is driven during the positive cycle and floats because of the direction of the diode 275, during the negative cycle where the signal is rectified. The microprocessor 304 detects this state and performs like it is connected to a heat pump system with a HP reversing valve (the reversing valve 263 energized for cooling or the reversing valve 264 energized for heating) driven for heating. As discussed above, the microprocessor 304 is configured to detect whether or not a specific signal input is active or inactive based on input signals received from the signal conditioning element 308 which is able to process five low-voltage electrical input signal states: 1) a ground or zero VAC signal, 2) a 24 VAC signal, 3) a floating signal, 4) a false positive stray voltage signal, and 5) rectified signal. The signal conditioning element 308 converts active analog HVAC control signal inputs from the thermostat to zero Volts Direct Current (VDC) digital HVAC control signals, and converts inactive analog HVAC control signals to non-zero VDC digital HVAC control signals used by the microprocessor 304.

(22) The microprocessor 304 performs several functions. In terms of timing, the microprocessor 304 keeps track of seconds and minutes by either monitoring the synchronous zero to +5 VAC 60 Hz square wave output from the AC-DC converter 303 referred to as the signal 345 which is a fifth digital timing HVAC control signal on the wire connection 830 to the microprocessor 304, or by counting microprocessor clock cycles. Each positive zero edge accounts for 1/60th of a second; therefore, sixty positive crossings occur each second. The seconds are then accumulated to keep track of minutes. The negative crossings are also monitored to provide timing for the WIFI or switching device 301.

(23) The efficient fan controller 211 draws power from the system transformer 210 (see FIG. 1-3). The switching device 301 can be standard relay type device, a reed relay or some other electro-mechanical device, and can also be a solid-state device such as an FET switch or a triac. In the event that an electro-mechanical switch is used, either an optional battery can be added to power the microprocessor 304 or the AC/HP Y input 215, the heat W input 216 or the COM B input 221 can provide power through the AC-DC converter when the switch is closed. The fan controller uses the Hot R 210b signal from the system transformer 210 and a triac to substitute for the WIFI or switching device 301 and does not require a battery.

(24) The microprocessor 304 continuously monitors inputs to determine if there is any change to the current system operation. The microprocessor 304 contains FLASH memory, which allows the unit to store the programming instructions and data when there is no power applied to the unit. The microprocessor 304 monitors the duration of the active or inactive signals from the thermostat or equipment control terminals 201 including: 1) the fan G input 214, 2) the AC/HP Y input 215, and/or 3) the heat W input 216. The microprocessor 304 adjusts the variable fan-off delay P2 based on the active or inactive analog signals representing the cooling cycle duration or the heating cycle duration including at least one cycle selected from the group consisting of: an on cycle and an off cycle. If the AC compressor or the heat source are operated for a short period of time (i.e., short cycle) and there is not much condensation stored on the evaporator or heat stored in the heat exchanger, then the fan relay 205 and the HVAC fan 206 operating time will be extended for a shorter period of time or not. Likewise, if the AC compressor operates longer allowing more condensate to be stored on the evaporator, or the heat source control 202 operates longer storing more heat in the heat exchanger, then the efficient fan controller 211 will energize the fan relay 205 and operate the HVAC fan 206 to run for a longer fan-off delay period of time after the AC compressor or the heat source have stopped.

(25) FIG. 5 shows a graph of a the sensible Energy Efficiency Ratio* (EER*) performance of an HVAC system in cooling mode with a method to improve energy efficiency and conserve energy. The method provides increasing sensible efficiencies from 5.5 to 5.9 EER* based on variable fan-off delays increasing from 3 to 5 minutes for AC compressor cycles 1-3 with durations of six to 10 minutes. During a 17-minute AC compressor cycle 4 with 8.5 minute fan-off delay, the sensible efficiency drops to 5.7 EER* due to continued fan operation with insufficient moisture on the evaporator coil. AC compressor cycle 5 turns on during the cycle 4 fan-off delay period P2 when the thermostat temperature exceeds 77 degrees Fahrenheit (° F.), which is the upper differential based on a 76° F. cooling setpoint resulting in no time between cycle 4 and cycle 5 where the fan and the AC compressor are both off (i.e., P115=P24). Furthermore, Cycle 5 turning on during the cycle 4 fan-off delay indicates insufficient evaporative cooling available to support the cycle 4 fan-off delay of 8.5 minutes. The method detects the AC compressor turning on during the cycle 4 fan-off delay and stores this FDD information. After the fifth 30-minute AC compressor cycle 5 the FDD method automatically reduces the cycle 5 fan-off delay to 5 minutes to increase the AC compressor Off time and increase the sensible efficiency to 6 EER* which is a 5% improvement compared to 5.7 EER* for cycle 4 that had no AC compressor Off time.

