Energy efficient HVAC system

Abstract

An energy efficient HVAC system. The system provides a bypass air supply duct, an air handler having a bypass air supply circuit, and an air supply unit that utilize return air as a source of heat that would otherwise need to be provided by a heating coil if the air, cooled sufficiently to service interior zones of a building, is too cold for servicing perimeter zones of the building, as is typically the case when the outside air temperature is low.

Claims

1. An air handler for ventilating and cooling a building having a return air duct for returning air from the building to the air handler, the air handler comprising a cold air circuit including at least one cold air supply fan and one or more dampers for drawing first return air from the return air duct and first outside air in a first, adjustable proportion to produce a first outside/return air mixture, and for propelling the first outside/return air mixture past a cooling coil adapted to provide for cooling or not cooling the first outside/return air mixture as needed or desired to provide a cold air supply of cold air at a cold air temperature T1 to the building, and a bypass air circuit including a bypass air supply fan and one or more dampers for drawing second return air from the return air duct and second outside air in a second proportion to produce a second outside/return air mixture, resulting in bypass air at a bypass air temperature T2, and for propelling the bypass air to the building as a bypass air supply in parallel with and separated from the cold air supply, wherein the air handler is configured to provide for selecting said second proportion as needed or desired for use in the bypass air supply, wherein the bypass air circuit includes an exhaust damper for exhausting bypass air propelled by the bypass air supply fan as needed to maintain neutral air pressure in the building.

2. The air handler of claim 1 in combination with at least one remotely located air supply unit, the air handler and air supply unit being in fluid communication through at least two additional ducts in the building, a cold air supply duct and a bypass air supply duct, the air handler adapted so that the cold air supply fan propels the first outside/return air mixture to the cold air supply duct and the bypass air supply fan propels the second outside/return air mixture to the bypass air supply duct, wherein the at least one air supply unit includes one or more variably adjustable air supply dampers for controlling the amounts of air allowed to flow through the at least one air supply unit from the bypass and cold air supply ducts so as to provide for mixing air from the bypass air supply duct with the cold air supply duct in an adjustable proportion as needed or desired to meet a heating or cooling requirement in the building.

3. The air handler and combination of claim 2, in further combination with a heater located downstream of the one or more air supply dampers and upstream of the return air duct for heating the air allowed to flow through the at least one air supply unit from the bypass air supply duct.

4. The air handler and combination of claim 3, in still further combination with a controller adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

5. The air handler and combination of claim 3, in still further combination with a controller adapted for controlling the bypass air supply fan in response to changes in the amount of airflow in the bypass air supply duct so as to maintain substantially constant pressure in the bypass air supply duct.

6. The air handler and combinations of claim 5, wherein the controller is further adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

7. The air handler and combination of claim 2, in further combination with a controller adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

8. The air handler and combination of claim 3, in further combination with a controller adapted for controlling the bypass air supply fan in response to changes in the amount of airflow in the bypass air supply duct so as to maintain substantially constant pressure in the bypass air supply duct.

9. The air handler and combinations of claim 8, wherein the controller is further adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

10. The air handler of claim 1, in combination with a controller adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

11. The air handler of claim 1, in combination with a controller adapted for controlling the bypass air supply fan in response to changes in the amount of airflow in the bypass air supply duct so as to maintain substantially constant pressure in the bypass air supply duct.

12. The air handler and combination of claim 11, wherein the controller is further adapted for controlling the cold air supply fan in response to changes in the amount of airflow in the cold air supply duct so as to maintain substantially constant pressure in the cold air supply duct.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a block diagram of a basic HVAC system according to the present invention.

(2) FIG. 2 is a schematic diagram of an air handler, shown in side elevation, of the system of FIG. 1 according to the present invention.

(3) FIG. 3 is a schematic diagram of a representative floor of a building, shown in plan, served by the system of FIG. 1.

(4) FIG. 4 is a schematic diagram of a typical prior art air handler, shown in side elevation, for comparison with the air handler of FIG. 2.

