Air handler unit fan installation and control method

Abstract

A method is provided including installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system. The method includes setting a control strategy for the variable frequency drive. The control strategy includes a drive speed for each mode of the HVAC system. The method includes operating the HVAC system in each mode and monitoring the HVAC system. The method includes adjusting the HVAC system or the control strategy of the variable frequency drive to increase drive speed based on monitoring the HVAC system.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/796,347, filed on Apr. 28, 2006, the disclosure of which is incorporated herein by reference.

FIELD

The present disclosure relates to an air handler unit fan installation and control method.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Heating, ventilating and air conditioning (HVAC) systems may generally be equipped with an air handler unit that may control the flow of air. Air handler units may traditionally be non-variable, in that the unit is equipped with a single or two speed fan that may operate at certain fixed or incremental speeds. Because such fans may only operate at either one or two fixed speeds, the fans may not be efficiently controlled to match HVAC system load. Consequently, use of such fans may result in excessive electrical energy consumption, excessive cycling of system components, and excessive system component wear.

Variable frequency drives (VFD) may be installed in the HVAC system air handler. Traditional retrofitting of a fixed speed air handler unit with a VFD may be accomplished by simply installing the VFD drive unit in line with the air handler unit power supply. Operating the air handler fans at variable speeds may result in increased energy efficiency, as well as decreased air flow in the HVAC system. In a cooling mode, when the air handler fans are operated at slower speeds, less air flow contacts evaporator coils resulting in low evaporator superheat temperatures and a risk of a refrigerant flood-back condition. Likewise, in a heating mode, less air flow contacting a heating element may result in a high heating element temperature and a risk of a heating element overheating condition.

SUMMARY

A method is provided and includes installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system. The HVAC system has a cooling unit for operating in at least one cooling mode. The method further includes setting a control strategy for the variable frequency drive. The control strategy includes a drive speed for each cooling mode of the at least one cooling mode. The method further includes operating the HVAC system in each cooling mode of the at least one cooling mode, monitoring a superheat of an evaporator of the cooling unit and comparing the superheat with a predetermined superheat threshold. The method further includes adjusting an expansion valve of the evaporator the control strategy of the variable frequency drive when the monitored superheat is less than the superheat threshold. The adjusting the expansion valve includes decreasing a flow of refrigerant in the evaporator and the adjusting the control strategy includes increasing the drive speed for at least one cooling mode.

Another method is provided and includes installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system. The HVAC system has a heating unit for operating in at least one heating mode. The method further includes setting a control strategy for the variable frequency drive. The control strategy includes a drive speed for each heating mode of the at least one heating mode. The method includes operating the HVAC system in each heating mode, monitoring a temperature of the heating unit, and comparing the temperature with a predetermined temperature threshold. The method further includes adjusting at least one of a setpoint of the heating unit and the control strategy of the variable frequency drive when the monitored temperature is greater than the temperature threshold. The adjusting the setpoint includes decreasing a heating capacity of the heating unit and the adjusting the control strategy includes increasing the drive speed for at least one heating mode.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a graph showing percent air flow versus percent fan speed;

FIG. 2 is a graph showing percent input power versant percent air flow and fan speed;

FIG. 3 is a schematic of an HVAC system;

FIG. 4 is a schematic of an air handler fan and a variable frequency drive;

FIG. 5 is a flow chart of a system check and initial setup;

FIG. 6 is a flow chart of a system operation adjustment and verification for cooling modes;

FIG. 7 is a flow chart of system operation adjustment and verification for heating modes;

FIG. 8 is a chart showing variable frequency drive control settings for a preset speed control strategy; and

FIG. 9 is a chart showing variable frequency drive control settings for a temperature based control strategy.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

A method for installing a variable frequency drive (VFD) unit in an air handler unit of an HVAC system may include installing a VFD in line with a power supply to an air handler unit fan. In this way, a previously single or two speed fan may be converted to operate as a variable speed fan to allow for more efficient fan operation. By monitoring and comparing system conditions after VFD installation, a VFD control strategy may be implemented and adjusted to minimize a probability of a refrigerant flood-back condition, in a cooling mode, or an overheating condition, in a heating mode, while maximizing energy efficiency.