(26) In another embodiment, the FDD algorithm determines a variable fan-off delay P2 based on the cooling cycle duration P4 including at least one cycle selected from the group consisting of: a cooling on cycle, and a cooling off cycle P11, or optionally, the Conditioned Space Temperature (CST) as measured by the thermostat (see FIG. 6).

(27) In another embodiment, the FDD method determines a variable fan-off delay P2 based on the heating cycle duration P3 including at least one cycle selected from the group consisting of: a heating on cycle, and a heating off cycle P11, or optionally, the CST as measured by the thermostat (see FIG. 7).

(28) For both of these embodiments, the variable fan-off delay P2 is based on the heating cycle duration P3 or the cooling cycle duration P4 in order to extend the fan-on operating time to improve energy efficiency. The off cycle time P11 is used to adjust the variable fan-off delay P2 to extend the off cycle time P11 and improve energy efficiency. If the variable fan-off delay P2 causes the off cycle time P11 to be less than the heating cycle duration P3 or the cooling cycle duration P4 indicating low heating or cooling capacity due to system faults or severe weather, then the P11 and the P3 or the P4 are used to reduce the P2. If the variable fan-off delay P2 causes the off cycle time P11 to increase relative to the P3 or the P4, then the P11 and the P3 or the P4 are used to increase the P2.

(29) The method monitors the cooling or heating off cycle time P11 and adjusts P2 based on P11 where P2 is adjusted up if P11 is increasing and P2 is adjusted down if P11 is decreasing. The adjustment is determined based on how far P11 is from P4 over time. If the rate of change of P11 with respect to time is decreasing, then the method reduces P2, and if the rate of change of P11 with respect to time is increasing, then the method increases P2. The method increases thermal comfort, extends off cycle times, reduces on cycle times, improves efficiency, and saves energy.

(30) FIG. 6 shows a graph of the sensible cooling application Energy Efficiency Ratio (EER*) 365, electric power 370, outdoor air temperature 368, thermostat temperature 369, and EER* for a HVAC system in cooling mode. FIG. 6 shows a first EER* curve 365 going from 0 to 5.3 EER* with no fan-off delay. The AC compressor and the fan are turned on when the thermostat temperature or the CST is at the upper cooling differential 372, and the AC compressor and fan are turned off when the thermostat temperature decreases to the lower cooling differential 371 a first time. FIG. 6 shows a second EER* curve 367 going from 0 to 6.2 EER* with a variable fan-off delay P2 of 4.33 minutes. The variable fan-off delay P2 may be based on a cooling cycle duration P4 including at least one cycle selected from the group consisting of: a cooling on cycle, and a cooling off cycle P11. A rate of change of the thermostat temperature with respect to time (dT/dt) is shown as the slope of the thermostat temperature 369.

(31) In another embodiment the thermostat cooling variable fan-off delay P2 is ended based on comparing a temperature measurement of a Conditioned Space Temperature (CST) to a CST threshold wherein the CST threshold is based on a previous measurement of the CST monitored during the current variable fan-off delay. The variable fan-off delay P2 is ended based on the CST as measured by the thermostat temperature 369 reaching at least one measurement threshold selected from the group consisting of: a measurement of the CST decreases to a minimum thermostat temperature after the cool source is turned off where the rate of change of temperature with respect to time (dT/dt) reaches an inflection point and is approximately equal to zero (dT/dt=0) plus or minus a confidence interval tolerance, the measurement of the CST increases to a cooling fan-off delay differential offset 374, and the measurement of the CST crosses a lower cooling differential 371 at least once after the cooling cycle.

(32) Operating individually or together, these FDD fan-off delay embodiments can be used to detect faults impacting energy efficiency performance, and recover and deliver additional sensible cooling energy from a cool source to improve efficiency and thermal comfort and reduce cooling system operating time to save energy.

(33) FIG. 7 shows a graph of a heating efficiency, an outdoor air temperature 358, and a thermostat temperature 359. The heating efficiency curve 355 reaches 63% and the heating efficiency curve 356 reaches 66% for the known control. The heat source is turned on when the CST or thermostat temperature decreases to the lower heating differential 360, the heat source is turned off when the thermostat temperature reaches the upper heating differential 361 a first time, and the fan operates for a fixed fan-off delay after the heat source is turned off. A measured rate of change of the CST or the thermostat temperature 359 versus time of operation is illustrated by the slope of the CST or the thermostat temperature 359.

(34) FIG. 7 also shows a heating efficiency curve 357 reaches 80.5% representing a heating system operating until the measurement of the CST reaches the upper heating differential 361 a first time where the heat source is turned off and the HVAC fan continues to operate for a variable fan-off delay time P2 based on the heating cycle duration P3 including at least one cycle selected from the group consisting of: a heating on cycle, and a heating off cycle P11.