(5) FIG. 5 is a schematic diagram of an air supply unit of the system of FIG. 1 associated with the floor of FIG. 3, according to the present invention.

(6) FIG. 6 is a block diagram of an enhanced HVAC system according to the present invention.

(7) FIG. 7 is a schematic diagram of an air handler, shown in side elevation, of the system of FIG. 6, according to the present invention.

(8) FIG. 8 is a schematic diagram of a representative floor of a building, shown in plan, served by the system of FIG. 6.

(9) FIG. 9 is a schematic diagram of an air supply unit of the system of FIG. 6 and associated with the floor of FIG. 8, according to the present invention.

(10) FIG. 10 is a schematic diagram of a preferred air supply unit of the system of FIG. 6 associated with the floor of FIG. 8, according to the present invention.

(11) FIG. 11 is a schematic diagram of an air handler, shown in plan, for use in a multizone HVAC system according to the present invention.

(12) FIG. 12 is a schematic diagram of the air handler of FIG. 11 shown in side elevation.

DESCRIPTION OF PREFERRED EMBODIMENTS

(13) FIG. 1 provides an overview of a basic HVAC system 10 according to the present invention. The system is typically used for heating, ventilating, and air conditioning commercial buildings, such as office buildings, but could be used in any desired application.

(14) With additional reference to FIG. 2, the system 10 includes an air handler 10a, which would typically be installed on the roof of the building, but which could be installed at any location that is in fluid communication with outside air. The system 10 is used generally for heating, ventilating, and/or air conditioning a (substantially) enclosed space, which will hereinafter be referred to as a building for convenience but with no loss of generality being implied, and for purposes herein the term outside air will refer to air that is outside the building.

(15) The air handler 10a is in fluid communication with three ducts that run within the building, a return air duct 12, a cold air supply duct 20, and a novel bypass air supply duct 24 that is part of a novel bypass air circuit according to the invention.

(16) With particular reference to FIG. 2, the air handler 10a receives return air from the building through a return air duct 12. The air is returned from the building through inlet grilles 11 inside the building (see FIG. 1).

(17) Some of the return air from the return air duct 12 is drawn into a cold air staging chamber 14 through a variable return air inlet damper 14a by negative pressure produced by a cooling fan 16. Outside air is also drawn into the cold air staging chamber 14 through a variable outside air inlet damper 14b by the same negative pressure. Respective position control of the return and outside air intake dampers provides for mixing the drawn return air and the drawn outside air in an adjustable proportion to produce an outside/return air mixture that the cooling fan 16 propels through, past, or across (hereinafter past) a cooling coil 18 and into the cold air supply duct 20.

(18) The air handler 10a also includes a bypass air circuit including a bypass air staging chamber 15 into which the return air is first drawn by negative pressure provided by a bypass fan 22. As noted above, some of this return air is drawn into the cold air staging chamber 14. The remaining return air, along with outside air that may be drawn through a bypass circuit outside air inlet toggling damper 15a (depending on whether the damper 15a is open or closed), is propelled by the bypass fan 22 into the bypass air supply duct 24. The damper 15a could be variable. Some of the air propelled by the fan 22 may be exhausted to the external environment through a variable exhaust damper 14c.

(19) The outside/return air mixture in the cold air staging chamber 14 varies according to the temperature circumstances, and so does the amount of cooling provided at the cooling coil. There must always be some outside air intake to provide for adequate ventilation, which is set by maintaining a minimum position on the outside air damper 14b.

(20) The exhaust damper 14c allows for exhausting an amount of return air equal to the quantity of outside air taken in, to maintain neutral air pressure in the building. The bypass circuit outside air inlet toggling damper 15a (which could alternatively be a variable damper) allows fresh air to enter the bypass air supply duct, to provide for adequate ventilation when only the bypass air supply duct (or the hot air supply duct discussed further below in connection with an enhanced HVAC system according to the invention) is utilized as described further below in connection with the toggling dampers of FIG. 5.

