Variable airflow energy efficient HVAC systems and methods

Abstract

Variable airflow energy efficient HVAC systems can be used to provide for a three duct air supply, a multi-zone air supply, and a two duct air supply. The systems include an air handling unit that defines a cooling circuit portion and an independent heating/bypass circuit portion. The two circuit portions may each have variable air dampers for admitting variable amounts of ambient and return air, and/or the cooling circuit portion and the heating/bypass circuit portion may each have an active cooling element. Corresponding methods are also disclosed.

Claims

1. An HVAC system for use in a building having a return air duct, the building being maintained at one or more temperatures, the system comprising: an air handling unit comprising a cooling circuit portion for providing a cold air airstream, a first return air damper for admitting return air from the return air duct into the cold air airstream, and a first cooling element disposed in the cold air airstream for cooling the cold air airstream, the air handling unit further comprising a bypass circuit portion for providing a bypass air airstream, independent of the cold air airstream, and a second return air damper for admitting return air from the return air duct into the bypass air airstream; and a controller configured to determine a hot ambient air condition where the temperature of the ambient air is higher than the one or more temperatures maintained in the building, close the second return air damper, and use the first return air damper to provide return air to the building in response to the hot ambient air condition.

2. The system of claim 1, the air handling unit further comprising an ambient air damper for admitting ambient air into the cold air airstream, wherein the controller is further configured to close the ambient air damper in response to the hot ambient air condition.

3. The system of claim 1, further comprising a second cooling element disposed in the bypass air airstream for cooling the bypass air airstream, and a second ambient air damper for admitting ambient air into the bypass air airstream, wherein the controller is further configured to open the second ambient air damper and use the second cooling element to cool the admitted ambient air in response to the hot ambient air condition.

4. An HVAC system for use in a building having a plurality of zones, the zones being maintained at one or more temperatures, each zone having at least one terminal unit and at least one air quality sensor, the HVAC system comprising: an air handling unit comprising a cooling circuit portion for providing a cold air airstream, a cooling element disposed in the cold air airstream, and a cooling circuit ambient air damper for admitting ambient air into the cold air airstream, and a bypass circuit portion for providing a bypass air airstream independent of the cold air airstream and a bypass circuit ambient air damper for admitting ambient air into the bypass air airstream; and a controller configured to determine a hot ambient air condition, where the temperature of the ambient air is higher than the one or more temperatures maintained in the zones, and to close the cooling circuit ambient air damper, and use the bypass ambient air damper for admitting sufficient ambient air into the bypass air airstream to ensure that the terminal units can provide for variable amounts of air as needed to satisfy the respective air quality sensors, in response to the hot ambient air condition.

5. A method for heating, ventilating, and air conditioning a building, wherein there is a lowest temperature needed in the building and a highest temperature needed in the building, the method comprising: providing a cold air airstream for distribution in the building; providing a bypass air airstream for distribution in the building, independent of the cold air airstream; determining a hot ambient air condition, where the temperature of the ambient air is higher than the highest temperature needed in the building; and cooling the cold air airstream to the lowest temperature needed in the building and cooling the bypass air airstream to the highest temperature needed in the building in response to the hot ambient air condition.

6. A method for heating, ventilating, and air conditioning a building having a return air duct, the building being, maintained at one or more temperatures, the method comprising: providing a cold air airstream for distribution in the building, and a first return air damper for admitting return air from the return air duct into the cold air airstream; providing a bypass air airstream for distribution in the building, independent of the cold air airstream, and a second return air damper for admitting return air from the return air duct into the bypass air airstream, determining a hot ambient air condition, where the temperature of the ambient air is higher than the one or more temperatures maintained in the building; and closing the second return air damper and using the first return air damper to provide return air to the building in response to the hot ambient air condition.

7. The method of claim 6, further comprising providing an ambient air damper for admitting ambient air into the cold air airstream, and closing the ambient air damper in response to the hot ambient air condition.