As shown in FIG. 1, fan air flow varies linearly with rotational fan speed. As shown in FIG. 2, fan input power consumption varies with the cube of rotational fan speed. Reducing fan speed may decrease power consumption. However, because fan air flow does not decrease as rapidly as power consumption, reducing fan speed may result in decreased power consumption and increased efficiency, while continuing to meet HVAC system load.

Generally, the method may include verifying that the VFD has been properly installed, verifying that associated HVAC equipment and system operation are not adversely affected by VFD control of the air handler fan speeds and air volumes, and utilizing control strategies and set-points available in the existing control system, or in new controls installed as part of the VFD installation, to create the optimal balance of energy savings and reliable system operation. In a cooling mode, to verify that the HVAC system and associated HVAC equipment are not adversely affected by installation of the VFD, a refrigerant temperature or pressure may be used to calculate a superheat temperature. The superheat temperature may be compared with a component manufacturer recommended operating superheat temperature, or threshold, to determine whether a refrigerant flood-back condition is probable. In a heating mode, a heating element temperature in the area of a high temperature limit safety switch sensor may be monitored to determine whether an overheat condition is probable.

With reference to FIG. 3, an HVAC system 10 may include a VFD 12 and an HVAC controller 14. VFD 12 may be connected to an air handler unit supply fan 16 which may control the flow of air in HVAC system 10. One or more additional fans 18 may also be used as additional supply fans or additional return fans. The additional fans 18 may also be configured with a corresponding VFD 20. VFD's 12, 20 may be connected to a single or three phase power supply 22. In FIG. 3, power supply 22 is shown as a three phase power supply.

With reference to FIG. 4, VFD 12, 20 may include a frequency modulator 24 connected to power supply 22 and controlled by a VFD controller 26. Frequency modulator 24 may include solid state electronics to modulate frequency of electrical power delivered by power supply 22. Generally, frequency modulator 24 may convert each phase of electrical power from AC to DC. Frequency modulator 24 may then convert each phase of electrical power from DC back to AC at a desired frequency. For example, frequency modulator 24 may directly rectify electrical power delivered by power supply 22 with a full-wave rectifier bridge. Frequency modulator 24 may then chop electrical power delivered by power supply 22 using insulated gate bipolar transistors (IGBT's) or thyristors to achieve the desired frequency. Other suitable electronic components may be used to modulate the frequency of electrical power delivered by power supply 22. VFD 12, 20 may also include a bypass module to allow inputted electrical power to be outputted directly without modulation by frequency modulator 24.

Rotational speed of fans 16, 18 may be controlled by a rotational speed of electric motors that drive fans 16, 18. Rotational speed of electric motors may be controlled by the frequency of electrical power received from the frequency modulator 24 of VFD 12, 20. For example, if power supply 22 operates at sixty hertz, fan speed may be varied by reducing frequency to a frequency less than sixty hertz. At thirty hertz, fan 16, 18 may operate at half speed or half capacity. In this way, VFD 12, 20 may vary speed and capacity of fans 16, 18 of HVAC system 10.

VFD controller 26 may control the output frequency of the frequency modulator 24. VFD controller 26 may monitor electrical current, voltage, and/or power and may communicate electrical data to HVAC controller 14. In this way, HVAC controller 14 may monitor electrical current, voltage, and/or power of VFD.

With reference again to FIG. 3, HVAC system 10 may include a heating unit 30 and a cooling unit 32. Heating unit 30 may include a boiler, gas pressure burner, atmospheric gas furnace, heat pump, or other elements to heat air flow of HVAC system 10. Cooling unit 32 may include a compressor, condenser, evaporator, or other elements to cool air flow of HVAC system 10. For example, cooling unit 32 may use chilled water, direct expansion (DX) or the like to cool air flow of HVAC system 10. HVAC system may include various dampers. For example, HVAC system may include a return air damper 34, a fresh air damper 36, an exhaust damper 38, and various heated/cooled space dampers. The components of HVAC system 10 operate to heat, cool, dehumidify or ventilate the heated/cooled spaces 40. While two spaces 40 are shown, a building may include many different spaces 40 in various HVAC system zones.

Other HVAC system components may selectively be installed. For example, HVAC system 10 may include duct mounted heating coils 42, terminal cooling coils 44, recovery wheels 46, desiccant wheels 48, and/or other HVAC devices. Variable air volume (VAV) boxes 49 and/or economizers 50 may also be installed. HVAC system components may generally be affected by variable volume air flow resulting from operation of VFD 12, 20 may require adjustment, as described in more detail below.