(21) Referring to FIGS. 1 and 3, the two supply ducts, i.e., the cold air supply duct 20 and the bypass air supply duct 24, and the return air duct 12, typically run inside the building to each floor F of the building. There are typically multiple floors. But to simplify the discussion, it may be assumed that all the floors in the building are the same or, equivalently, that there is only one floor F, which is referenced specifically as F.sub.10 in FIGS. 1 and 3 to signal its association with the system 10. It is to be understood that there may be any number of floors, and that the floors need not be treated alike by the system 10.

(22) FIG. 4 shows a typical prior art air handler 110a, for comparison with the air handler 10a of FIG. 2. In the air handler 110a the return air is drawn into a first cold air staging chamber 114 under negative pressure produced by a return/exhaust fan 36. Some of the return air is allowed to exit the air handler to the external environment through a variable air valve, or damper, namely exhaust damper 114c; and the remaining return air proceeds through a variable restriction damper 114a to a second cold air staging chamber 116 wherein, under negative pressure produced by a cooling fan 16, the remaining return air is mixed with outside air drawn through a variable outside air inlet damper 114b.

(23) Respective position control of the exhaust, outside air inlet, and restriction dampers provides for mixing the remaining return air and the drawn outside air in an adjustable proportion to produce an outside/return air mixture that the fan 16 propels past a cooling coil 18 and into the cold air supply duct 20.

(24) Again, the outside/return air mixture varies according to the temperature circumstances, and so does the amount of cooling provided at the cooling coil. There must always be some outside air intake to provide for adequate ventilation, which is set by maintaining a minimum position on the outside air damper 114b. The exhaust damper 114c allows for exhausting an amount of return air equal to the quantity of outside air taken in, to maintain neutral air pressure in the building.

(25) A bypass air circuit according to the present invention including the bypass air supply chamber 15, the toggling damper 15a, and the bypass air fan 22 could be added to the air handler 110a in parallel with the existing circuit, in which case the function of the exhaust damper 14c would be provided by the existing exhaust damper 114c. The penalty relative to the air handler 10a is the need for an additional fan, namely, the return/exhaust fan 36.

(26) Referring back to FIG. 3, the cold and bypass air supply ducts 20 and 24 feed what is typically a large number of air supply units (ASU) at the floor of the building. To simplify the discussion, only two air supply units are shown for the floor F.sub.10, namely ASU.sub.1, which in this example is an interior zone air supply unit, and ASU.sub.2, which in this example is a perimeter zone air supply unit, it being understood that there may be any number of interior and/or perimeter zone air supply units ASU, and that the construction of all the air supply units is preferably the same.

(27) FIG. 5 shows an air supply unit ASU.sub.10 for use with the air handler 10. Air from the cold air supply duct 20 is allowed to enter the air supply unit through a cold air supply damper Da; and air from the bypass air supply duct 24 is allowed to enter the air supply unit through a bypass air supply damper Db. Position control of the cold and bypass air supply dampers Da and Db allows for selecting air from either the cold air supply duet, if cooling is needed, or the bypass air supply duct if heating is needed. The term toggling mode will refer to a mode of operation of a damper to de-select a duct, i.e., to shut off the airflow from that duct.

(28) If cooling is needed and the bypass air supply duct is de-selected (or toggled off), the amount of airflow from the cold air supply damper Da is adjusted to match the zone requirement for cooling.

(29) If heating is needed and the cold air supply duct is de-selected, heating may be provided by any known means, referred to generally as a heater and indicated as 33 in FIG. 5, which would typically be a heating coil as in the prior art. There is generally no need to limit the airflow through the bypass air supply duct, and greater airflow has the advantage of providing for greater ventilation. While greater airflow also increases the demand on the bypass air supply fan, much of the work done by the bypass air supply fan results in heating the air, reducing the need for heating by the heater.

