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Make-Up Air Units: An Important Component in Achieving Energy Efficiency

Posted on March 5, 2014 by HSC


Energy efficiency, resident comfort and utility costs all largely depend on the age, quality and maintenance of your heating and ventilation equipment. One of the most important and perhaps least understood components of this equipment is the make-up air system (MUA).

Functions of a Building MUA

The MUA is an essential part of a building’s mechanical system. The components of a MUA are typically a box composed of a casing, air filters to remove contaminants, supply fans (a wide variety of fan types) and heating and cooling coils, for which the heat source can be from a number of different sources. These components perform the functions of a MUA.

An MUA supplies one hundred percent outside air to condition the space and does not recirculate air. In contrast, an air handler unit is able to mix a percentage of return air and outside air, filter it, and then supply it to the space. An air handler unit has a mixing chamber, where outside air is combined with return air, whereas a MUA does not.

Often located on the roof of the building, a MUA is designed to condition the air entering the building and to counteract the effect of building exhaust systems, which can remove large quantities of air and cause negative pressure. Negative pressure makes outside doors hard to open, exhaust fans function poorly and allows cooking odours to travel into the common spaces. A standard MUA brings in fresh air into corridors to help maintain positive pressure. The MUA also ensures efficient performance of exhaust systems, improves indoor air quality, reduces work load on your HVAC system and can save you money on your energy bills.

When to Replace a MUA

A MUA unit consumes a significant amount of energy, and older MUAs will likely cost more to operate than newer, energy efficient equipment. A new, high quality MUA has a lifespan of approximately 20 years, although older versions can fail at 10 to 12 years. One of the challenges in using a MUA is to know when to replace equipment; waiting until equipment fails can be very costly and inconvenient since a typical replacement can easily take more than two months.

If your MUA unit is over 12 years old, now may be a good time to consider replacing it. Discuss with your contractor the unique needs of your project and ensure that they select the most appropriate system.

Energy Efficiency Features to Consider

When installing a MUA, your contractor should consider overall duct design and energy efficiency features. These latter features include 90+% efficient units, improved premium efficiency motors (check the fan efficiency grade (FEG) rating of the fan) and direct drive type with variable frequency drives which can result in higher efficiency and lower overall noise. Also if existing MUA units are running 24/7, there may be an opportunity to reduce energy by scheduling to run at lower speeds during off peak periods. Ensure that the installation allows space for maintenance and includes the proper commissioning. And finally it is an excellent idea to take advantage of gas company incentives – these can lower your up-front costs and significantly shorten the payback.

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