Duct mounted heating coils 42 or terminal cooling coils 44 may provide additional heating or cooling to the heated/cooled space 40. VAV box 49 may control air volume delivered to heated/cooled space 40. Economizer 50 may be used to allow cooling with outside air. Recovery wheel 46 may be used to exchange heat between exhaust air and the fresh air makeup. For example, if exhaust air is warmer than fresh air makeup, exhaust air may be used to warm fresh air makeup. If exhaust air is cooler, exhaust air may be used to cool fresh air makeup. Desiccant wheel 48 may be used to remove moisture from return air stream.

HVAC system 10 may be operated in a number of different modes. In a heating mode, return air may be heated with heating unit 30. In a cooling mode, return air may be cooled with cooling unit 32. In an economizer mode, cooling may be effectuated by use of fresh outside air. In economizer mode, return air may be completely exhausted to the outside, while fresh air may be taken in. In a dehumidification mode, return air may be cooled to remove moisture with cooling unit 32, but may also be heated with heating unit 30. In ventilation mode, air may be circulated with a mix of fresh outside air.

HVAC system 10 may be configured with a number of sensors. HVAC system 10 may include a supply/discharge air temperature sensor 52, a heating air-off temperature sensor 54, a cooling air-off temperature sensor 56, a mixed air temperature sensor 58, a return air temperature sensor 60, and heated/cooled space temperature sensors 62. In addition, static pressure sensors may be installed in various locations of the HVAC system. Signals generated by the various sensors may be received and monitored by HVAC controller 14 and used to control operation of HVAC system.

Return air temperature sensor 60 may measure the temperature of return air before any exhaust or fresh air dampers. Mixed air temperature sensor 58 may measure the temperature of the return air stream before it enters cooling unit 32 or heating unit 30, and after any exhaust dampers 38 or fresh air dampers 36. Cooling air-off temperature sensor 56 may measure the temperature of air leaving the cooling unit 32. Heating air-off temperature sensor 54 may measure the temperature of air leaving heating unit 30. Supply/discharge air temperature sensor 52 may measure the temperature of air after all heating and cooling units.

The method may include a system check and initial setup routine, as shown in FIG. 5, a system operation adjustment and verification routine for cooling modes, as shown in FIG. 6, and a system operation adjustment and verification routine for heating modes, as shown in FIG. 7.

With reference to FIG. 5, the system check and initial setup method 500 may start in step 501. In step 502, HVAC system sensors may be checked and calibrated where needed. In step 502, the location and accuracy of return air temperature sensor 60, mixed air temperature sensor 58, cooling air-off temperature sensor 56, heating air-off temperature sensor 54, and discharge/supply air temperature sensor 52 may be confirmed where present.

In step 502, the accuracy of these sensors during full and part speed fan operation, for all available HVAC operating modes may be checked. When present, and to be used as part of the VFD control strategy, the location and accuracy of any static pressure sensors may be checked. Further, the accuracy of these sensors during full and part speed fan operation, for all unit operating modes available may be verified.

Other sensors may additionally be used as part of the HVAC system control strategy and may also be checked and/or calibrated in step 502. For example, electrical current transducers, or other electrical sensors may be used. Further, VFD 12, 20 may include electrical sensors that may be checked and/or calibrated in step 502.

In step 504, existing air handler operation may be checked and current HVAC system design and equipment may be reviewed. Any downstream heating or cooling devices, for example duct mounted heating coils 42, additional furnaces, terminal cooling coils 44, VAV boxes 49, economizers 50, recovery wheels 46, desiccant wheels 48, and other devices likely to be affected by VFD operation, may be identified and checked.

Further in step 504, the source of heating and cooling, i.e., (gas pressure burner, atmospheric gas furnace, chilled water, direct expansion (DX), etc.) where present may be identified. If DX cooling is used, a coil type may be determined. For example, the DX coil type may be split or full face. Additionally, the number of coil rows may be identified. Whether the rows are separate or intertwined may also be determined. Configuration of the coil type may affect VFD operation. For example, a split face coil type may include divided coil areas, such as a low coil and a high coil. If a split face configuration is used, during HVAC system operation all of the return air does not contact a cooling coil as it passes cooling unit 32. When VFD 12, 20 is operated at a lower capacity speed, and when the split face configuration is such that the return air load does not entirely contact the coil face, a risk of a refrigerant flood-back condition may be more likely. The control strategy may be adjusted, as discussed in more detail below, to address such a situation and reduce the risk of a refrigerant flood-back condition.