(30) There is a minimum airflow needed to provide for adequate ventilation. It may be that cooling is required, but not very much, and (with the bypass air supply duct de-selected) if air from the cold air supply duct is passed into the zone at the minimum airflow, too much cooling would result. To avoid this, the cold and bypass air supply dampers Da and Db are operated together in a mixing mode of operation. In the mixing mode of operation of the dampers Da and Db in the ASU.sub.10, position control of the dampers provides for mixing air from the bypass air supply duct with air from the cold air supply duct in an adjustable proportion to produce a cold/bypass air mixture. While not allowing for the full range of control, just one mixing damper (e.g., the damper Db) could be used to provide for this mixing.

(31) While it may generally be preferable, when cooling is required and the minimum ventilation requirement can be satisfied by varying the airflow from the cold air supply to duct to satisfy the cooling requirement, it may be a desirable alternative to always maintain a constant airflow through the air supply units, and operate the cold and bypass air supply dampers Da and Db in the mixing mode to regulate the temperature of the outlet air. The main advantage of the constant airflow strategy is that it provides for maximum ventilation, whereas the variable airflow strategy conforms to prior art practice and has the advantage of minimizing the power requirements of the fan.

(32) In summary, when cooling is required in the air supply unit ASU.sub.10, position control of the dampers Da and Db provides for either (A) de-selecting the bypass air supply duct (toggling damper Db off) and thus providing an unmixed air supply from the cold air supply duct (through damper Da), and adjusting the airflow of the unmixed cold air supply as needed or desired to meet the zone cooling requirement (varying the airflow through damper Da), or (B) mixing air from the bypass air supply duct with air from the cold air supply duct (operating both dampers Da and Db in mixing mode) in an adjustable, desired proportion to provide a mixed air supply at the volume of airflow needed or desired to meet the zone cooling requirement.

(33) Conversely, when heating is required in the air supply unit ASU.sub.10, position control of the dampers Da and Db provides for de-selecting the cold air supply duct (toggling damper Da off) and thus providing an unmixed bypass air supply from the bypass air supply duct at an airflow determined by control of the damper Db.

(34) It is convenient that the portion of the air supply unit indicated as P is a standard, commercially available part, referred to as a terminal unit.

(35) Returning again to FIG. 3, the cold/bypass air mixture is further scattered into the spaces served by the air supply units by respective diffusers D, here D.sub.1 and D.sub.2. There may be any number of the diffusers D in fluid communication with a given air supply unit.

(36) As is standard practice, associated with each air supply unit ASU.sub.10 is a temperature sensor T, here T.sub.1 and T.sub.2. Each temperature sensor defines a zone within the space defined by the floor F. The temperature sensor is part of a temperature control circuit for the air supply unit, which measures the temperature in the zone, and which may or may not allow for an occupant of the zone to set the desired temperature in the zone. There may be any number of air supply units governed by the same temperature sensor, acting in concert.

(37) One or more electrical or electronic control modules, referenced in FIG. 1 as controller C.sub.10, receive electrical signals from the temperature sensors, and produce electrical signals providing the aforementioned position control of the dampers at both the air handler units and the air supply units, which are suitably adapted for such electrical position control.

(38) The rotational velocity of the cold air supply fan 16 is also controlled by the controller C.sub.10 to maintain a designated pressure in the cold air supply duct 20, and the rotational velocity of the bypass air supply fan 22 is controlled to maintain a designated pressure in the bypass air supply duct 24. It will be appreciated by persons of ordinary skill that other strategies for controlling the fans could be employed.

(39) The controller C.sub.10 may include any number of programmable computers or computing modules, or hardwired electrical device or devices, localized or distributed. The structure and manner of operation of the temperature sensors and controller follows standard practice, applied according to the teachings herein.

