Makeup air is supplied to replace exhausted air. It can be conditioned and can come from outside or indoor areas. It’s required for high-CFM systems with fuel-burning appliances present. Combustion air is supplied to ensure adequate oxygen for fuel-burning appliances. It can come from inside and/or outside.
Difference Between Makeup and Combustion Air: Overview
Makeup air and combustion air are similar in that they both are necessary to ensure good air quality in the home and efficient appliance performance.
However, makeup air and combustion air do this in different ways (they have different purposes with common goals).
Makeup air is necessary to replace the air expelled from exhaust vents of appliances and can be regulated in terms of temperature.
Combustion air, on the other hand, is needed to ensure that combustion appliances have a ready supply of air (oxygen) for the combustion of fossil fuels to be completed.
What is Makeup Air?
The Purpose of Makeup Air
In most homes, air is expelled from various appliances such as bathroom exhaust fans, clothes dryers, gas furnaces, or range hoods.
These appliances can expel air at rates of up to 1400 CFM (cubic feet per minute). The purpose of makeup air is to replace the air in an interior space that has been removed via exhaust fans.
Without makeup air, a room can develop a negative pressure system that cannot be balanced without intervention. The negative pressure system has many adverse effects, including:
- Pulling air can be backward through water-heater flues or wood-burning chimneys in a process known as backdrafting. Backdrafting can be dangerous or even life-threatening, as some appliances produce toxic gases like carbon monoxide, which can build up and harm inhabitants.
- Pulling sewer gases from drains. These gases are unpleasant and can also pose a threat to health.
- Preventing hot, moist air or smoke from being exhausted. A negative air pressure system is a vacuum. It is extremely hard to pull anything out of a vacuum. So, the negative pressure can inhibit the function of the exhaust system.
When It Is Required
In homes that are not well-sealed, makeup air would typically enter through random cracks and gaps in doors, windows, baseboards, etc.
However, as houses become more energy efficient and tightly sealed, it may be necessary to set up a system to deliberately provide makeup air, especially during the operation of high-CFM exhaust appliances.
Section M1503.6 of the International Residential Code (IRC) says that makeup air is required for kitchen exhaust systems if:
- There are appliances that burn gas, liquid, or fuel and do not have a direct vent or mechanical draft
- The exhaust system is rated 400 CFM or more.
Section G2439.5 of the IRC says that makeup air is only required with gas-fuelled dryers that exhaust more than 200 CFM, and Section G2439.5.1 says that makeup air is also required for closet-installed gas dryers.
Makeup air should be inputted at an equal rate to the outflow and the unit must be equipped with at least one damper.
Makeup air is not needed for exhaust systems that are only used for room cooling purposes when windows or other air inlets are open.
How Makeup Air Is Provided
When air is removed from exhaust systems, it creates negative pressure or depressurization.
Air will typically move down a pressure gradient from a region of high pressure to one of low pressure.
Makeup air can be allowed to flow in passively by opening a window or it will simply flow in through unsealed cracks, down the pressure gradient caused by appliance exhaust systems.
In well-sealed homes and especially in houses in cold or hot climates, this can be problematic for temperature regulation in the home because cold or warm air will flow in, offsetting air conditioning. It may also not be sufficient to balance the air lost from a high-CFM device.
Hence, makeup air units can be installed to balance the depressurization caused by exhaust systems.
Makeup air units have a damper that regulates the flow of air and is located in the duct and wired to a pressure regulator.
The pressure regulator works by opening or closing the damper depending on the amount of air that is exhausted, and a wall cap lets in fresh air from the outside.
For well-sealed homes this system is sufficient, but in climates with extreme cold or warm conditions, a heating or cooling system within the makeup air unit can be installed to warm or cool the air before it enters the house.
Makeup air can also be transfer air, provided from other rooms in the house, such as the attic, although there are some drawbacks to this technique.
Makeup air is supplied to the room containing the exhaust system or to an adjacent room with adequate openings between the two spaces.
What is Combustion Air?
The Purpose of Combustion Air
Appliances that burn fossil fuels, such as oil furnaces or gas fireplaces, require air to carry out combustion safely and efficiently.
Without air, combustion cannot take place, because it requires oxygen for the chemical reaction. But combustion air is not provided to prevent this situation because it is unlikely to present in a home.
Additionally, combustion appliances do not remove air to the same extent as exhaust systems.
Combustion does, however, use up the oxygen in the air and there are two different types of combustion, depending on the amount of oxygen available for the chemical reaction at any given time.
Complete combustion is when there is enough oxygen for the reaction to produce carbon dioxide. Incomplete combustion is when there is only enough oxygen to produce carbon monoxide.
I assume you know enough about these two byproducts to know that carbon monoxide is far more dangerous.
Furthermore, incomplete combustion releases less energy (heat) than complete combustion. As heating is vital to the performance of gas water heaters, furnaces, etc., this makes the appliances less effective.
So, combustion air is a way of ensuring that there is always sufficient oxygen for safe and effective combustion.
When It Is Required
Combustion air must be provided for all fuel-burning appliances.
Section M1701.1 of the IRC states that combustion air should be supplied for solid fuel-burning appliances as per manufacturer instructions, and oil-fired appliances should be provided with combustion air as per NFPA 31.
Sections G2407.5, G2407.6, and G2407.7 cover combustion air requirements for gas-powered appliances.
How Combustion Air Is Provided
While it may sometimes be sufficient for makeup air to be provided through cracks and gaps in the building envelope, combustion air supply is seldom left to chance in the same way. Instead, it is deliberately provided.
Combustion air can be provided in several ways through crawlspace vents, exterior wall vents, or roof vents, depending on where the appliances are located.
Combustion air can be provided to the room in general, using vents and sometimes ducts, it can be provided to the area immediately surrounding the appliance via direct vents, or it can be supplied through fixed ducts attached to the appliance air inlet.
Combustion air is not conditioned when it is brought into the home and the systems are not required to have a backdraft damper in place.
Table of Differences Between Makeup and Combustion Air
|Makeup air||Combustion air|
|Purpose||Replaces air exhausted from the house by exhaust systems.||Provides air for combustion in appliances that burn fossil fuels.|
|When it is required||Kitchens with fuel-burning appliances and exhaust systems of 400+ CFM.|
Rooms containing gas dryers exhausting 200+ CFM.
|Whenever there are fuel-burning appliances present in the home.|
|How it is provided||Transfer air from other rooms or makeup air units with ducting. The air is supplied in the room or an adjacent room.||From indoor and/or outdoor areas via vents and/or ducts.|
The air is supplied to the room in general, to the area immediately around the unit, or into the unit directly.
|Temperature control||Heating/cooling systems can be included.||Heating/cooling systems not typically included.|
|Damper||Required according to IRC.||Not required according to IRC.|