(35) In another embodiment the heating variable fan-off delay P2 is optionally based on the CST as measured by the thermostat temperature 359 reaching at least one threshold selected from the group consisting of: the measurement of the CST reaches a maximum temperature beyond the upper heating differential 361 after the heat source is turned off where the rate of change of the temperature with respect to time (dT/dt) reaches an inflection point and is approximately equal to zero (dT/dt=0) plus or minus a confidence interval tolerance, the measurement of the CST decreases to heating fan-off delay differential offset 363, and the measurement of the CST crosses the upper heating differential 361 at least once after the heating cycle.

(36) The CST thresholds for heating and cooling can be adjusted based on at least one duration selected from the group consisting of: the heating cycle duration P3, the cooling cycle duration P4, and the off cycle P11. The method can improve HVAC cooling and heating efficiency by providing a variable thermostat differential to provide longer operating times where the variable differential is based on the heating cycle duration P3, the cooling cycle duration P4, and the off cycle P11.

(37) Operating individually or together, these FDD embodiments can be used to detect faults impacting energy efficiency performance, and recover and deliver additional sensible heating energy from a heat source to improve efficiency and thermal comfort and reduce heat source operational time to save energy.

(38) FIG. 8 shows a known control 11 operating from 0 to 60 minutes for a HVAC system with a fan-on setting causing continuous fan power and increasing HVAC energy use. FIG. 8 also shows the FDD method 12 operating from 0 to 110 minutes where the FDD method detects a fan-on setting or monitors active or inactive signals present on a thermostat or equipment control terminals to determine if the HVAC thermostat fan control has been set to the fan-on setting. FIG. 8 shows the FDD method 12 performs at least one method: monitors, detects, reports a fan-on setting, overrides a fan-on setting and turns off the HVAC fan for a fraction of a fan-on setting duration, and reduces HVAC energy use. FIG. 8 shows the FDD method allows the fan-on setting and four HVAC cycles for a first time duration 12a of 60 minutes. FIG. 8 shows the FDD method overrides the fan-on setting and turns off the HVAC fan for a second time duration 12b of 17.3 minutes from 60 minutes to 77.3 minutes or 15.7% of the fan-on setting duration of 110 minutes. The FDD method allows a HVAC cycle and a fan-off delay for a third time duration 12c of 12.3 minutes from 77.3 minutes to 88 minutes. The FDD method overrides the fan-on setting and turns off the HVAC fan for a fourth time duration 12d of 22 minutes from 88 minutes to 110 minutes or 20% of the fan-on setting duration of 110 minutes. The FDD method may also include an optional user interface 305 (shown in FIG. 4) which may be used to configure the microprocessor 304 to enter a user-selected TFT value. The fan-on setting may comprise at least one fan-on setting selected from the group consisting of: a continuous hourly fan-on setting greater than 0 minutes to 60 minutes, a continuous daily fan-on setting of 1 hour to 24 hours, and a continuous fan-on setting greater than 24 hours (on a scheduled basis). The FDD method detects at least one fan-on setting selected from the group consisting of: an active fan signal with neither an active cooling signal nor an active heating signal from the thermostat, the active fan signal and an inactive heating signal from the thermostat, the active fan signal and an inactive AC/HP compressor signal from the thermostat, the active fan signal without an active fan-off delay signal after the active cooling signal or after the active heating signal from the thermostat, or the active fan signal and an active gas heating signal from the thermostat or equipment terminal or the presence of the fan-on setting without a thermostat call for cooling or without a thermostat call for heating. By turning off the fan-on setting during the occupied or the unoccupied period the FDD method reduces over ventilation and HVAC energy by 10 to 90%.

(39) FIG. 9 provides a flowchart of the FDD method to determine an HVAC system type and operating mode and fan control including detecting a fan-on setting and provide a variable fan-off delay. The FIG. 9 flowchart starts at the FDD method step 501. Step 502 accumulates the heating off cycle duration or the cooling off cycle duration P11. The FDD method uses the off cycle duration P11 to decrease the variable fan-off delay P2, if P11 is less than the heating on cycle (P3-P11) or the cooling on cycle (P4-P11) minus a tolerance based on a third coefficient times the heating on cycle duration (P3-P11) or a first coefficient times the cooling on cycle duration (P4-P11) where the third coefficient varies as a function of the heating on cycle duration (P3-P11) and the first coefficient varies as a function of the cooling on cycle duration (P4-P11).