(40) The HVAC system 10 is more energy efficientand under very cold conditions it is much more energy efficientthan the basic prior art HVAC system under the aforementioned transitional temperature circumstances (B) and (C). Under temperature circumstances (B) and (C), the prior art system expends energy for heating that is unnecessary in the system 10 because the system 10 utilizes heat energy already present in the return air (provided by people and heat-generating equipment). Specifically, whenever there is a zone that needs air at a temperature that is higher than T.sub.LOWER, the prior art HVAC system requires an expenditure of energy to heat the air; whereas the system 10 doesn't need to use any energy to heat the air unless and until there is a zone that needs air at a temperature higher than that of the bypass air, which is typically within one or two degrees F. of T.sub.DESIRED.

(41) Accordingly, defining T.sub.MAX as being the required air temperature of a given quantity of air expelled at a given air supply unit (as called for the by the temperature sensor for the zone the air supply unit serves), the higher the temperature T.sub.MAX greater than T.sub.LOWER but less than or equal to T.sub.BYPASS (or T.sub.DESIRED plus or minus one or two degrees, depending on the amount of outside air mixed with the return air), the greater the efficiency provided by the system 10, which is to eliminate the need to heat the given quantity of air from T.sub.LOWER to T.sub.MAX. The greatest efficiency is reached when T.sub.MAX equals or exceeds T.sub.BYPASS (or T.sub.DESIRED as modified by mixing outside air with return air), in which case the prior art HVAC system must raise the temperature of the given quantity of air the maximum amount relative to the system 10 (T.sub.BYPASST.sub.LOWER), before the system 10 begins to consume heating energy as well (to raise the temperature of the given quantity of air above T.sub.BYPASS).

(42) An enhanced HVAC system 30 according to the present invention is shown in FIG. 6. With additional reference to FIG. 7, the system 30 employs an air handler 30a having many of the same features as the air handler 10a of the basic system 10. The differences are that the air handler 30a has a heating coil 34 past which bypass air from the bypass air staging chamber 15 is propelled by the bypass air supply fan 22 and supplied to a hot air supply duct 32.

(43) FIG. 8 shows the floor of FIG. 3 modified for use with the system 30, now referenced as F.sub.30. Again, it is to be understood that there may be any number of floors, and that the floors need not be treated alike by the system 30. There are now three supply duets running from the air handler 30a to the floor F.sub.30, the cold air supply duct 20, the bypass air supply duct 24, and a hot air supply duct 32. These three ducts feed air supply units ASU.sub.30a or ASU.sub.30b as next described. As for the system 10, the air supply units have associated temperature sensors T, here again T.sub.1 and T.sub.2, defining two zones to which these air supply units belong, and air exiting the air supply units is further scattered into the spaces served thereby by respective diffusers D, here again D.sub.1 and D.sub.2.

(44) FIG. 9 shows an air supply unit ASU.sub.30a that could be used with the air handler unit 30. The air supply unit ASU.sub.30a is the same as the air supply unit ASU.sub.10 (FIG. 5) except that it is adapted to receive air from the hot air supply duct 32 in addition to being adapted to receive air from the cold and bypass air supply ducts 20 and 24. As in the air supply unit ASU.sub.10, the air supply unit ASU.sub.30a uses dampers (Da, Db, Dc) on each of the air supply ducts that may be operated in toggling, mixing, or variable airflow modes as needed or desired to satisfy the heating/cooling requirements at the zone.

(45) FIG. 10 shows a preferred air supply unit ASU.sub.30b having a portion P thereof that can be the standard terminal unit mentioned above. Here again, there are two dampers (Da, Db) that can be operated in either toggling, mixing, or variable airflow modes as needed or desired. The damper Db is used to control the flow from the bypass air duct 24. A toggling damper TD, not part of the standard terminal unit, is also provided for selecting air from either the cold air supply duct 20 or the hot air supply duct 32.