8. The method of claim 6, further comprising providing a cooling circuit ambient air damper for admitting ambient air into the cold air airstream, and closing the cooling circuit ambient air damper in response to the hot ambient air condition.

9. The method of claim 8 wherein the building has a plurality of zones, each zone having at least one terminal unit and at least one air quality sensor, the method further comprising providing a bypass circuit ambient air damper for admitting ambient air into the bypass air airstream, and using the bypass air ambient air damper for admitting sufficient air into the bypass air airstream to ensure that the terminal units can provide for variable amounts of air as needed to satisfy the respective air quality sensors, in response to the hot ambient air condition.

10. The method of claim 6, further comprising providing an ambient air damper and using the ambient air damper for admitting ambient air into the bypass air airstream in response to the hot ambient air condition.

11. The method of claim 10, wherein there is a highest temperature that is needed in the building, the method further comprising cooling the bypass air airstream to said highest temperature.

12. A method for heating, ventilating, and air conditioning a building having a plurality of zones, the zones being maintained at one or more temperatures, each zone having at least one terminal unit and at least one air quality sensor, the HVAC system comprising: providing a cold air airstream for distribution in the building, and a cooling circuit ambient air damper for admitting ambient air into the cold air airstream; providing a bypass air airstream for distribution in the building, independent of the cold air airstream, and a bypass circuit ambient air damper for admitting ambient air into the pass air airstream; determining a hot ambient air condition, where the temperature of the ambient air is higher than the one or more temperatures maintained in the zones; and closing the cooling circuit ambient air damper, and using the bypass circuit ambient air damper for admitting sufficient air into the bypass air airstream to ensure that the terminal units can provide for variable amounts of air as needed to Satisfy the respective air quality sensors, in response to the hot ambient air condition.

13. A method for heating, ventilating, and air conditioning a building having a return air duct and a plurality of zones, the zones being maintained at one or more temperatures, each zone having at least one terminal unit and at least one air quality sensor, comprising: providing a cold air airstream for distribution in the building; providing a bypass air airstream for distribution in the building, independent of the cold air airstream, and a return air damper for admitting return air from the return air duct into the bypass air airstream; determining a hot ambient air condition, where the temperature of the ambient air is higher than the one or more temperatures maintained in the zones; and closing the return air damper in response to the hot ambient air condition, the method further comprising providing a cooling circuit ambient air damper for admitting ambient air into the cold air airstream, and closing the cooling circuit ambient air damper in response to the hot ambient air condition, the method further comprising providing a bypass circuit ambient ambient air damper for admitting sufficient air into the bypass air airstream to ensure that the terminal units can provide for variable amounts of air as needed to satisfy the respective air quality sensors, in response to the hot ambient air condition.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a plan view of a floor of a building, showing a zone with three supply ducts and a terminal unit for admitting airflow into the zone from the three supply ducts in a three duct air supply embodiment of a variable airflow, energy efficient HVAC system according to the present invention.

(2) FIG. 2 is a schematic, elevation view of an air handling unit for use in combination with the three supply ducts and terminal unit of FIG. 1.

(3) FIG. 3 is a schematic, elevation view of an air handling unit for use in a multi-zone embodiment of a variable airflow, energy efficient HVAC system according to the present invention.

(4) FIG. 4 is a schematic, plan view of the air handling unit of FIG. 3.

(5) FIG. 5 is a schematic, plan view of a floor of a building, showing multiple air supply ducts, and a representative zone, for use with the air handling unit of FIGS. 3 and 4.

(6) FIG. 6 is a schematic, elevation view of an air handling unit for use in a two duct air supply embodiment of a variable airflow, energy efficient HVAC system according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(7) It was noted above that the air handling units of typical commercial HVAC systems have heating and cooling elements that heat or cool the air as needed to maintain the desired air temperatures in the zones. Those elements are often provided in the form of coils through which a chilled or heated fluid, such as water, or a refrigerant, or an electrical current, is passed, and will be referred to herein as heating coils and cooling coils for ease of discussion, it being understood that other types of heating and cooling elements can be used.

