Variable Frequency Drive (VFD) on Make-up Air Unit

Make-up Air Unit (MUA) is a Code requirement and certain minimum levels must be provided in order to ensure occupant comfort and safety. ASHRAE Standard 62.1 governs building ventilation and is referenced by the Ontario Building Code (OBC).

ASHRAE 62 at the moment contains only recommendations for minimum suite ventilation, but these recommendations will become requirements as the Ontario Building Code seeks to improve building energy efficiency. The minimum requirement for the air rate is as follows:

0.06 CFM/ft2 in dwelling units
0.06 CFM/ ft2 in common corridors
5 CFM/person in dwelling units

Adding Variable Frequency Drive (VFD) to the Make-up Air Unit (MUA) is a simple way to reduce building’s energy consumption. The idea is to reduce the amount of heat/cool supplied to common areas. Variable Frequency Drive (VFD) typically turn down supply air volumes during non-peak hours, such as overnight or in the middle of the day, a schedule is set as:

Peak Supply (90% of full CFM*):
Breakfast time 2 hours
Lunch time 2 hours
Dinner time 4 hours
Non-Peak Supply (60% of full CFM*): Remaining time 16 hours

Installing Variable Frequency Drive on Make-up Air Unit’s maintains positive pressure through corridors and odours are kept inside the suites. Exterior air and moisture is limited from infiltrating the building and reduces the possibility of condensation on windows and frames, and also results in up to 50% of energy savings as Variable frequency drives reduces gas or electrical consumption of the MUA unit by reducing the amount of energy required to heat/cool the fresh air being supplied to the building.

An annual savings of $5,520 and Incentive of $5800 was achieved on a Variable Frequency Drive installation done recently.