(40) The FDD method also uses the off cycle duration P11 to increase the variable fan-off delay P2, if P11 is greater than the heating on cycle (P3-P11) or the cooling on cycle (P4-P11) plus a tolerance based on a fourth coefficient times the heating on cycle (P3-P11) or a second coefficient times the cooling on cycle (P4-P11) where the fourth coefficient varies as a function of the heating on cycle (P3-P11) and the second coefficient varies as a function of the cooling on cycle (P4-P11). If P11 is within a range of P3+/−the tolerance (defined by the first and second coefficients), then the FDD method does not adjust P2 which is based on the heating cycle duration P3 or the cooling cycle duration P4. For the gas furnace, the FDD method provides a fan-on delay P1 before the fan is energized to a ventilation fan speed higher than a lower heating ventilation fan speed normally used for heating when a fan relay is energized after a short delay to allow the heat exchanger (HX) to reach operating temperature.

(41) FIG. 9 at step 503 detects whether or not a fan is active by itself and operating continuously without a thermostat call for heating or cooling. If step 503 is Yes (Y), then the method proceeds to step 519 Go to Fan-On FDD method the step 951 (FIG. 12). In FIG. 12, the FDD method diagnoses the fan-on setting, and determines whether or not to provide an optional FDD alarm fan-on message, override the fan-on setting, and de-energize the fan relay (or thermostat fan G signal) and turn off the HVAC fan to save energy. For a thermostat controlling a gas furnace, the heat W signal is energized without the fan G signal during the heating cycle, and for a thermostat fan-off delay the method proceeds to the step 515 Go to the heating fan control step 601 (FIG. 10). For a thermostat controlling a heat pump, electric resistance, or hydronic heating system, the method proceeds to step 515 Go to the heating fan control step 601 (FIG. 10). For a thermostat controlling a cooling system, the method proceeds to the step 516 and the step 701 cooling fan control (FIG. 11). Otherwise, the FDD method goes to step 504 to determine if the fan and the AC/HP compressor or the fan and the heat are active. If the step 504 is Yes (Y), then the method goes to step 510 to determine if the heat signal is active. If the step 504 is No (N), then the method proceeds to step 516 Go to Cooling fan control the step 701 (FIG. 11).

(42) At step 510, if the heat signal is active simultaneously with the fan signal, then the method proceeds to step 511 to determine which system is active including at least one system selected from the group consisting of: a heat pump heating, an electric heating, or a hydronic heating system. If step 511 determines the HP input 234 signal is active, then the method proceeds to step 517 to set the HP flag. If step 511 determines the HP input 234 signal is not active, then the method proceeds to step 512 to set electric or hydronic heat flag. After steps 512 or 517, the method proceeds to step 513 to accumulate heating on cycle time P3, and proceeds to step 515 Go to the heating fan control step 601 (FIG. 10).

(43) FIG. 9 steps 504 through 508 determine if gas furnace heating is active (with no fan signal). If the step 505 is yes (Y), the HP input 234 signal is active, then the method loops back to step 518 clear fan off flag. If the step 505 is No (N), the HP input 234 signal is not active, then the method proceeds to step 506. If step 506 determines No (N) the heat signal is not active, then the method loops back to step 518 clear fan off flag. If step 506 determines Yes (Y), the heat signal is active, then method proceeds to step 507 to set gas furnace flag, step 508 to accumulate heating cycle P3 and step 509 to evaluate if fan on delay P1 expired and if not, loops back to step 508 to continue to accumulate heating on cycle P3 time. If step 509 determines Yes (Y), fan on delay P1 has expired, then proceeds to step 515 Go to the heating fan control step 601 (FIG. 10). In some embodiments, heat pump operation is established by connecting HP Rev signal to hot side of the system transformer with reversing valve normally energized for cooling or a wire with a diode for a heat pump with reversing valve normally energized for heating.

(44) FIG. 10 shows a heating fan control FDD method. The heating fan control step 601 is the beg inning of the method. Step 602 energizes a switching device which connects 24 VAC to a fan relay to turn on a HVAC fan. Step 603 is the entry of a loop that operates continuously while the thermostat is calling for heating regardless of system type. At step 603, the method accumulates the duration of the heating on cycle P3 and optionally monitors CST until the thermostat is satisfied and discontinues the call for heating. Step 604 is used to determine whether or not a flag is set for HP heating, electric heating, or hydronic heating system based on the step 512 or the step 517 of FIG. 9.