Further in step 504, operation of all existing HVAC system equipment may be verified for each HVAC system operating mode. HVAC system 10 may be constant run or run on demand only during building unoccupied periods. Operating mode information may be collected primarily as documentation of conditions prior to VFD installation, for use as a basis of comparison to evaluate or diagnose any operation problems. Where available, the existing building control system data, such as elapsed run hours for each fan, each stage of heat, each stage of cooling, and each stage of dehumidification may be checked and recorded.

Further in step 504, steady state full speed fan motor volts, amps, and kW power may be measured and recorded. Additionally, supply air volume, return air volume, fresh air makeup volume, store pressurization, and outside temperature may be checked and recorded.

In step 506, VFD 12, 20 may be installed in the air handler and VFD operation may be verified. Alternatively, VFD 12, 20 may be installed prior to step 502 above and operated in a bypass mode or full speed mode during steps 502 and 504. Steady state full speed fan motor volts, amps and kW power may be measured and recorded. Additionally, supply air volume, return air volume, fresh air makeup volume, store pressurization, and outside temperature may be measured and recorded.

Further in step 506, with VFD 12, 20 controlling the fan 16, 18 at full speed, a voltage waveform may be checked with an oscilloscope at the electric motor connections. If voltage excursions above a voltage limit, for example seven hundred and fifty volts, are detected, all connections between the fan motor and the VFD outputs may be checked to determine whether loose or dirty connections or pitted disconnect or contactor points of electrical contact surfaces are present. In some cases, installation of an output filter on the VFD may be required.

In step 508, a VFD control system or strategy may be set and/or initialized. VFD control may be based on a preset-based speed control or a temperature based speed control, and/or associated set points. Referring now to FIG. 8, in a preset-based speed control system, VFD 12, 20 may be programmed, or HVAC controller 14 may be programmed to control VFD 12, 20, to operate fan 16, 18 at preset speeds indicated in the table. VFD control may depend on the number of stages present. For example, one, two, three, four, or more, stages of cooling may be present. If, for example, three stages of cooling are present, VFD 12, 20 may be operated at sixty percent during stage one, seventy-five percent during stage two, and ninety percent during stage three. As a further example, if two stages of heating are present, VFD may be operated at forty percent in stage one and sixty percent in stage two.

With reference to FIG. 9, VFD control may also be set according to a temperature based speed control system corresponding to supply air temperature. VFD 12, 20 may be programmed, or HVAC controller 14 may be programmed to control VFD 12, 20, to operate fan 16, 18 at the initial setting indicated in the table of FIG. 9, using a supply air temperature-based speed control strategy and set-points. For example, when on call for AC or dehumidification, the drive speed may start at the cooling base speed setting, such as sixty percent, and may ramp the speed and air flow up linearly if and when supply air temperature declines below the ‘start ramp-up’ temp set-point. The ramp-up may be completed to the maximum speed, for example one hundred percent, by the ‘end of ramp-up’ temp set-point.

When on call for heat, the drive speed may start at the heating base speed setting indicated in the table of FIG. 9, for example forty percent, and may ramp speed up linearly if and when the supply air temperature rises above ‘start ramp-up’ temp set-point. The ramp-up may be completed to maximum speed, for example ninety percent, by the ‘end of ramp-up’ temp set-point. When on call for economizer, the drive speed may be set at the economizer base speed setting, for example sixty percent. When there is no call for AC, dehumidification, heating or economizer, the drive speed may be set at ventilation-only base speed, for example thirty-five percent.

With reference again to FIG. 5, after VFD control is set in step 508, the system check and initial setup routine may end in step 708.

With reference now to FIGS. 6 and 7, VFD operation may be checked in each cooling and heating mode. Problems with respect to low superheat temperatures or high heating element temperatures, if any, may be identified for trouble-shooting and system adjustment, as necessary.

In FIG. 6, a system operation adjustment and verification routine for cooling modes 600 is shown, and begins in step 601. in step 602, HVAC system 10 is set to an initial cooling mode. For example, an initial mode may be stage one of four stages. In step 604, HVAC system 10 is allowed to reach equilibrium before the routine proceeds. In step 606, fan motor volts, amps, motor kW, air flow, supply air temperature, fresh air makeup volume, and building pressurization may be measured and recorded.

