Makeup air and combustion air—it’s easy to get confused between them. Yet, to provide a safe operational environment for gas appliances, it’s important to understand the difference. The fact that you’re trying to find out if gas appliances need makeup air means your thoughts are on the right track.
Makeup air is not actually required for gas appliances, but it can still be important to their continued and safe functioning. If this statement brings up more questions for you, don’t worry! You’ll find all the answers you need below.
Gas appliances don’t need makeup air. They require a combustion air supply. Makeup air is linked to exhaust systems that remove air from a house. This can negatively affect gas appliances, so often, if exhaust systems and gas appliances are present together, makeup air is required.
Gas Appliances Don’t Need Makeup Air
Gas appliances do not need makeup air.
I always find that explanations help facts to stick in my mind, so let me explain why gas appliances don’t need makeup air.
The purpose of makeup air is to replace the air that is removed from a room by exhaust systems. When the rate at which this air is high enough, then depressurization of the room occurs.
Depressurization is when more air leaves the house than enters it, and the pressure outside of the house becomes greater than that inside the house, essentially creating a vacuum indoors. This vacuum pulls air in from any place and via any route that it can to try and balance the pressure.
The result is a smorgasbord of issues, including gurgling or smelly drains, poorly functioning exhaust systems, overworked HVAC systems, and backdrafting.
This can happen whether or not combustion appliances are present. As such, makeup air is provided for exhaust systems, not fuel-burning appliances.
Gas appliances might be closely associated with exhaust systems, which is why there can be confusion over what system actually requires the makeup air. For example, gas ovens and range hoods.
Here, the makeup air is required for the range hood, not the oven itself. However, the presence of the oven is likely to still factor into the equation, as you will see in a moment.
Exception: Gas Dryers
Before I get to the relationship between makeup and gas-burning appliances, I need to point out an exception to the rule that I have just explained.
Gas dryers are both combustion appliances and exhaust systems. This means that these gas appliances may require makeup air. However, it is their function as appliances that take air in from the room, use it, and exhaust it to the outdoors that make them qualify for the need for makeup air.
Moreover, they only need the makeup air under certain circumstances.
The International Residential Code (IRC) states, in Section G2439.5, that any dryer capable of exhausting air at or over 200 CFM requires a makeup air supply.
Additionally, if you installed your dryer in a closet enclosure, there must be a makeup air supply as per Section G2439.5.1 of the IRC.
Makeup Air and Gas-Burning Appliances
So, fuel-burning appliances do not need makeup air. However, the presence of fuel-burning appliances often necessitates makeup air for exhaust systems because of the influence that removing air from the room has on the movement of combustion air.
Compromised Appliance Function
According to Section G2407.4 of the IRC, makeup air must be provided where dryers, exhaust fans, and kitchen ventilation can interfere with the operation of gas appliances.
This means that makeup air must be supplied to a space where the negative pressure system can impact how gas appliances function.
Depressurization can pull combustion gas byproducts back into the house, exposing you to the harmful and potentially fatal gases.
The low-pressure system can also interfere with the supply of combustion air, promoting incomplete combustion as opposed to complete combustion. (We will talk more about what this means later.)
Mechanically-Supplied Combustion Air
Regarding mechanically supplied combustion air, Section G2407.9.1 states that makeup air must be supplied where an exhaust fan is installed. For example, if you are mechanically providing combustion air to your furnace, makeup air will be required to replace the air an exhaust system removes.
If you have a system bringing in air at a set rate, and this is the only source of combustion air, then having another system in that vicinity that exhausts air from the room means that combustion air may end up being exhausted or at least pulled away from the combustion appliance to balance the depressurization.
As the mechanical air supply is delivered at a fixed rate and cannot respond to the demand for more air, this leaves you at risk of insufficient combustion air in the appliance and all the issues associated therewith.
400+ CFM Range Hoods
The next point has a few different factors that can make it harder to follow, so let me simplify the information in Section M1503.6 of the IRC:
When you have one or more fuel-burning appliances (without direct-vent or mechanical draft venting system) in your home, the installation of a range hood with an exhaust capability of over 400 CFM needs a mechanical makeup air supply rate equal to the exhaust rate.
Direct-vent means the appliance has a sealed combustion system: combustion air is taken directly from the outdoors, and waste products are ejected directly outdoors. If these were in place, the combustion air and process would be protected from the inside air system, negating the need for makeup air.
A mechanical draft venting system is a system that mechanically removes flue gases. This means that there is an air pressure system (since it uses drafts) that removes the waste products, which will keep them out of the house regardless of other factors inside.
Again, this makes makeup air immaterial to the proper functioning of the gas appliance.
200+ CFM Dryers
Dryers that exhaust air at a rate of 200 CFM or more require a supply of makeup air to combat the effects of depressurization, particularly in the presence of gas-burning appliances.
This is to prevent the loss of air from the room, creating situations in which the combustion gases are pulled from the appliance vents and released into the breathing air. It is also to ensure that there is no interference with the supply of combustion air.
This applies to all vented dryers, not just gas-burning ones.
Ventless dryers do not need makeup air because they do not continually remove air from the room. Instead, they pull air in once and cool and reheat this same air throughout the dryer cycle.
Gas-Burning Appliances Need Combustion Air
So, gas appliances do not need makeup air. What they do need is combustion air. This is also an air supply but serves a different purpose than makeup air.
Combustion air is essential to any fuel-burning appliance. This is because these machines need oxygen to complete the chemical reaction of the combustion process that results in the ignition of the fuel (gas, liquid, or solid) and the production of thermal energy.
Complete combustion, which is the goal, occurs when there is a sufficient amount of oxygen to produce the most heat possible from the reaction (meaning it is efficient) and to produce carbon dioxide as waste.
If you do not have sufficient combustion air (whether there isn’t enough or a negative pressure system is diverting it), there is not enough oxygen. This results in incomplete combustion. So, combustion still takes place, heat is still produced, and a carbon byproduct is still released.
However, the amount of heat generated by incomplete combustion is low compared to that of complete combustion. In addition, the carbon byproduct is a different form. Instead of carbon dioxide, incomplete combustion produces carbon monoxide.
Carbon monoxide is not a desirable byproduct since it can be so dangerous in your home. It is flammable and prone to explosions and displaces oxygen in your breathing air.
On top of this, your appliance’s efficiency is lower. This translates to longer run times, increasing your machine’s workload and decreasing its lifespan.
Signs That Makeup Air Is Required
So, what if you already have a gas appliance in the same space as an exhaust system?
Well, there are some signs to watch out for that indicate that makeup air is required and to ensure that a combustion air supply is serving its purpose and the byproducts are being vented from your home safely.
If you smell gas in the same room as the appliance or notice your appliances aren’t as effective as they should be, you might need makeup air.
The gas smell comes from backdrafting.
If your clothes are coming out of the dryer damp or the furnace/heater is not warming the house/room, this can be because incomplete combustion produces less energy, meaning that there is less heat for the appliance to use.
There are also 5 other signs elsewhere in the house that indicate a makeup air problem (due to the negative air pressure system):
- Smoke, grease, and steam problems in kitchens and bathrooms as a result of the exhaust systems performing poorly.
- Widespread and unexplained drafts through the house.
- Noisy, gurgling, and backflowing drains indicate that air is being pulled into the house through the drains.
- Pulling air from drains, toilets, exhaust systems, chimneys, and vents can result in bad smells in the house.
- Higher utility bills from the longer running times of appliances that are made less efficient.