(45) If step 604, determines No (N), the HP, electric, or hydronic heating flag is not set, then the method proceeds to step 605 to determine if the gas furnace heat signal is active (from thermostat W heat terminal), and if Yes (Y), then the method loops back to step 603 to accumulate the heating cycle duration and optionally monitor CST. If the step 605 is No (N), the gas furnace heat signal is not active, then the method proceeds to step 606. At step 606 the method calculates the heating variable fan-off delay P2 based on at least one heating system type selected from the group consisting of: a gas furnace heating system with a flag set in step 507, an electric resistance heating system or a hydronic heating system with the flag set in step 512, and heat pump heating system with the flag set in step 517; or the heating variable fan-off delay P2 is based on at least one heating cycle duration selected from the group consisting of: a heating on cycle duration P3, and a heating off cycle duration P11; or optionally the heating variable fan-off delay P2 is based on the measurement of the CST reaching at least one threshold selected from the group consisting of: the measurement of the CST increases to the maximum thermostat temperature 362 beyond the upper heating differential 361 after the heat source is turned off where the rate of change of the temperature with respect to time (dT/dt) reaches an inflection point and is approximately equal to zero (dT/dt=0) plus or minus a confidence interval tolerance, the measurement of the CST decreases to heating fan-off delay differential offset 363, and the measurement of the CST crosses the upper heating differential 361 at least once after the heating cycle. The variable fan-off delay increases from zero to a maximum and/or decreases to a minimum or zero as a function of the heating cycle duration or the cooling cycle duration or HVAC system type such as a direct-expansion cooling system with at least one heating system type selected from the group consisting of: the gas furnace heating system, the hydronic heating system, the electric resistance heating system, or the heat pump heating system. The heating cycle duration or the cooling cycle duration or the HVAC system type are determined and/or based on thermostat settings and/or measurements of signals present on thermostat or equipment terminals.

(46) Alternatively, if step 604, determines Yes (Y) the heating system is a HP, electric or hydronic heating system then the method proceeds to step 611, and if the HP compressor signal or the heat signal are active Yes (Y), then the method returns to step 603 and accumulates the heating cycle duration P3 and optionally monitors the CST. If step 611 determines No (N), the HP compressor or heat signals are not active, then the method proceeds to step 606. For a thermostat not providing a fan-off delay for a HP, electric or hydronic heating system, the FDD method skips from the step 611 to the step 608 to de-energize the fan relay and turn off the HVAC fan. At step 606 the method calculates the variable fan-off delay P2 or the variable fan-off delay P2 is based on CST (as discussed above). The method uses different algorithms with different coefficients to calculate a unique variable fan-off delay P2 for heat pump heating compared to electric/hydronic heating or gas furnace heating. In step 606, the method calculates the variable fan-off delay P2 for each heating system and each heating cycle duration using the different algorithms with the different coefficients depending on the flag set in the step 507, the step 512, or the step 517. After step 606, the method proceeds to step 607 and continues to loop and operate the HVAC fan 206 for the variable fan-off delay time P2 until the time delay P2 has expired or CST reaches a threshold. At step 607 after the variable fan-off delay time P2 has expired or CST has reached the threshold, the method proceeds to step 608 to de-energize the fan relay and turn Off the fan, step 609 to store heating on cycle P3 and Off cycle P11 and optionally CST, and step 610 Go to start the FDD method step 501 (FIG. 9).

(47) FIG. 11 shows a cooling fan control method. The step 701 is the beginning of the method. The step 703 energizes a switching device which connects 24 VAC to a fan relay to turn on a HVAC fan. Step 705 is the entry of a loop that operates continuously while the thermostat is calling for cooling regardless of system type. At the step 705, the method accumulates the duration of the cooling on cycle P4 and optionally monitors the CST until the thermostat is satisfied and discontinues the call for cooling. If the step 707 determines the cooling or fan signal is active Yes (Y), then the method continues in the loop and accumulates the duration of the cooling on cycle P4.

(48) If step 707 determines No (N), the cooling or fan signal is not active, then the method proceeds to step 709 to calculate the cooling variable fan-off delay P2 based on a cooling cycle duration including at least one cycle selected from the group consisting of: a cooling on cycle P4, and a cooling Off cycle P11. The cooling variable fan-off delay P2 may also be based on a measurement of the Conditioned Space Temperature (CST) reaching at least one threshold selected from the group consisting of: the measurement of the CST decreases to the minimum thermostat temperature 373 beyond the lower cooling differential after the cool source is turned off where the rate of change of temperature with respect to time (dT/dt) reaches an inflection point and is approximately equal to zero plus or minus a confidence interval tolerance, the measurement of the CST increases to cooling fan-off delay differential offset 374, and the measurement of the CST crosses the lower cooling differential 371 at least once after the cooling cycle. In another embodiment, the FDD method compares the cooling off cycle time P11 to the cooling on cycle time P3 in order to determine whether or not to adjust the variable fan-off delay and decrease P2 if P11 is less than the P4 lower tolerance and increase P2 if P11 is greater than the P4 upper tolerance. For a thermostat not providing a fan-off delay, the FDD method skips from the step 707 to the step 713 to de-energize the fan relay and turn off the fan.

(49) After step 709, the method proceeds to step 711 and continues to loop and operate the HVAC fan for the variable fan-off delay time P2 until the variable fan-off delay time P2 has expired or the measurement of the CST reaches the at least one threshold described above. At step 713 after the variable fan-off delay time P2 has expired, the method de-energizes the fan relay and turns off the fan. At step 715 the method stores the cooling cycle duration P4, the off cycle time P11 and optionally stores the CST, and proceeds to step 717. At step 717 the method goes to Start 501 (FIG. 9).

