The control of air flow or ventilation in buildings is important for maintaining suitable indoor air quality and subsequently maintaining a comfortable and healthy interior environment. Indoor air quality deteriorates with time for a variety of reasons and must be continually removed, or exhausted, and replaced (“made up”) with conditioned ambient air.

What is Exhaust Air?

Air that is deliberately removed from the building envelope and rejected to the environment is referred to as exhaust air. Building ventilation systems are equipped with exhaust fans that work to draw air out of the buildings from strategic locations where low quality, moist or polluted air is likely to accumulate. Typical examples of exhaust air systems in the domestic environment include bathroom extractor fans to prevent moisture build up and associated mold issues, and kitchen extractor hoods for removing cooking smoke, odors and de-oxygenated air.

Natural Replenishment of Exhaust Air

The continual removal of air via exhaust fans reduces the indoor pressure and creates a pressure difference between the interior of the building and the outdoor environment. Air flow is driven by pressure difference with air flowing from regions of high to low pressure. Ambient air will therefore be drawn in naturally through any openings in the building envelope (doors, windows, vents or leaks) from which air is mechanically exhausted.

Uncontrolled natural replenishment of exhaust air can be an effective and energy efficient ventilation tool in certain situations when the ambient air is of suitable quality. However, in situations where the ambient air is too hot / cold, moist or contains unacceptable levels of contaminants (dust or pollution for example), it may be undesirable to allow this air to enter the building naturally.

Makeup Air Units

In sealed buildings, the exhausted air must be replaced by mechanical means, typically through ventilation fans that draw ambient air into the ventilation system. Ambient air can either replace all of the exhaust air, or some of the interior air can be recirculated after some conditioning. Recirculation is typically more energy efficient but is not acceptable in some applications.

Makeup Air Units (MAUs) act to replenish the exhaust air completely with conditioned ambient air (no recirculation). MAUs draw ambient air into the building via a series of conditioning phases (filtration, temperature and humidity adjustment) and replace exhausted air completely.

The Importance of Balance

For optimal comfort and energy efficiency, the exhaust and make-up systems must be well-balanced to provide stable building interior pressure. A building with a poorly balanced ventilation system will experience either an accumulation or reduction in internal gauge pressure, both of which are undesirable.


By Michael Owen, PhD

Michael is a mechanical engineer working in academia. His research encompasses various aspects of fluid dynamics and heat transfer with a strong emphasis on industrial heat exchangers.