Variable Frequency Drives in Single Zone Rooftop HVAC Units as Installed by Commercial HVAC Contractors

It is our understanding that VFDs are now required in single zone units in California and that other states may follow. ASHRAE standard 90.1-2010 requires VFDs in single zone, direct expansion (DX) units with cooling capacities of greater than 9.1 tons. If you are not familiar with the term, DX units are your typical package rooftop units which have internal compressors. ASHRAE standard 18931-2009 also requires VFDs in single zone DX units with cooling capacities of greater than 9.1 tons and, in addition, two stages of cooling in units with cooling capacities of greater than 5.4 tons. This may provide additional savings because having multiple stages may allow you to use lower blower speeds than with single stage, on/off, compressors. You may check with your local code authority to see if these standards have been adopted by your jurisdiction, and/or consult with commercial HVAC specialists

As with any equipment modification, there are a few things you need to be aware of when having qualified commercial HVAC contractors install VFDs on single zone units. From the service contractors’ viewpoint, the most immediate issue is to ensure that the low-speed setting moves enough air across the coil in the cooling mode to prevent “flood back,” a condition where the refrigerant does not absorb enough heat and returns to the compressor in liquid form. In severe cases, flood back can damage even the newest scroll compressors. Concern for this may be the reason that both ASHRAE standards will accept a low setting of two-thirds of full speed. Since even slower speeds may work in many applications and provide greater savings, the safest approach would be to check each unit individually. Existing problems external to the unit such as undersized or restricted duct work, closed diffusers or dampers, leaks between the supply and return ducts and other issues may not allow for sufficient air flow when the blower speed is reduced. Insufficient air flow would also be a problem in the heating mode because it may result in the short cycling of the safety devices, overheating of the heat exchangers, and possibly an increase in the air temperature above the safe design temperature for that equipment.

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