(50) FIG. 12 shows the Fan-on FDD method to monitor for a continuous fan-on setting operation. The FDD method is used to turn off the HVAC fan if a user-selected fan-on setting or a thermostat fan switch is in the fan-on position. The FDD method comprises monitoring a fan-on time and diagnosing the fan-on time is greater than a Threshold Fan-on Time (TFT) where the FDD method performs at least one action selected from the group consisting of: providing an optional FDD alarm fan-on message at step 964, overriding the fan-on setting to save energy, and de-energizing the fan relay to override the thermostat fan-on setting and turning off the HVAC fan at step 965 to save energy. The TFT or the override duration may be at least one value selected from the group consisting of: greater than 0 minutes to 60 minutes for a continuous hourly fan-on setting, 1 hour to 24 hours for a continuous daily fan-on setting, greater than 24 hours for a continuous fan-on setting, a fraction of the fan-on duration, a fraction of the fan-on setting duration, and a user-selected value. The FDD method continues to monitor HVAC system parameters during the off cycle. Step 951 is the start of the FDD Fan-on method with the fan on and no thermostat call for heating or cooling. At Step 953, the FDD method initiates a continuous loop to accumulate a fan-on operating time or a fan-on duration F6.

(51) At Step 955 the method determines whether or not the “Fan off flag set” is Yes (Y) or No (N). If step 955 is Yes (Y), then the FDD method continues to step 964 to provide an optional FDD alarm message reporting a fan-on setting where the optional FDD alarm message is selected from the group consisting of: a software display message, an email message, a text message, or other communication method. The method continues to step 965 to override the fan-on setting, de-energize the fan relay (or thermostat fan G signal), and turn off the fan. At step 965, the FDD method performs at least one method of overriding the fan-on setting selected from the group consisting of: turning off the HVAC fan during an unoccupied period, turning off the HVAC fan for a fraction of a fan-on duration, turning off the HVAC fan for a fraction of the fan-on setting duration, turning off the HVAC fan for 0 to 100% of the fan-on duration, and turning off the HVAC fan for a user-selected fan-off duration that does not interfere with a fan operation during a thermostat call for cooling or heating or a fan-off delay (see FIG. 8). The Fan off flag is set in step 967 based on step 959 determining that F6 is greater than or equal to the Threshold Fan-on Time (TFT) or the override duration. If step 955 is No (N), then step 957 energizes the fan relay (or thermostat fan G signal) and turns on the HVAC fan, and transitions to step 959. At Step 959, the fan controller determines if the fan-on operating time or the fan-on duration F6 has met or exceeded the Threshold Fan-on Time (TFT) or the override duration. The TFT or the override duration may be set to 60 minutes or greater to provide about 8 to 10 air changes per hour depending on occupant discretion (typical air filters are 25% effective at removing airborne particles). The TFT or override duration may also be set to a fraction of a fan-on setting duration including a continuous hourly fan-on setting greater than 0 minutes to 60 minutes, a continuous daily fan-on setting of 1 hour to 24 hours, and a continuous fan-on setting greater than 24 hours. The step 959 determines if the fan-on operating time or the fan-on duration F6 is greater than the TFT or if F6 is greater than the override duration.

(52) If step 959 determines F6 is greater than or equal to the TFT or F6 is greater than or equal to the override duration for the fan-on setting, then the FDD method proceeds to Step 967 and the Fan off flag is set to indicate F6 has met or exceeded the TFT or the override duration. If step 959, is No (N), then the method continues to step 960. At step 960 if there is a thermostat call for cooling, then the method proceeds to step 962 Go to cooling fan control the step 701 (FIG. 11) to turn on the HVAC fan during a thermostat call for cooling. If there is no thermostat call for cooling, then the method proceeds to step 961. At Step 961, the FDD method determines if there is a thermostat call for heating. If step 961 is Yes (Y), then the FDD method proceeds to step 968 Go to the heating fan control step 601 (FIG. 10), to turn on the HVAC fan during a thermostat call for heating. If there is no thermostat call for heating or cooling, the method continues to Step 963 and determines if the fan signal is active. If step 963 determines Yes (Y) the fan signal is active, then the method loops back to Step 953. If step 963 determines No (N), the fan signal is not active, then the method proceeds to step 969 Go to start the FDD method step 501 (FIG. 9).

(53) If occupied or unoccupied continuous fan-on operation is turned off prior to reaching the TFT at step 955, then the FDD method performs at least one action selected from the group consisting of: de-energizing the fan relay (or thermostat fan G signal) to turn off the HVAC fan at step 965, and monitoring the HVAC system parameters during the off cycle to continually check for faults. Adjusting the TFT allows the FDD method to determine whether or not the thermostat fan-on setting was selected by occupants to circulate air and improve air quality.