(46) In the air supply unit ASU.sub.30b, when cooling is required, the toggling damper TD closes off the flow of air from the hot air supply duct 32, passing air from the cold air supply duct 20 to the damper Da. Then, position control of the dampers Da and Db provides for either (A) de-selecting the bypass air supply duct (toggling damper Db off) and thus providing an unmixed air supply from the cold air supply duct (through damper Da), and adjusting the airflow of the unmixed cold air supply as needed or desired to meet the zone cooling requirement (varying the airflow through damper Da), or (B) mixing air from the bypass air supply duct with air from the cold air supply duct (operating both dampers Da and Db in mixing mode) in an adjustable, desired proportion to provide a mixed air supply at the volume of airflow needed to meet the zone cooling requirement.

(47) Conversely, when heating is required, the toggling damper TD closes off the flow of air from the cold air supply duct 20, passing air from the hot air supply duct 32 to the damper Da. Then, position control of the dampers Da and Db provides either for (A) de-selecting the bypass air supply duct (toggling damper Db off) and thus providing an unmixed air supply from the hot air supply duct (through damper Da), and adjusting the airflow of the unmixed hot air supply as needed or desired to meet the zone heating requirement (varying the airflow through the damper Da), or (B) mixing air from the bypass air supply duct with air from the hot air supply duct (operating both dampers Da and Db in mixing mode) in an adjustable, desired proportion to provide a mixed air supply at the volume of airflow needed or desired to meet the zone heating requirement.

(48) As for the system 10, it may generally be preferable to use the variable airflow strategy (A) when cooling is required and the minimum ventilation requirement can be satisfied, but it can be a desirable alternative to use the constant airflow strategy (B). Again, the main advantage of the constant airflow strategy is that it provides for maximum ventilation, whereas the variable airflow strategy conforms to prior art practice and has the advantage of minimizing the power requirements of the fan.

(49) Also as for the HVAC system 10, in the HVAC system 30 one or more electrical or electronic control modules, referenced in FIG. 6 as C.sub.30, receive electrical signals from the temperature sensors, and produce electrical signals providing the aforementioned position control of the dampers at both the air handler and the air supply units, which are suitably adapted for such electrical position control.

(50) The rotational velocity of the cold air supply fan in the system 30 is also controlled by the controller C.sub.30 the same as in the system 10. The rotational velocity of the bypass air supply fan 22 in the system 30 is controlled to maintain a designated pressure in the hot air supply duct 32 and the bypass air supply duct 24. It will be appreciated by persons of ordinary skill that other strategies for controlling the fans could be employed.

(51) Like the controller C.sub.10, the controller C.sub.30 may include any number of programmable computers or computing modules, or hardwired electrical device or devices, localized or distributed. The structure and manner of operation of the temperature sensors and controller follows standard practice, applied according to the teachings herein.

(52) It may be noted that, in a variation of the system 30, there may be a separate parallel circuit, each with its own dedicated fan, for providing the bypass and hot air supplies.

(53) As an extension of the aforementioned refinement to the basic prior art HVAC system described previously, having both hot and cold air ducts, some prior art HVAC systems provide separate air ducts for each zone. These are often referred to as multizone systems, though the distinction is not actually in the number of zones but rather in the number of ducts, particularly the number of ducts emanating from the air handler, at least one for each zone in the building. Each duct carries air at the desired temperature and flow rate to the zone, the temperature and flow rate being determined at the air handler, the flow rate being adjusted by use of dampers, and the temperature being adjusted by use of heating and cooling coils the same as in the refined basic system.

(54) The principles employed in the system 30 can also be extended to a multizone embodiment, where the functions of selecting air from one of three bypass, cold, and hot air ducts according to the variable airflow strategy (A), or mixing air from the bypass duct with air from either the cold or hot air supply ducts according to the constant airflow strategy (B), which in the system occurs at the air supply units within the zone are performed at the air handler instead.

(55) More particularly with reference to FIG. 11 showing, in plan, an air handler 40a of a multizone HVAC system 40 according to the present invention, there are multiple zone air supply ducts 42 emanating from the air handler, each duct 42 supplying air to one particular zone in the building.

