Planning ventilation systems can be exhausting (pardon the pun!) and even dangerous. One thing that you have to look out for is locating your makeup air vent far enough away from your exhaust vents.
The International Residential Code (IRC) is rather specific when it comes to how close makeup air intakes can be to exhaust outlets. Here are the rules and why they exist.
Gravity (or passive) makeup air units need to be at least three feet away from exhaust openings. Mechanical (or active) makeup air units need to be at least 10 feet away from exhaust openings unless the opening is above the air intake vent; in such cases, a distance of three or more feet is acceptable.
Exhausts and Makeup Air Units Need to Be Distanced?
When thinking about air intake and exhaustion, you can imagine makeup air and exhaust systems as opposites.
Makeup air units are essential for providing fresh air drawn from the outside into spaces that lose a significant amount of air through exhaust systems like range hoods. This prevents depressurization from occurring, which can lead to things like backdrafting.
Exhaustion is an important function of many appliances, such as gas appliances and bathroom fans.
Gas appliances will use exhaust vents to channel harmful chemicals and volatile organic compounds (VOCs) from combustion out of the home, and bathroom fans exhaust steam, which can be damaging to your bathroom and lead to the growth of mold.
Basically, exhaust systems rid the house of several pollutants that would damage your home and/or threaten your health. You do not want these substances brought right back into the house!
Therefore, it is very important to distance the two to allow them to both function properly and safely.
Gravity Makeup Air Units
One type of makeup unit that is available that does not use fans or any mechanics to move air is a gravity makeup air unit.
As mentioned, when air is expelled from a room faster than it can be renewed, then a depressurization effect occurs. A gravity makeup air unit capitalizes on this by providing a path for fresh air to enter the house, traveling down the pressure gradient.
What this means is that because the inside of the room will have a lower pressure than the outside, air will be drawn in to equalize it.
For air exhaust openings, the IRC highlights the following restrictions about where air exhaust openings can terminate:
“Not less than 3 feet (914 mm) from gravity air intake openings, operable windows, and doors.”
IRC M1504.3
This means that the intake of any gravity makeup air unit needs to be three feet or more away from any exhaust openings.
When separated by this distance, there will not be enough of a pull from air entering the gravity makeup air unit to draw any of the harmful gasses from the exhaust vents and back into the house. This makes the makeup air unit efficient and safe to operate.
Mechanical Makeup Air Units
On the other hand, another type of unit you can get is a mechanical makeup air unit. These work by using fans to actively draw air in from the outside and into your home.
This makes them more efficient than gravity makeup air units; however, the increased efficiency can come with its own drawbacks, particularly when looking at where to place the intake.
While a mechanical unit is better at drawing air into a building, it is also able to pull harmful gasses in from a much greater distance away than a gravity unit.
The IRC says the following about where air exhaust openings can terminate when it comes to mechanical makeup air units:
“Not less than 10 feet (3048 mm) from mechanical air intake openings except where the exhaust opening is located not less than 3 feet (914 mm) above the air intake opening.”
IRC 1504.3
Because mechanical air units are able to draw in air from a much greater distance away, any exhaust vents will need to be installed much farther away to prevent any harmful uptake.
However, there is an exception to this rule and it exists where the exhaust vent is above the makeup air unit intake. When it is above, the exhausted air rises as soon as it leaves the vents, traveling further from the makeup air unit, so the lesser distance is acceptable.
Sources
https://blog.totalhomesupply.com/make-up-air-units-ultimate-guide/