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Pause at the door as you enter the building. Crack the door open just enough to slip a pencil in. Place your face near the opening and you should be able to feel a gentle breeze in your face from inside to outside. Then check from inside and you should feel nothing. In most homes you will likely sense no breeze either way. These are balanced conditions for residential and commercial buildings.
During hours of occupancy commercial buildings, such as schools and office buildings have bathroom exhaust fans pulling air out of the building. The exhausted air needs to be made up. The makeup air must be ‘conditioned’ for temperature, sometimes humidity and ‘controlled’ to enter in a predictable fashion. This is achieved with makeup air and/or supply fans.
Otherwise, makeup air would migrate into the building through penetrations where cables, drains, vents and pipes enter, or anywhere else it can. Freezing air entering around water pipes such as through gas water heater vents provides a clear illustration of why we need to control and condition the incoming makeup air.
In the past few years home air handlers have included the introduction of outside air into the home to freshen and makeup air exhausted by bathroom fans, dryer exhaust and range hoods.
Improperly installed wood and pellet stoves can drive up your energy bill. They need a considerable amount of makeup air, which typically comes from under the tub and sinks, through the dryer, the range hood, lights and bathroom exhaust. None of these are places you want this air to come from.
In some cases sealing your windows may threaten your pipes.
So think of it this way. Anything that takes air out of your house or building on a continuous basis has to have an appropriate means of safely making that air up. In commercial buildings it is common practice to introduce more air into the building than is taken out so controlled air will escape from the leaks rather than having uncontrolled air entering through them.
The differences are slight, so a door blowing open may indicate a failed exhaust fan and a door sucking shut may be a sign of a failed makeup fan. If you know this, you can check the pulse of the building as you walk through the door.
Exceptions are restaurants and science labs. Restaurant design seldom includes adequate makeup air, or if it does the handy mechanic will defeat it in attempt to save energy. Science labs want to keep experimental creatures and processes inside the lab. We simply can’t have Frankenstein escaping through the pipe chase.