(8) The term cooling will be used herein to refer to passing air over or through a cooling coil (or other cooling element) for cooling the air; and the term heating will be used herein to refer to passing air over or through a heating coil (or other heating element) for heating the air. The significance of such heating and cooling is that it requires an expenditure of energy.

(9) Three examples of variable airflow energy efficient HVAC systems according to the present invention will be described herein, starting with a three duct air supply system 10.

Three Duct Air Supply System

(10) FIG. 1 shows a zone 12 of a building 11 in which the three air supply duct system 10 is employed. The building may be any building, but is typically a multi-storey office building. There are typically multiple zones on a single floor of the building, and zone 12 is shown as being representative. The zone 12 is shown as being an exterior zone, but it can be either an interior or exterior zone.

(11) The three duct system 10 can be characterized by the use of three ducts to serve the HVAC needs of the zone 12, the ducts being referenced here as A, B, and C. Also shown are a standard temperature sensor 12a and a standard air quality sensor 12b for the zone 12. All the zones would normally have the same sensors.

(12) FIG. 2 shows an air handling unit 14 for the three duct system 10, which in this example is mounted on top of the building 11. The air handling unit 14 provides airflow to the building through the ducts A, B, and C. According to the invention, the air handling unit 14 can be thought of as being divided into two circuit portions: a cooling circuit portion 14a, and a heating/bypass circuit portion 14b. The two circuit portions are divided at the line L, which corresponds to a barrier between the two circuit portions that prevents air from being exchanged between the two circuit portions.

(13) The cooling circuit portion 14a is for cooling the zones with cool or cold air as needed, and the heating/bypass circuit portion 14b is for both ventilating the zones and heating the zones with warm or hot air as needed.

(14) With reference to both FIGS. 1 and 2, a full circuit, of which the two circuit portions 14a and 14b of the air handling unit 14 are integral parts, is defined by airflow from the air handling unit, through the air supply ducts into the zone 12, and back to the air handling unit 14 through a return air duct D.

(15) As for all zones in the building, it is necessary to satisfy both the temperature sensor and the air quality sensor.

(16) To facilitate these requirements according to the invention, each of the two circuit portions 14a and 14b of the air handling unit 14 is preferably provided with an ambient air damper, referenced here as AAD.sub.14a and AAD.sub.14b. These air dampers may be of the standard type known and commonly used in the art to allow for varying, selectably controlled airflow. As provided according to the invention, these air dampers allow for variable, modulated amounts of ambient air from outside the building to enter the respective circuit portions of the air handling unit. When the dampers are closed, the variable airflow is at its minimum, which is typically essentially zero.

(17) In addition, each of the two circuit portions 14a and 14b of the air handling unit 14 is preferably provided with a return air damper, referenced here as RAD.sub.14a and RAD.sub.14b. These may be the same types of air dampers as the ambient air dampers. As provided according to the invention, the return air dampers allow for variable, modulated amounts of return air from the return air duct D to enter the respective circuit portions of the air handling unit.

(18) Thus, suitable operation of the ambient air damper AAD.sub.14a and the return air damper RAD.sub.14a in combination can provide for any desired mixture of ambient and return air as input to the cooling circuit portion 14a of the air handling unit 14; and likewise, suitable operation of the ambient air damper AAD.sub.14b and the return air damper RAD.sub.14b in combination can provide for any desired mixture of ambient and return air as input to the heating/bypass circuit portion 14b of the air handling unit.