(54) If the heating signal or the cooling signal are detected or the thermostat call for heating or the thermostat call for cooling are detected during what was previously the occupied or unoccupied continuous fan-only operation and prior to reaching the TFT, then the FDD method performs at least one action selected from the group consisting of: energizing the fan relay (or thermostat fan G signal) to continue energizing the HVAC fan, and monitoring the HVAC system parameters, waiting for the completion of either the heating cycle duration P3 or cooling cycle duration P4 while continuing to energize the HVAC fan, and upon completion of either the heating cycle duration P3 or the cooling cycle duration P4, performing at least one action selected from the group consisting of: determining a variable fan-off time delay P2 based on the heating cycle duration P3 (including the heating on cycle and/or the heating off cycle) or the cooling cycle duration P4 (including the cooling on cycle and/or the cooling off cycle), energizing or continuing to energize the fan relay and the HVAC fan for the variable fan-off delay P2, waiting for the completion of the variable fan-off time delay P2, and de-energizing the fan relay (or thermostat fan G signal) and turning off the HVAC fan at the end of the variable fan-off delay P2.

(55) The FDD method for controlling the HVAC fan is based on comparing a current measurement of a HVAC parameter to a previous measurement of a HVAC parameter, and if a fault is detected, then performing at least one action selected from the group consisting of: turning off a fan-on setting, reporting a FDD alarm fan-on, overriding a fan-on setting, and determining a variable fan-off delay P2. Calculating the variable fan-off delay duration may be based on at least one HVAC parameter selected from the group consisting of: the variable fan-off delay P2, an off cycle time P11, a thermostat call for heating duration, a heating cycle duration P3 including at least one heating cycle selected from the group consisting of: a heating on cycle time, and a heating off cycle, a cooling cycle duration P4 including at least one cooling cycle selected from the group consisting of: a thermostat call for cooling duration, a cooling on cycle time, and a cooling off cycle. Operating the fan for the variable fan-off delay after a cooling cycle or operating the fan for the variable fan-off delay after a heating cycle and ending the variable fan-off delay may also be based on at least one method selected from the group consisting of: comparing a current measurement of a Conditioned Space Temperature (CST) to a previous measurement of the CST during the variable fan-off delay, the measurement of the CST crosses a heating fan-off delay differential offset, the measurement of the CST crosses an upper heating differential at least once after the heating cycle, the measurement of the CST crosses a cooling fan-off delay differential offset, the measurement of the CST crosses a lower cooling differential at least once after the cooling cycle, and the measurement of the CST reaches an inflection point where the rate of change of the measurement of the CST with respect to time equals zero plus or minus a confidence interval tolerance. The rate of change of the measurement of the CST is defined as a difference in temperature between at least two measurements of the CST divided by a difference in time between the at least two measurements of the CST.

(56) A current HVAC parameter is compared to a previously monitored HVAC parameter to determine whether or not the current HVAC parameter is outside a tolerance threshold value sufficient to indicate that a fault has been detected and this fault is impacting energy efficiency performance by more than 5%. If the fault is detected and determined to impact energy efficiency performance by more than 5%, then the FDD output is used as a basis to initiate at least one action. The actions preferably include detecting, reporting and overriding a fan-on setting to save energy, and turning off a fan. A continuous fan-on setting increases fan energy use or heating or cooling energy use by accidentally or intentionally being left on for a long period of time. The actions may also comprise providing or adjusting the variable fan-off delay P2. These actions are preferably based on HVAC parameters including, for example: detecting a HVAC fan is controlled by a fan-on setting and the fan is operating for a continuous fan-on duration with or without a thermostat call for heating or cooling. Providing a variable fan-off delay at the end of a heating cycle to improve energy efficiency or providing a variable fan-off delay at the end of a cooling cycle to improve energy efficiency may be based on HVAC parameters comprising a heating cycle duration; a cooling cycle duration; a conditioned space temperature; or a rate of change of the HVAC parameters with respect to time.

(57) The FDD method is based on at least one of the following HVAC parameters: the variable fan-off delay P2, a heating cycle duration P3 including the heating on cycle time only or the heating on cycle time and off cycle time, a heating off cycle time P11, a cooling cycle duration P4 including the cooling on cycle time only or the cooling on cycle time and the cooling off cycle time, a cooling off cycle time P11, a indoor air temperature, an outdoor air temperature (OAT), a conditioned space temperature (CST), a rate of change of CST with respect to time, an air temperature measurement, a return air temperature (RAT), a supply air temperature (SAT), a temperature rise (TR) across a heat exchanger defined as the supply air temperature minus the return air temperature, a temperature split (TS) across an evaporator defined as the return air temperature minus the supply air temperature, a thermostat temperature, a rate of change of thermostat temperature with respect to time (dT/dt), a compressor electrical power (W), a fan electrical power (W), a sound level (Decibel dB), a vibration (Hz), an airflow (cfm), an air velocity (f/s), a refrigerant pressure (psig), and a refrigerant system temperature (degrees Fahrenheit F).