In step 608, evaporator coil outlet superheat may be measured and recorded. In step 610, the measured superheat is compared with a desired superheat to determine whether measured superheat is too low. For example, desired superheat may be determined based on manufacturer recommendations for a particular evaporator used with cooling unit 32. When measured superheat is less than desired, a refrigerant flood-back condition may be likely.

When measured superheat is too low in step 610, HVAC system adjustments are made in step 612 to increase superheat and reduce the likelihood of a refrigerant flood-back condition. Four HVAC system adjustment options are generally shown in step 612. First, an expansion valve of an evaporator of cooling unit 32 may be adjusted to increase superheat in the current cooling mode. An evaporator may have an mechanical or electronic expansion valve that may be adjustable to control a flow of refrigerant in the evaporator. Proper adjustment of the expansion valve may increase superheat. For example, adjusting the expansion valve to decrease refrigerant flow may increase superheat.

Second, a VFD control strategy may be modified to increase superheat. For example, VFD speed may be incremented in the current mode to increase air flow through cooling unit 32, thereby increasing the volume of air contacting evaporator coils and increasing superheat. For example, the speed percent setting, as shown in FIG. 8, or the base VFD percent, as shown in FIG. 9, may be increased for the current cooling mode.

Third, operation of other HVAC components may be checked or modified in the current mode. For example, air flow may be measured to see if the air flow is lower than anticipated at any given preset speed set-point. Additionally, HVAC system may be checked for blocked filters, blocked or collapsed ductwork, fire doors partially or fully closed, and other air flow obstruction. Additionally, HVAC system may be checked for slipping fan belts or damaged fan sheaves.

Fourth, program interlocks may be modified. For example, a program interlock may be set such that operation of duct mounted terminal cooling coils 44 may be required in the current cooling mode. Program interlocks are established such that specified HVAC system components are forced to cycle when the HVAC system enters a particular mode.

After adjusting HVAC system operation in step 612, HVAC system is allowed again to reach equilibrium in step 604. Steps 606, 608, 610, and 612 are repeated until a desirable superheat is attained in step 610. When superheat is not too low in step 610, the routine checks for additional cooling modes in step 614. When additional cooling modes remain, the routine cycles to the next cooling mode in step 616, and waits for HVAC system 10 to reach equilibrium again in step 604. Steps 604 to 614 are repeated until all cooling modes have been checked. In step 614, when no additional modes are available, the routine proceeds to step 618. In step 618, the routine determines whether all cooling modes have been checked without any adjustment. Adjustments made in subsequent modes may affect operation in a previous mode. Therefore, in step 618, when adjustments were made in any cooling mode in the last round of checks, the routine returns to step 602 and starts with the initial cooling mode again. When in step 618, all cooling modes have been checked without adjustment, the routine ends in step 620.

In FIG. 7, a system operation adjustment and verification routine for heating modes 700 is shown, and begins in step 701. in step 702, HVAC system 10 is set to an initial heating mode. For example, an initial mode may be stage one of four stages. In step 704, HVAC system 10 is allowed to reach equilibrium before the routine proceeds. In step 706, fan motor volts, amps, motor kW, air flow, supply air temperature, fresh air makeup volume, and building pressurization may be measured and recorded.

In step 708, heating unit temperature is measured and recorded in the area of any high temperature limit safety switch sensor. Heating unit 30 may be equipped with a high temperature limit safety switch that deactivates heating unit 30 when the heating unit is too hot such that there is a risk of damage to heating unit. In step 710, the measured temperature is compared with a temperature limit, such as a manufacturer recommended temperature limit. When measured heating element temperature is too high, an overheat condition may be may be likely.

When measured temperature is too high in step 710, HVAC system adjustments are made in step 712 to decrease heating element temperature and reduce the likelihood of an overheating. Four HVAC system adjustment options are generally shown in step 712. First, a heating unit 30 setpoint may be adjusted such that heating unit 30 does not provide as much heat. When the heating unit temperatures are too high, operation of the set-points for the heating unit and high limit switches may be checked. Comparison may be made between the set-points and the manufacturer's recommended set-points for actual application. Comparison may be made between the set-points and user defined set-points.