(19) The present inventor has identified three ambient air conditions pertinent to the operation of the system 10 as follows: (1) Cold ambient air conditions, where the ambient air is cold enough that none of the zones in the building require cooling; (2) Hot ambient air conditions, where the ambient air is hotter than the temperature(s) maintained in the zones, so that all the zones will require at least some cooling; and (3) Mild ambient air conditions, where the ambient air is colder than the temperature(s) maintained in the zones, but not cold enough to avoid the need for at least some cooling. These three different ambient air conditions allow for defining the air flowing in the ducts A, B, and C.

(20) A cooling circuit fan CCF in the cooling circuit portion 14a of the air handling unit 14 creates negative pressure therein for drawing ambient air through the ambient air damper AAD.sub.14a, and for drawing return air through the return air damper RAD.sub.14a, and positive pressure downstream for expelling the mixture into the duct A for delivery to the zone 12.

(21) In addition, the cooling circuit portion 14a of the air handling unit has a cooling circuit cooling coil CC.sub.14a for cooling the mixture as needed to satisfy all the building's cooling requirements. But the cooling coil CC.sub.14a may be off (i.e., no cooling as defined herein) and the air may still be cold enough to satisfy the temperature sensor 12a under Cold or Mild ambient air conditions. In any case, the duct A will be referred to hereinafter as the cold air duct because it supplies cool or cold air as needed, and the output of the cooling circuit portion 14a of the air handling unit may be referred to as a cold deck CD.sub.14.

(22) Corresponding to the cooling circuit fan CCF, a heating/bypass circuit fan H/BCF in the heating/bypass circuit portion 14b of the air handling unit 14 creates negative pressure therein for drawing ambient air through the ambient air damper AAD.sub.14b, and for drawing return air through the return air damper RAD.sub.14b, and positive pressure downstream for expelling the mixture into the ducts B and C for delivery to the zone 12.

(23) Preferably, the air handling unit has just the two fans, CCF and H/BCF.

(24) The heating/bypass circuit portion 14b of the air handling unit has a heating coil HC.sub.14 for heating the mixture flowing into the duct C as needed to satisfy all the building's heating requirements. Accordingly, the duct C will be referred to hereinafter as the hot air duct because it supplies hot air as needed, and the output of the heating/bypass circuit portion 14b downstream of the heating coil HC.sub.14 may be referred to as a hot deck HD.sub.14.

(25) Also in addition, the heating/bypass circuit portion 14b of the air handling unit has a heating/bypass circuit cooling coil CC.sub.14b for cooling the mixture flowing into the duct B under certain conditions as will be explained further below. The bypass air duct B will be referred to hereinafter as the bypass air duct, and the output of the heating/bypass circuit portion 14b downstream of the cooling coil CC.sub.14b may be referred to as a bypass deck BD.sub.14.

(26) Turning back to FIG. 1, each zone has associated therewith at least one terminal unit 13 downstream of the air handling unit 14, and upstream of the zone 12, that selects, mixes, and modulates, i.e., provides for variable amounts, of air from the ducts A, B, and C as needed to satisfy the temperature sensor 12a and the air quality sensor 12b in the zone.

(27) The standard terminal unit 13 is a 2 input-device, and may be employed in combination with a 2-position or toggling damper 13a upstream of the terminal unit, that toggles between the cold air duct A and the hot air duct C, and thereby presenting one, but not the other, of these air supplies as one of the two inputs to the terminal unit. The bypass air duct B is provided as the other input to the terminal unit 13.

(28) Turning back to FIG. 2, a cold air duct pressure sensor CDPS.sub.10 may be provided for sensing the pressure in the cold air duct A, as part of a cooling fan rotational velocity control circuit CFRVC for controlling this pressure. The sensor CDPS.sub.10 may be positioned anywhere that allows for this sensing, but is preferably positioned between one-half and three-quarters of the way toward the downstream end of the duct. The rotational velocity of the cooling circuit fan CCF is modulated by the cooling fan rotational velocity control circuit CFRVC to create the desired pressure.

