Make Up Air Units
A Make Up Air Unit (MUA) can be found on all types of buildings from office towers, industrial factories, apartment and condominium buildings. This type of unit can provide fresh air, filtered air, heated air, air conditioning or be used to pressurize a building to replace air exhausted from other sources. In the manufacturing industry a MUA unit would be used to replace the air exhausted by large exhaust fans that are rejecting heat or fumes. Without the MUA the building would experience a severe negative pressure that would make opening any doors difficult and could also create dangerous conditions with any of the other gas burning appliances in the building. In apartment building or condominiums the MUA unit is common for conditioning the hallways and lobbies for human comfort, but more importantly to pressurize the hallways. This pressurization of hallways controls odors from the individual suites by preventing air from the suites coming into the higher pressure zone. If you live or have visited a condo you will recognize that when the door to a suite is opened that the air movement is from the hallway into the suite. Depending on the size of the building there may be more than one unit. The Fire department can control the fan operation of these units from the alarm panel in the lobby and must have this control to regulate the spread of smoke in the case of a fire in the building. Most MUA units inject 100% outside air into a building unlike the standard furnace system that re circulates the same air over and over. If you can imagine outside air at minus 10 degrees Celsius being injected into a building you can imagine the vast amount of heat that must be added in one pass over the heat exchanger to raise the temperature to one of human comfort. This is
what makes these units the "gas guzzlers" of our industry. For a comparison the average home furnace may use 80,000 BTU's of gas whereas the average MUA for a condo could easily consume 2,000,000 to 5,000,000 BTU's. The cost of this fuel is part of the monthly condo fees that the owners pay.
There are two basic types of MUA units... Direct Fired and Indirect Fired. In A direct fired unit there is no heat exchanger so all the products of combustion from burning the natural gas enter the building with the heat produced. This style of unit has limited useful operation due to the toxic gases that are introduced into a building that could affect human occupation. They are common in manufacturing facilities where they would be interlocked with a paint spray booth and the air exhausted from the booth is almost equal to the air introduced by the MUA. They would also be used in
agriculture where the products of combustion of the gas can facilitate faster plant growth and livestock farming for chickens and pigs due to the high volume of air exhausted from barns to control odors. All MUAs' for buildings designed for human occupancy have a heat exchanger to separate the heat from the products of combustion and are called "Indirect Fired" MUA units. In condominiums these units are typically on the roof, but can also be in basement mechanical rooms connected to an air shaft to ground level.
One typical location for MUA that is unfiltered and unheated would be in the underground parking garages where large exhaust fans remove the fumes from car engines and MUA enters through large motorized dampers and louvered intakes that are connected to air shafts up to grade level. Many newer condominium buildings have added air conditioning to the make-up air units and in the same comparison as the fuel consumption for heating 100% outside air the cooling capacity is also oversized to provide a 20 degree drop in air temperature in one pass through the unit. There is a simple example I like to use when explaining to customers about building pressure and the effect of even a small exhaust fan that is left
running when not needed. The average residential bathroom fan will exhaust 100 cubic feet of air per minute (CFM). In one hour the same fan will remove 100 x 60 or 6,000 CFM. The same fan left running for 8 hours while you are at work would remove 100cfm x 60minutes x 8hours or 48,000 cubic feet of air. An average house of 2,000 square feet with 8 foot ceilings has a total of 2,000 x 8 or 16,000 cubic feet of air. Doing the math you can see that 48,000 / 16,000 = 3 complete air changes in your house from the little bathroom fan. This reminds me of the children's story about the "Little Engine That Could". The air that this little fan has exhausted has to be replaced and it re-enters the house through doors, windows, cracks and results in you paying to reheat or re-cool the air at a considerable cost. In the average home your MUA unit is the entrance door. If you have ever felt an inrush of cold air in the winter when greeting someone at the door, the reason is the lower or negative pressure in the home compared to outdoors. There are many things in the average home that exhausts air from the structure from exhaust fans, range hoods over the stove, clothes dryers,
water heaters, furnaces, fireplace chimneys etc. All these items remove much more air than the average bathroom exhaust fan. On an average day you would not notice the effect, but during extremes in temperature from very cold to very hot you will notice discomfort. See our section under air conditioning to learn helpful hints on reducing the load on your furnace and air conditioner. The cost to operate a MUA unit is obviously high and yet there are methods to minimize the expense. Many condominium managers have added timers on the MUA units to cycle the unit off when the bulk of the owners are sleeping (11:00 PM to 5:00 AM). Others have added a device that slows down the fan speed to reduce the air entering the structure. This method is a VFD or "Variable Frequency Drive" with a sensor that will adjust the fan speed from
approximately 30% to 80% to maintain a slight positive pressure in the building rather than running the fan at full speed all the time even when there is no demand. The VFD will reduce the electrical cost of operating the large fan motor and reduces the gas consumption because less air flow requires less heat or less cooling energy. There are government or hydro grants available from time to time that will offset a considerable portion of the installation cost of VFD's and there is also a very reasonable payback in the savings on energy.