Make Up Air Units and Variable Frequency Drives

Corridor Fresh Air Units or Make-Up Air Units (MUA) are found in virtually all High Rise Multi-Residential Buildings. Older buildings, including rental properties, often have these units with no heating or air conditioning of the air introduced into the building. In newer buildings and in virtually all condominiums, these units do provide heating and cooling of the fresh air.

The purpose of the MUA units is threefold. First, the units slightly pressurize the building in order to prevent infiltration of outdoor air through the building envelope. Second, the units provide, as the name implies, ‘make up air’, to replace air that has been exhausted from the building by in suite exhaust fans. The third purpose is to provide for ‘air changes’, or fresh air for comfort and respiration.

The Canadian Mortgage and Housing Corporation (CMHC) have done numerous studies on make up air units, and have found some very interesting results. One of the things that were found was that this air often leaks through the building envelope in unintended ways.Air can leak through door openings, envelope penetrations, elevator shafts, stairwells, and other places where the air can find its way directly to the outside. The result is that Fresh Air Units can be tremendous energy ‘wasters’. When fans are sized during the construction phase, they are often sized using the assumption that a certain number of exhaust fans will be running at the same time. This is referred to as the diversity factor. Clearly, at night and at other off-peak times, the actual use of exhaust fans will be well below the diversity factor used to size the fans.

One excellent solution is the use of Variable Frequency Drives (VFD). The fan can be operated on a time of day schedule, speeding up and slowing down as needed. Variable Frequency Drives today are much more reliable than they have been in the past and their increased use has driven installation costs down. Provident Energy has supplied and installed hundreds of VFDs and we highly recommend their use in a wide range of applications.

For more information regarding VFD applications for your building or to arrange a free energy assessment, please contact the sales department at Provident Energy at 416-736-0630, extension 5.