Even if you don’t know much about ventilation in a home, you will likely know that bringing outside air into your nicely conditioned house is not a good idea. Who wants icy winter air piercing through the warm blanket created by the heating system, or cloying hot air suffocating the effects of the AC?
Makeup air is (most often) outside air that is brought into the house. One of the benefits of makeup air (although not its main purpose) is to prevent unconditioned drafts from being pulled into the house. It would not make a whole lot of sense of it to achieve this by bringing in unconditioned air itself!
Not all makeup air units condition the makeup air. Some only temper cold air. Others can both heat or cool the air, depending on the outdoor temperature. Units that can cool air can also dehumidify it. Humidification is possible, although it is not a common feature.
Different Types of Makeup Air Units
There are various types of makeup air units available. These have different capabilities and you can choose which ones suit your needs best.
Even though the range of makeup air units is varied, they can be classed into the following categories:
- Active or passive.
- Non-conditioning or conditioning.
The second grouping is what we are most interested in here, but before we get to these, I just want to briefly touch on active versus passive because you can get either version with non-conditioning and conditioning makeup air units.
Active makeup air units use a fan to pull air into the home. With these units, you are more able to control the amount and rate of air pulled in.
Passive makeup air units, on the other hand, utilize holes cut into the exterior of the home and linked to the room containing the exhaust system. When exhausted air leaves behind a negative pressure system, air is drawn in through these holes. It is passive because air naturally moves down the pressure gradient.
There are also makeup air units that are designed with different users in mind. There are industrial, commercial, as well as domestic makeup air units.
Makeup air units that don’t condition the air can be active or passive units.
By non-conditioning, I mean that the unit does not heat, cool, or add/remove moisture from the air. The air is simply brought into the home as is.
Such units are most commonly used in temperate climates that experience mild temperatures at either extreme.
They are also acceptable to use in houses that have no other conditioning. In other words, if your air is not heated by a heating system, cooled by an AC, humidified, or dehumidified, then adding in more unconditioned air is not going to have the same ramifications that it would in conditioned homes.
Another situation in which these units would be acceptable would be if the amount of makeup air needing to be supplied was small or at least small in comparison to the size of the room/space.
This would mean the makeup air would not have as big an influence over the general room air condition anyway.
There is seldom any point to insulating makeup air ducts carrying non-conditioned makeup air.
Units That Temper Cold Air
Makeup air units that temper cold air take cold air from outside and warm the air so that it is not as cold when it enters the space.
Part of the system through which the air passes on its way into the house contains coils filled with water or glycol (when ambient temperatures regularly drop below water’s freezing point). Liquid propane, natural gas, or electricity are used to heat the liquid in the coils.
As air passes over them, heat from the coils is transferred to the air, tempering it so that it is not as cold as outside air when it is released indoors. This allows the makeup air unit to be used in winter without putting added strain on your heating systems.
If the air was not tempered, then your furnace would have to work harder to maintain the temperature to which your thermostat is set.
This can have more immediate negative effects, like higher utility bills each month. Or the effects can be seen in a few years’ time when the heating system starts to fail prematurely.
While makeup air ducts don’t have to be insulated, those carrying tempered makeup air will benefit from the insulation.
Heating and Cooling Units
Some units can both heat and cool the makeup air that is brought into the home and do so depending on the air temperature outside. How a makeup air unit heats the air was just described above.
The cooling system is similar in that the air passes over coils. However, these coils are filled with a cooled refrigerant that absorbs heat from the air passing over the coils. The cooled air is then released into the house.
Having a makeup air unit with both heating and cooling capabilities helps reduce the strain on air conditioning units and furnaces, particularly in less temperate climates, where you can experience extreme heat and extreme cold in the same year.
Such units are also well-suited to homes that tend to condition the air extremely using the HVAC system. If you have paid a lot of money to keep your house at fall temperatures in the middle of the summer, then you don’t want makeup air to undo all the cooling.
Do Any Makeup Air Units Remove/Add Moisture?
Another aspect of the condition of the air that may upset the balance in your home is the humidity level. If you are bringing in extremely humid air, then your house can start to feel like a tropical island. Moreover, your furniture and textiles can start to suffer.
Happily, makeup air units can also add and/or remove moisture from the air.
Moisture is removed by cooling the air. As you probably remember from high school science, cold air holds less water than hot air, so as the air is cooled, water condenses and is removed. The dehumidified air is then released into the house.
To add moisture to the air, makeup units must be equipped with a specialized system that supplies water to the makeup air unit (connects to the home’s water supply). This water is then subjected to vibrations that cause it to break up into very fine droplets.
The dehumidifying aspect of a makeup air unit is present in any unit that cools the air, but a humidifying aspect is one that is not automatically included with a unit that heats or cools.
Makeup air units that add moisture to the air are not too common. These units are normally used in arid climates where the air is exceedingly dry.