(29) Likewise a bypass air duct pressure sensor BDPS.sub.10, and a hot air duct pressure sensor HDPS.sub.10, may be provided for sensing the pressures in the bypass air duct B and the hot air duct C, respectively. The sensors BDPS.sub.10 and HDPS.sub.10 may be positioned anywhere that allows for this sensing, but are preferably positioned between one-half and three-quarters of the way toward the downstream ends of the respective ducts. The sensors BDPS.sub.10 and HDPS.sub.10 are parts of a heating/bypass fan rotational velocity control circuit H/BFRVC for controlling the pressure in the ducts B and C, where the sensor with the lowest pressure governs. The rotational velocity of the heating/bypass circuit fan H/BCF is modulated by the heating/bypass fan rotational velocity control circuit H/BFRVC to create the desired pressure.

(30) Briefly returning to FIG. 1, in addition, one of the floors of the building may have a building pressure sensor BPS positioned therein. And turning back to FIG. 2, an exhaust air damper EAD in the heating/bypass circuit portion 14b of the air handling unit 14 may be provided to allow for exhausting air from this circuit portion if the pressure at the building pressure sensor BPS is too high.

(31) Some principles of operation of the system are dependent on the ambient air conditions, as follows:

(32) Cold Ambient Air Conditions:

(33) Cold Air Duct A: by suitable control of the ambient air damper AAD.sub.14a in combination with the return air damper RAD.sub.14a, ambient outside air is mixed with return air from the return air duct D and admitted into the cooling circuit portion 14a of the air handling unit 14 as needed to obtain an air mixture that is cold enough to satisfy the air temperature sensor(s) 12a of the zone requiring the lowest temperature. The cooling coil CC.sub.14a is off, because, by definition, the ambient air is cold enough to obtain the desired low temperature.

(34) Bypass Air Duct B: by suitable control of the ambient air damper AAD.sub.14b in combination with the return air damper RAD.sub.14b, ambient outside air is mixed with return air from the return air duct D and admitted into the heating/bypass circuit portion 14b of the air handling unit 14 as needed to satisfy the air quality sensor(s) 12b at the zone requiring the most ventilation, to ensure that airflow from the bypass air duct can satisfy the ventilation needs at all the zones.

(35) Hot Air Duct C: the same air mixture as for the bypass air duct B is heated to a high temperature (about 160-165 degrees F.), by use of the heating coil HC.sub.14 to ensure that airflow the hot deck HD.sub.14 into the hot air duct can satisfy the heating needs at all the zones, and also to minimize the amount of ambient airflow (i.e., air that will require a heating energy expenditure) from the hot air duct that will be required. Alternatively, the air provided to the hot deck could be heated to the highest temperature needed in the building.

(36) Terminal Units:

(37) The terminal unit 13 for the zone 12 admits into the zone an amount of airflow from the bypass air duct B that is needed to satisfy the air quality sensor(s) 12b, and admits an additional amount of airflow from either the cold air duct A or the hot air duct C (by use of the toggling damper 13a), depending on whether the zone 12 needs cooling or heating, that is needed to satisfy the temperature sensor(s) 12a. The two airflows (i.e., either from the bypass air duct and the cold air duct, or from the bypass air duct and the hot air duct) can simply be added to one another as needed to satisfy both kinds of sensors.

(38) Hot Ambient Air Conditions:

(39) Cold Air Duct A: by suitable control of the ambient air damper AAD.sub.14a in combination with the return air damper RAD.sub.14a, 100% return air from the return air duct D is admitted into the cooling circuit portion 14a of the air handling unit 14, without being mixed with ambient air, and is subsequently cooled by use of the cooling coil CC.sub.14a, to the lowest air temperature needed in the building.