Second, a VFD control strategy may be modified to increase airflow and decrease heating unit temperature. For example, VFD speed may be incremented in the current mode to increase air flow through heating unit 30, thereby increasing the volume of air contacting heating unit 30 and decreasing temperature. For example, the speed percent setting, as shown in FIG. 8, or the base VFD percent, as shown in FIG. 9, may be increased for the current heating mode.

Third, operation of other HVAC components may be checked or modified in the current mode. For example, air flow may be measured to see if the air flow is lower than anticipated at any given preset speed set-point. Additionally, HVAC system may be checked for blocked filters, blocked or collapsed ductwork, fire doors partially or fully closed, and other air flow obstruction. Additionally, HVAC system may be checked for slipping fan belts or damaged fan sheaves.

Fourth, program interlocks may be modified. For example, a program interlock may be set such that operation of duct mounted terminal heating coils 44 may be required in the current heating mode.

After adjusting HVAC system operation in step 712, HVAC system is allowed again to reach equilibrium in step 704. Steps 706, 708, 710, and 712 are repeated until a desirable temperature is attained in step 710. When temperature is not too high in step 710, the routine checks for additional heating modes in step 714. When additional heating modes remain, the routine cycles to the next heating mode in step 716, and waits for HVAC system 10 to reach equilibrium again in step 704. Steps 704 to 714 are repeated until all heating modes have been checked. In step 714, when no additional modes are available, the routine proceeds to step 718. In step 718, the routine determines whether all cooling modes have been checked without any adjustment. Adjustments made in subsequent modes may affect operation in a previous mode. Therefore, in step 718, when adjustments were made in any heating mode in the last round of checks, the routine returns to step 702 and starts with the initial heating mode again. When in step 718, all heating modes have been checked without adjustment, the routine ends in step 720.

Further, HVAC system operation may be reviewed for smooth and reliable operation. Additional adjustments to strategies or set-points may be made as required to secure efficient system operation. These may include adjustments to allow lower speed set-points in one or more operating modes, operation of chillers at evaporator suction and chilled water temperatures, and operation of heating appliances at lower firing rates.

As shown in FIGS. 6 and 7, the process is iteratively repeated as necessary as adjustments are made and the affects of those adjustments are reviewed.

Adjustments continue until the lowest total energy usage in each operating mode is obtained while maintaining a reduced risk of refrigerant flood-back. For example, as shown in FIGS. 6 and 7, HVAC system 10 is checked in each mode for excessive heating unit 30 temperatures and for low superheat temperatures.

Additionally, HVAC system 10 may be checked to determine whether additional efficiencies may be gained by slowing fan operation even further. Fan speed may be initially set high, at or near full speed. Then, fan speed may be gradually lowered, while allowing HVAC system to reach equilibrium. At equilibrium, evaporator superheat for cooling modes and heating element temperatures for heating modes may be checked and compared with corresponding thresholds to determine whether fan speed may be lowered further without adverse effects. In this way, efficient HVAC system operation is achieved.

Claims

1. A method comprising:

installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system, said HVAC system having a cooling unit for operating in at least one cooling mode;

setting a control strategy for said variable frequency drive, said control strategy including a drive speed for each cooling mode of said at least one cooling mode;

operating said HVAC system in each cooling mode of said at least one cooling mode;

monitoring a superheat of an evaporator of said cooling unit and comparing said superheat with a predetermined superheat threshold, for operation of said HVAC system in each cooling mode of said at least one cooling mode;

adjusting at least one of an expansion valve of said evaporator and said control strategy of said variable frequency drive when said monitored superheat is less than said superheat threshold;

wherein said adjusting said expansion valve includes decreasing a flow of refrigerant in said evaporator and said adjusting said control strategy includes increasing said drive speed for at least one cooling mode.

2. The method of claim 1, further comprising:

adjusting additional HVAC system components to increase air flow within said HVAC system when said monitored superheat is less than said superheat threshold.

3. The method of claim 1, further comprising:

modifying program interlocks for said HVAC system when said monitored superheat is less than said superheat threshold;

wherein said modifying said program interlocks includes setting an HVAC system component to cycle based on a current mode of said HVAC system.

4. The method of claim 1 wherein said setting said control strategy includes setting a preset speed control strategy.

5. The method of claim 4 wherein said control strategy includes a maximum mode and a minimum mode, and wherein said setting said preset speed control strategy includes setting said maximum mode at about ninety percent capacity speed and said minimum mode at about sixty percent capacity.