(58) In one embodiment during cooling, if the AC compressor off time P11 minus the variable fan-off delay time P2 from the previous cooling cycle, is less than a minimum time period, then an FDD algorithm based on the cooling off cycle time P11 will reduce the fan-off delay P2. In another embodiment during cooling, the AC compressor off time P11 is the target value to maximize, and the variable fan-off delay P2 is the process variable. The error is the difference between the P11 and P4 divided by P2 and defined as e(t)=(P11−P4)/P2 where the goal is to achieve an error between zero and 1 (i.e., off cycle time equal to or greater than cooling on cycle time, and the difference between the off cycle time and the cooling on cycle time is less than P2). The FDD method uses a Proportional Integral Differential (PID) control equation to reduce the error by adjusting the value of P2 based on the cooling cycle duration including at least one cooling cycle selected from the group consisting of: the cooling on cycle, and the cooling off cycle.

(59) In another embodiment during heating, if the furnace off time P11 minus the fan-off delay time P2 from the previous heating cycle is less than 0.5 minutes, then an FDD algorithm based on the cooling off cycle time P11 will reduce the fan-off delay P2. In another embodiment during heating off time P11 is the target value to maximize, and the variable fan-off delay P2 is the process variable. The error is the difference between the P11 and the heating cycle duration P3 divided by P2 and defined as e(t)=(P11−P3)/P2 where the goal is to achieve an error between zero and 1 (i.e., off cycle time equal to or greater than heating on a temperature split across an evaporator (return air minus supply air temperature), a temperature rise across a heat exchanger (supply air minus return air temperature), outdoor air temperature, cycle time, and the difference between the off cycle time and the heating on cycle time is less than P2). The FDD method uses a Proportional Integral Differential (PID) control equation to reduce the error by adjusting the value of P2 based on the heating cycle duration including at least one heating cycle selected from the group consisting of: the heating on cycle, and the heating off cycle.

(60) The FDD algorithm may be used to detect whether or not occupied or unoccupied fan-on operation is greater than a time limit (e.g., 0 minutes to 60 minutes or 1 hour to 24 hours, or a longer period of time depending on indoor air quality and health issues for example 7 to 10 days, etc.) then the detection method will turn off the fan using at least two methods: 1) if time limit has expired during an inactive heating cycle or an cooling cycle, then turn the fan to off; and 2) if time limit has expired, during an active heating cycle or an inactive cooling cycle, then turn the fan to off after a current heating cycle or a fan-off delay or after a current cooling cycle or the fan-off delay P2.

(61) In another embodiment, an FDD algorithm may be used to measure the return air temperature and the supply air temperature to determine the Temperature Split (TS) (return minus supply) for cooling or the Temperature Rise (TR) (supply minus return) for heating. The FDD method can use these HVAC parameters to evaluate the current sensible cooling capacity or current heating capacity compared to threshold values and determine when to turn the fan to off during the variable fan-off delay P2 whether or not to provide an FDD error message regarding low cooling or heating capacity.

(62) In another embodiment, an FDD algorithm can be used in a thermostat to measure the CST or the rate of change of the CST with respect to time (dT/dt). For cooling, if the current cooling CST minus the average CST during the variable fan-off delay period, is greater than the FDD threshold of 0.1 to 0.2° F., then the method will turn the low voltage G signal to the fan relay to off to turn the fan off. For heating, if the current heating CST minus the average CST during the variable fan-off delay period, is less than the FDD threshold of 0.1 to 0.2° F., then the method will turn the low voltage G signal to off to the fan relay to turn the fan off.

(63) In another embodiment, an FDD algorithm may be used in a thermostat to calculate the rate of change of the CST with respect to time (dT/dt), and when the dT/dt reaches an Inflection Point (IP) of zero plus or minus a confidence interval tolerance, then the method will turn the low voltage G signal to the fan relay off to turn the fan off. For example, if during cooling fan-only operation dT/dt>zero plus an FDD.sub.tolerance then turn the fan off during the cooling fan-only period. If during heating fan-only operation dT/dt<zero minus an FDD.sub.tolerance then turn the fan off during the heating fan-only period.

(64) As described herein, other embodiments may use sound, vibration, temperature, airflow (velocity), or refrigerant temperature or pressure or power measurement sensors to detect AC compressor operation during the fan-off delay or within a specific time (i.e., 0.5 minutes) after the end of the fan-off delay to set an FDD and adjust the fan-off delay for the next cooling or heating cycle to improve efficiency and thermal comfort.

(65) While the method herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the method set forth in the claims.