(40) Bypass Air Duct B: by suitable control of the ambient air damper AAD.sub.14b in combination with the return air damper RAD.sub.14b, 100% ambient air from outside the building is admitted into the heating/bypass circuit portion 14b of the air handling unit 14 without being mixed with return air, and is subsequently cooled by use of the cooling coil CC.sub.14b, just enough to bring the air temperature down to the hottest air temperature needed in the building, to ensure that the airflow from the bypass air duct will not need to be heated.

(41) Hot Air Duct C: the same mixture as for the bypass air duct B, with the heating coil HC.sub.14 for the heating/bypass circuit portion of the air handling unit 14 turned off because, by definition, no heating is needed anywhere in the building under Hot ambient air conditions.

(42) Terminal Units:

(43) The terminal unit 13 for the zone 12 admits into the zone an amount of airflow from the bypass air duct B that is needed to satisfy the air quality sensor(s) 12b, and admits an additional amount of airflow from the cold air duet A (by use of the toggling damper 13a) that is needed to satisfy the temperature sensor(s) 12a. The two airflows (i.e., from the bypass air duct and from the cold air duct) can simply be added to one another as needed to satisfy both kinds of sensors.

(44) Mild Ambient Air Conditions:

(45) Cold Air Duct A: by suitable control of the ambient air damper AAD.sub.14a in combination with the return air damper RAD.sub.14a, 100% ambient air is admitted into the cooling circuit portion 14a of the air handling unit 14, and cooled by use of the cooling coil CC.sub.14a to the lowest temperature needed in the building, to ensure that airflow from the cold air duct can satisfy all the cooling needs of the zones that need cooling.

(46) Bypass Air Duct B: by suitable control of the ambient air damper AAD.sub.14b in combination with the return air damper RAD.sub.14b, ambient air is mixed with return air and admitted into the heating/bypass circuit portion 14b of the air handling unit 14 as needed to satisfy the ventilation needs of the zone requiring the most ventilation, to ensure that airflow from the bypass air duct can satisfy the ventilation needs of all the zones.

(47) Hot Air Duct C: the same mixture as for the Bypass Air Duct is heated to a high temperature (about 160-165 degrees F.) by use of the heating/bypass heating coil HC.sub.14. Again, as an alternative, this air could be heated to the highest temperature needed in the building.

(48) Terminal Units:

(49) The terminal unit 13 for the zone 12 admits into the zone an amount of airflow from the bypass air duct B that is needed to satisfy the air quality sensor(s) 12b, and admits an additional amount of airflow from either the cold air duct A or the hot air duct C (by use of the toggling damper 13a), depending on whether the zone 12 needs cooling or heating, that is needed to satisfy the temperature sensor(s) 12a. The two airflows (i.e., either from the bypass air duct and the cold air duct, or from the bypass air duct and the hot air duct) can simply be added to one another as needed to satisfy both kinds of sensors.

(50) Additional Operating Principles:

(51) The cooling circuit fan CCF, in the cooling circuit portion 14a of the air handling unit 14, may be turned off if no zone requires cooling, to realize additional energy savings.

(52) On the other hand, the heating/bypass circuit fan H/BCF, in the heating/bypass circuit portion 14b of the air handling unit should be continuously on during occupied hours of the building, to ensure the necessary ventilation.

(53) Control Circuits:

(54) Control circuits, which may be referred to individually or collectively as a controller (referenced in FIGS. 2, 3, and 6 as CTL) that provide for system operation as described above are known in the art, and may take the form of one or more programmable general or special purpose computers.

Multi-Zone Air Supply System

(55) FIG. 3 shows an air handling unit 140 for a multi-zone air supply system 100 according to the present invention. FIG. 3 corresponds to FIG. 2 for the three duct system 10. And there is an additional plan view of the air handling unit 140 in FIG. 4, which shows that instead of the three ducts shown in FIG. 2 for the three duct supply system 10, the system 100 may have any number of ducts MD.sub.N (twelve being shown).

(56) Except for the differences noted below, the multi-zone air supply system 100 may be the same and be operated the same as the three duct air supply system 10 as described above.