(56) FIG. 12 shows the air handler 40a in side elevation. Comparison of the air handler 40a as shown in FIG. 12 with the air handler 30a of the system 30 as shown in FIG. 7 reveals that the two are essentially the same, except that the air handler 40a includes, for each of the ducts shown in FIG. 11, a damper set 44 that may provide for the same selecting and mixing functions that were described above in connection with the system 30 as being performed at the air supply units.

(57) More particularly, the functions of the cold, bypass, and hot air ducts in the system 30 are replaced by corresponding cold, bypass, and hot air chambers, referenced as 46, 48, and 50 respectively, in the air handler 40a. The damper set 44 associated with a given zone air supply duct 42 is adapted to provide for at least one of two modes of air supply to the zone air supply duct, either (A) selecting an unmixed air supply from one of the cold, hot, and bypass air chambers, or (B) mixing air from the bypass air chamber with air from either the hot or cold air chambers in an adjustable proportion to provide a mixed air supply.

(58) The prior art multizone systems do not provide for varying the airflow. Likewise it is believed to be preferable to provide only for the constant airflow strategy (B) in multizone systems according to the present invention, so that the damper sets would not be adapted to vary the airflow according to strategy (A). This has the usual advantage of maximizing ventilation, but also allows for use of the same damper sets that are used in the prior art, accompanied by simpler control.

(59) The cold, bypass and hot air chambers referred to in connection with the multizone system 40 correspond to the cold, bypass, and hot air supply ducts referred to in connection with the systems 10 and 30. The term air supply is used herein as a generic term to refer to either an air supply duct or an air chamber.

(60) When using the constant airflow strategy (B), it remains desirable, in all HVAC systems according to the present invention, to provide for controlling the rotational velocities of the cold air supply fan and the bypass air supply fan. The rotational velocity of each supply fan is increased or decreased to match the airflow requirements for the corresponding supply. That is, during cool weather, as the outside air temperature decreases, the cooling load also decreases, so a reduced amount of cold air is required with a corresponding increase in the need for warm air. Conversely, during warm weather, as the outside air temperature increases, the cooling load increases, so an increased amount of cold air is required with a corresponding decrease in the need for warm air. Where there are separate fans providing the cold and warm air supplies, each needs to be controlled to vary its throughput to provide for a substantially constant total airflow.

(61) When using the variable airflow strategy (A), the airflow is varied as needed or desired to meet a heating or cooling requirement, because varying the airflow under strategy (A) is not always needed; for example, where unmixed air is selected from the bypass supply. If heating is needed and bypass air is selected, it is preferable to vary the output of the heating coil than to vary the airflow to meet the heating requirement.

(62) It will be understood that, in any HVAC system according to the invention, any number of fans could be used to perform the functions of the cold and bypass air supply fans as described herein; accordingly, the terms cold air supply fan and bypass air supply fan as used herein refer to any number of fans performing the described or recited functions.

(63) It will be understood that HVAC systems according to the invention may be used for heating, ventilating, air conditioning, and cooling (where return air temperature is reduced by mixing in outside air), alone or in any combination.

(64) It will also be understood that, while the cold and bypass air circuits of HVAC systems according to the invention are shown one on top of the other at the air handler in the Figures, this particular mounting relationship or configuration is not essential, and any mounting relationship or configuration may be employed that provides for the indicated fluid flow paths.

(65) For purposes herein, for any two points A and B served by a HVAC system according to the invention, point B is defined to be downstream of point A, and therefore point A is defined to be upstream of point B, if the pressure of the air supplied by the system is higher at point A than at point B.

(66) All of the cold, bypass, and hot air supplies described herein are intended to be provided at distinctly different temperatures. Preferably, the temperature of the air of the cold air supply is at least 5 degrees lower than that of the air of the bypass air supply, and the temperature of the air of the hot air supply (if provided) is at least 5 degrees higher, when the air handler is operating normally, or during what is referred to in the art as occupied hours of operation.

(67) While specific HVAC methods and systems have been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention.

(68) 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.