We all know the importance of a supply of air for gas-fuelled appliances, including gas fireplaces, even if the reason for this supply is not fully understood. If you look into the building code regulations regarding this, you come across two terms: makeup air and combustion air.
These are actually referring to two different things, and even though they are both mentioned in the chapter on gas fuel, only one of them is required for gas fireplaces. Let’s clarify this further.
Gas fireplaces do not need makeup air. Makeup air is the air brought into the house to replace air removed by exhaust systems. Gas fireplaces do, however, require a supply of air. This is known as combustion air, and it is separated from makeup air in when, how, and why it is brought in.
Makeup Air Replaces Exhausted Air
Makeup air is necessary to fill the negative pressure created when appliances with exhaust systems, such as range hoods, bathroom fans, and dryers, actively remove air from the house’s interior.
On their own, gas fireplaces don’t require makeup air.
However, according to Section G2407.4 of the International Residential Code (IRC), exhaust fans as well as appliances such as clothes dryers and kitchen ventilation systems should not interfere with appliances fuelled by gas, like gas fireplaces.
So, if exhaust systems are present in the same room as a gas fireplace, regulations require makeup air to be provided to avoid interference.
Interference can manifest in two main ways.
Firstly, if there is no makeup air, the exhaust systems can create negative pressure in the room, which pulls air back out of the gas fireplace vents to try and balance the pressure.
This is called backdrafting and it is very dangerous when it occurs with appliances that generate harmful combustion byproducts that are supposed to be removed from the house through the vents/chimneys.
Secondly, if the combustion air supply is not directly fed into the appliance but is, instead, delivered more generally into the room in which the gas fireplace is installed, then the combustion air will be prevented from entering the appliance and serving its purpose.
Instead, it will act as makeup air to balance the uneven pressure system.
But gas fireplaces need this combustion air, which is what we will be discussing shortly.
Makeup Air Can Become Combustion Air
If you have a device that is exhausting air from the room containing the gas fireplace, then the air pulled in from outside by the makeup air unit can then be drawn into the appliance’s vicinity and be used as combustion air.
However, you can need makeup air without having a gas fireplace and you can have a gas fireplace without a makeup air unit if there are no exhaust systems nearby. This is why the air supplies are given different terms and are differentiated in terms of their function.
Gas Fireplaces Need Combustion Air
For combustion to occur, three things are needed: fuel, heat, and air.
While makeup air replaces the air expelled from exhaust vents, combustion air is supplied specifically for the combustion of fossil fuels to be carried out.
The difference between complete and incomplete combustion, i.e., safe and efficient combustion and dangerous and inefficient combustion, is the amount of oxygen available for the reaction.
If combustion air is not supplied, the gas produced by burning fuel is carbon monoxide as opposed to carbon dioxide. You don’t need me to tell you which of these two gases is more toxic!
In addition, incomplete combustion produces less heat, which is the purpose of a gas fireplace. This is what makes gas fireplaces inefficient when combustion air is lacking.
One other negative consequence of a lack of combustion air is backdrafting.
Yes, combustion air is needed to ensure a proper supply of oxygen for burning the gas, but the fact that the appliance draws air in means that the area around the fireplace can suffer from negative air pressure, which results in backdrafting.
In a way, this means that combustion air acts as makeup air for the fireplace, but technically, makeup air, which is associated with exhaust systems, is not required by gas fireplaces.
Sources of Combustion Air
Combustion air can be supplied from indoors, outdoors, or a combination of the two.
According to Section G2407.5 of the IRC, indoor combustion air can be provided from rooms that are adjacent or otherwise connected to the room containing the gas fireplace.
The connections must be permanent (i.e., they cannot be coverable by doors) or they can be specially installed combustion air openings.
As stated by Section G2407.6 of the IRC, outdoor combustion air is provided through three-inch-plus openings (vents) or ducts between the outdoors and the space containing the gas fireplace.
Air pulled in from a ventilated attic is also considered to be outdoor combustion air because of the free communication between the attic and the outdoors.
Indoor and outdoor air can also be used in combination to provide combustion air as long as they fulfill the IRC regulations as per Section G2407.7.
Indoor combustion air should be provided at a minimum volume of 50 cubic feet per 1000 Btu/h (British thermal unit per hour) of the gas fireplace input rating.
Alternatively, or if the air infiltration rate is confirmed to be less than 0.40 air changes per hour, the required volume can be determined using specific calculations.
Apart from this, the amount of combustion air required is represented by the specifications of the openings to be provided.
When the rooms supplying indoor combustion air for the fireplace are located on the same floor/story, then:
- There must be two openings.
- The opening must be permanent.
- The openings must provide a free area of at least 100 square inches or 1 square inch for every 1,000 Btu/h of the fireplace, whichever is greater.
- One opening must start less than 12 inches from the top of the wall between the two spaces, and the other opening must start less than 12 inches from the bottom.
When the rooms supplying indoor combustion air for the fireplace are located on a different floor/story, then:
- There must be one to two openings.
- The openings must be permanent.
- There must be at least two square inches of free space for every 1,000 Btu/h of the fireplace.
When outdoor combustion air for the fireplace is provided by two openings, then:
- The openings must be permanent.
- One opening must start less than 12 inches from the top of the room housing the fireplace and the other opening must start less than 12 inches from the bottom.
- These openings can lead directly to the outdoors, to ducts that lead outdoors, or into spaces that communicate directly with the outdoors, like ventilated attics.
- If the ducts are vertical, the free area must be at least 1 square inch for every 4,000 Btu/h of the fireplace.
- If the ducts are horizontal, the free area must be at least 1 square inch for every 2,000 Btu/h of the fireplace.
When outdoor combustion air for the fireplace is provided by one opening, then:
- The opening must be permanent.
- The opening must start less than 12 inches from the top of the enclosure.
- The opening must lead directly to the outdoors or to ducts that lead outdoors.
- The minimum required free area is 1 square inch for every 3,000 Btu/h of the fireplace.
Ways of Providing Combustion Air for Gas Fireplaces
As you can see from the above section, indoor combustion air can be supplied through two permanent openings that connect the room containing the gas fireplace with another space, either above, below, or next to the room.
Outdoor combustion air can be provided through one or two permanent vents that directly connect the room containing the gas fireplace with the outdoors or a space that freely communicates to the outdoors.
Alternatively, outdoor combustion air can be provided through one or two ducts that connect the room in which the gas fireplace is installed with the outside and which are permanently open.