(57) As shown in FIG. 5, each duct MD is dedicated to serving a particular zone, such as the duct MD.sub.12 for the zone 12, and FIG. 3 shows how the air handling unit 140 feeds the ducts MD.sub.N.

(58) In particular, the air handling unit 140 defines a cold deck CD.sub.140, a bypass deck BD.sub.140, and a hot deck HD.sub.140. Each duct MD.sub.N mates to the cold deck CD.sub.140 through a respective cold deck damper CDD.sub.N and cold deck damper controller CDDC.sub.N for controlling the cold deck damper for the duet MD.sub.N. Each duct MD.sub.N mates to the bypass deck BD.sub.140 through a respective bypass deck damper BDD.sub.N and bypass deck damper controller BDDC.sub.N for controlling the bypass deck damper BDD.sub.N. And each duct MD.sub.N mates to the hot deck HD.sub.140 through a respective hot deck damper HDD.sub.N and hot deck damper controller HDDC.sub.N for controlling the hot deck damper for the duct MD.sub.N.

(59) A cooling circuit coil CC.sub.140a in the multi-zone system may correspond identically to the cooling circuit coil CC.sub.14a in the cooling circuit portion of the three duct system 10; a cooling circuit coil CC.sub.140b in the heating/bypass circuit portion 140b of the multi-zone system 100 may correspond identically to the cooling circuit coil CC.sub.14b in the heating/bypass circuit portion of the three duct system 10; and a heating/bypass heating coil HC.sub.140 in the heating/bypass circuit portion 140b may correspond identically to the heating/bypass heating coil HC.sub.14 in the system 10.

(60) Referring back to FIG. 2 and as explained previously, the three duct air supply system 10 may include pressure sensors CDPS.sub.10, BDPS.sub.10, and HDPS.sub.10 for sensing the pressure in the cold air duct A, the bypass air duct B, and the hot air duct C, respectively, and these pressure values may be used to control the rotational velocities of the fans serving these ducts. The corresponding functions may be provided in the multi-zone system 100 by use of pressure sensors in the air handling unit 140, CDPS.sub.100, BDPS.sub.100, and HDPS.sub.100, for sensing the pressures in the cold deck CD.sub.140, the bypass deck BD.sub.140, and the hot deck HD.sub.140, respectively.

(61) The three dampers CDD.sub.N, BDD.sub.N, and HDD.sub.N for the duct MD.sub.N serving a particular zone N are operated to achieve the same mixing of airflow that would otherwise have taken place at the terminal unit for the same zone in the three duct system as described above.

Two Duct Air Supply System

(62) FIG. 6 shows an air handling unit 240 for a two duct air supply system 200 according to the present invention. The three duct system 10 may be converted to the two duct system 200 simply by eliminating the hot air duct C in the three duct system 10, and replacing the heating coil HC.sub.14 in the three duct system with separate heating coils (not shown) for each zone, which may be upstream (in the bypass air duct), downstream, or part of, the terminal unit serving that zone, and if heating is needed at a particular zone, controlling the heating coil for that zone to heat bypass air from the bypass air duct as needed to satisfy the temperature sensor for the zone. And in all other respects, the two duct air supply system may be the same and be operated the same as the three duct air supply system 10 as described above. For example, the cooling coils CC.sub.240a and CC.sub.240b of, respectively, a cooling circuit portion 240a and a heating/bypass circuit portion 240b of the air handling unit 240 may correspond identically to the cooling coils CC.sub.14a and CC.sub.14b of, respectively, the cooling circuit portion 14a and the heating/bypass circuit portion 14b of the air handling unit 14.

(63) It is to be understood that, while some specific variable airflow energy efficient HVAC systems and methods have been shown and described as preferred, this specification is not intended to describe all the variations that may be employed and recognized by persons of ordinary skill as being consistent with the principles and practice of the invention.

(64) The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.