6. The method of claim 5 wherein said control strategy includes an intermediate mode, and wherein said setting said present speed control strategy includes setting said intermediate mode at about seventy five percent capacity.

7. The method of claim 1 wherein said setting said control strategy includes setting a temperature based control strategy.

8. The method of claim 7 wherein said setting said temperature based control strategy includes setting a base capacity and a maximum capacity for said variable frequency drive and setting a start ramp-up temperature and an end ramp-up temperature.

9. The method of claim 8 wherein said base capacity is about sixty percent, said maximum capacity is about one hundred percent, said start ramp-up temperature is about fifty five degrees Fahrenheit and said end ramp-up temperature is about forty five degrees Fahrenheit.

10. A method comprising:

installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system, said HVAC system having a heating unit for operating in at least one heating mode;

setting a control strategy for said variable frequency drive, said control strategy including a drive speed for each heating mode of said at least one heating mode;

operating said HVAC system in each heating mode of said at least one heating mode;

monitoring a temperature of said heating unit and comparing said temperature with a predetermined temperature threshold, for operation of said HVAC system in each heating mode of said at least one heating mode;

adjusting at least one of a setpoint of said heating unit and said control strategy of said variable frequency drive when said monitored temperature is greater than said temperature threshold;

wherein said adjusting said setpoint includes decreasing a heating capacity of said heating unit and said adjusting said control strategy includes increasing said drive speed for at least one heating mode.

11. The method of claim 10, further comprising:

adjusting additional HVAC system components to increase air flow within said HVAC system when said monitored temperature is greater than said temperature threshold.

12. The method of claim 10, further comprising:

modifying program interlocks for said HVAC system when said monitored temperature is greater than said temperature threshold;

wherein said modifying said program interlocks includes setting an HVAC system component to cycle based on a current mode of said HVAC system.

13. The method of claim 10 wherein said setting said control strategy includes setting a preset speed control strategy.

14. The method of claim 13 wherein said control strategy includes a maximum mode and a minimum mode, and wherein said setting said preset speed control strategy includes setting said maximum mode at about sixty percent capacity speed and said minimum mode at about forty percent capacity.

15. The method of claim 14 wherein said control strategy includes an intermediate mode, and wherein said setting said present speed control strategy includes setting said intermediate mode at about fifty percent capacity.

16. The method of claim 10 wherein said setting said control strategy includes setting a temperature based control strategy.

17. The method of claim 16 wherein said setting said temperature based control strategy includes setting a base capacity and a maximum capacity for said variable frequency drive and setting a start ramp-up temperature and an end ramp-up temperature.

18. The method of claim 17 wherein said base capacity is about forty percent, said maximum capacity is about one ninety percent, said start ramp-up temperature is about eighty five degrees Fahrenheit and said end ramp-up temperature is about one hundred and five degrees Fahrenheit.

19. A method comprising:

installing a variable frequency drive unit to drive a previously non-variable air handler fan of an HVAC system, said HVAC system having a cooling unit for operating in at least one cooling mode and a heating unit for operating in at least one heating mode;

setting a control strategy for said variable frequency drive, said control strategy including a drive speed for each cooling mode of said at least one cooling mode and each heating mode of said at least one heating mode;

operating said HVAC system in each cooling mode of said at least one cooling mode;

monitoring a superheat of an evaporator of said cooling unit and comparing said superheat with a predetermined superheat threshold, for operation of said HVAC system in each cooling mode of said at least one cooling mode;

adjusting at least one of an expansion valve of said evaporator and said control strategy of said variable frequency drive when said monitored superheat is less than said superheat threshold;

operating said HVAC system in each heating mode of said at least one heating mode;

monitoring a temperature of said heating unit and comparing said temperature with a predetermined temperature threshold, for operation of said HVAC system in each heating mode of said at least one heating mode;

adjusting at least one of a setpoint of said heating unit and said control strategy of said variable frequency drive when said monitored temperature is greater than said temperature threshold,

wherein said adjusting said expansion valve includes decreasing a flow of refrigerant in said evaporator, said adjusting said control strategy includes increasing said drive speed for at least one heating or cooling mode, and said adjusting said setpoint includes decreasing a heating capacity of said heating unit.

20. The method of claim 19 wherein setting said control strategy for said variable frequency drive includes setting at least one of a temperature based control strategy and a preset speed control strategy.