Allowing fresh air into the home to replace exhausted air is quite important for the house. However, when it occurs in what would be considered to be an uncontrolled manner, the migration of outside air into your home isn’t always a positive experience.
To discern whether makeup air cools or heats the home, it must first be understood where it comes from and what, if anything, the makeup air units do to that air before releasing it into your home.
Makeup air can heat or cool the house incidentally. This isn’t makeup air’s purpose, and the heating/cooling is typically undesirable (it heats the house in summer and cools it in winter). To counter this, some makeup air units temper incoming air. Others actively heat or cool it.
The Purpose of Makeup Air
Makeup air’s purpose is to replace the air that has been actively forced to exit the home by exhaust systems. This is done regardless of the temperature of the air.
The function of makeup air units is not to heat or cool an area through the addition of this air. Rather, makeup air is used to prevent the issues of negative pressure systems.
Negative pressure systems, also known as low-pressure systems, are areas that cause the surrounding air to move toward them.
You may have felt the presence of a negative pressure system if you have put your hand up to the gap in a door and felt air blowing onto you. In this case, you are in a low-pressure area since the air is moving toward you.
Negative pressure systems occur when large amounts of air are removed from the home over a relatively short space of time, creating a void. Nature resists voids (vacuums) at all costs, so a negative pressure system is formed to replace this air.
Without makeup air, a low-pressure system in a home can cause a variety of issues.
Air from outside can enter the home through cracks and gaps (uncontrolled entry). This can widen these spaces, creating structural problems. It can also cause unpleasant drafts in the home.
Moreover, not only air from outside will enter the home. Air that is meant to exit the home may be drawn in by a negative pressure system.
This means that air in plumbing pipes and exhaust vents can reenter the home. This is known as backdrafting.
The air in plumbing pipes can be smelly, and exhaust reentering the home can undermine the work of your exhaust vents, causing them to work harder and require more power.
However, the most dangerous form of backdrafting is that which affects combustion appliances—combustion gases can be extremely toxic.
Where Does Makeup Air Come From?
The origin of makeup air will determine whether or not conditioning is necessary to prevent certain unwelcome effects (we will discuss these in the next section).
Some makeup air is pulled from adjacent rooms in the house. For example, makeup air in the kitchen may be pulled from the adjacent dining room. This air does not require conditioning because it is already roughly the same temperature as the air in the room containing the exhaust system.
However, since the house is a relatively closed system, unless windows are open or the house isn’t well-sealed, taking air from another area of the house may not fix the problem.
Sometimes, makeup air is deliberately pulled (using makeup air units) from well-ventilated areas of the home, such as a ventilated attic. This means that the air is coming from outside the building envelope, but it is often more temperate than air coming directly from outside.
However, the majority of makeup air systems bring air in from the outside.
Outside air, because of its differing temperatures and possible pollutants, must often be conditioned or tempered to ensure the fewest negative effects of this supply of air.
Problems With Untempered/Unconditioned Makeup Air
If makeup air entering your home alters the inside temperature, there are a variety of problems that can arise.
For one, cold air entering a home in winter or warm air entering a home in summer can make the air feel uncomfortable.
Not only can unconditioned and untempered makeup air be physically uncomfortable, but it can also make you sweat when you open up your next utility bill.
With your HVAC unit working to keep the air at a comfortable, preset temperature, the entry of outside makeup air will undermine the HVAC system’s work and force it to work harder to counter the effects of the makeup air.
An HVAC system that has to work harder is one that will use more energy, increasing electricity costs.
Additionally, when an HVAC system has to work harder, it can put its various parts under considerable strain.
It is likely that an HVAC under strain will fail faster than one put under less strain. You might not notice the financial repercussions of this immediately, but when you have to start repairing and replacing parts in a couple of years’ time, the costs will add up really quickly.
Another part of the air’s condition that can cause issues if not dealt with is the moisture content of the air—the humidity.
Outside air that is too dry or too humid can make your home feel uncomfortable. Highly humid air is no less disagreeable. Furthermore, air that is too humid or too dry can wreak havoc on your furniture and paint.
Makeup Air Units Can Temper the Air
One way that makeup air units can allow outside air to enter the home without raising energy bills is through a tempering process.
Tempering is a process that controls the temperature of incoming air but not necessarily through the addition or removal of heat.
Instead, the makeup air can be added to a flow of already-cold air if the intent is to cool the incoming outside air. Or the makeup air can be added to a flow of already-warm air if the intent is to heat up the incoming outside air.
Additionally, humid air can feel warmer than dry air. To reduce the apparent warmth of humid outside air, the tempering process can condense this water and remove it.
Makeup air units that temper the air are typically linked up to your HVAC system.
Some Units Heat/Cool the Air
Rather than using the somewhat indirect process of tempering, some makeup air units directly heat or cool the air.
For heating, makeup air units generally use heating coils with direct gas lines.
For cooling, makeup air units may use processes such as hydronic or evaporative cooling, although, some also use cooling coils.
You can read about how these work in Makeup Air | Is It Conditioned?
These units are very useful for areas that experience extreme temperatures, whether they are high or low.
Unfortunately, it seems to be rare to have a makeup air unit that can both heat and cool air. As such, you may have to choose between heating your makeup air and cooling it.
Additionally, makeup air units that cool or heat are often more expensive than normal makeup air units that don’t condition, temper, heat, or cool since these are added features.
Makeup Air Units Can Heat/Cool House Incidentally
Because we have established where makeup air comes from and what temperature it enters the home at, we can now discuss whether makeup air has an effect on the temperature of the air in your home.
The answer to this depends on whether your makeup air is tempered, conditioned, heated, or cooled.
If the temperature of the makeup air is changed to better match the temperature of the air in your home, it is unlikely that it will have much of an effect on the air in your home.
However, if the outside air comes in as-is, it may heat or cool your house incidentally.
This is more often a problem than a perk.
For example, if your home is being cooled through air conditioning, it is most likely warmer outside. The warm outside air entering as makeup air will be warmer than the cooled air and will warm up your home.