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Make-up Air | When Is It Required for a Furnace

I am cautious when it comes to fuel-burning appliances. There are strict regulations for all of them, but it can also get confusing if you don’t work with all the different systems frequently.

So, if your gas stove has a makeup air supply, does that mean that your furnace needs it too? Well, it may help get you started on the right track to find out that your gas stove does not need makeup air. It’s the range hood that necessitates the new supply of air. Now, let’s dive a little deeper!

Makeup air isn’t required for furnaces. The air furnaces take in and exhaust is deliberately supplied (combustion air). Makeup air is for systems that exhaust a significant amount of the general air from a room. If an exhaust system is in the furnace room, makeup air may be needed to prevent backdrafting.

Makeup Air Will Never Be Required for a Furnace

Makeup air is associated with exhaust systems, not combustion systems (like your furnace). While both exhaust systems and combustion systems remove air from a room, the key difference, and the reason why furnaces don’t need makeup air, lies in where this air comes from.

With a range hood (an exhaust system), the exhausted air starts as ambient kitchen air, so the loss of this air will lower the air volume and pressure in the kitchen, causing all manner of issues. These issues are addressed through the addition of makeup air.

With a furnace, the exhausted air starts as combustion air, which is deliberately supplied to the furnace, so the ambient room air is left relatively unchanged in terms of volume and pressure. As such, you do not end up with the same issues caused by exhaust systems, and makeup air is not necessary.

Furnaces Need Combustion Air

As we just touched on, furnaces do require the supply of air—just not makeup air.

Because they are fuel-based, the International Residential Code (IRC) requires the provision of a combustion air supply for a furnace, regardless of what gas or fuel is used to power the appliance.

Additionally, there are several types of furnaces listed in the IRC, and all of them need combustion air.

Here’s where you can find the requirements for the various furnaces:

Woman opening floor heater vent

Adhering to these regulations will allow your furnace to operate as efficiently and safely as possible.

What Is Combustion Air?

While both are added to the home, makeup and combustion air differ.

Combustion air is necessary for fuel-burning appliances. Air must be present for combustion to take place, meaning that air is needed for the gas to ignite and produce heat. A chemical reaction takes place during combustion, and this reaction requires oxygen.

This air must be provided specifically for the purpose of igniting the fuel, and must not be taken from your breathing air.

Why Furnaces Need Combustion Air

As I said, for your furnace to light and produce heat, it needs the oxygen supplied by combustion air.

In addition, if there is insufficient oxygen present during combustion, you can end up with incomplete combustion.

With complete combustion, the byproducts are water, carbon dioxide, and heat. Incomplete combustion yields water, carbon monoxide, and a lesser amount of heat.

So, incomplete combustion is not as efficient, but it also releases a toxic and potentially fatal gas.

Exhaust Systems Near Furnaces Need Makeup Air

Now, just because furnaces don’t need makeup air does not mean that it is not important when it comes to the functioning of the furnace. We can see this in Section G2407.4 of the IRC:

Old style kitchen with 2 pendant lights on the top of the island, make up air and stove exhaust

“Where exhaust fans, clothes dryers and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.”

What this means is that an exhaust system has the ability to interfere with a furnace’s function, but this cannot be permitted.

In order to remedy the situation, the room containing—or even connected to the room containing—the exhaust system is provided with a makeup air so that it isn’t interfering with the supply and exhaust of combustion air.

How Exhaust Systems Can Interfere With Furnaces

If you think about it, an exhaust system (which removes air from a space) is actually working against the furnace, which needs air to function properly and safely.

If there is a pressure imbalance around the furnace due to the exhaust system, then the supplied combustion air may be “taken” and used to equalize the pressure imbalance in the room instead of fuelling complete combustion.

The ability of an exhaust system to interfere with this will depend on how the furnace is supplied with combustion air.

Additionally, when depressurization occurs around a furnace, then all those unpleasant and even dangerous gases produced during combustion are sucked out of the furnace exhaust in a process called backdrafting.


The Effects of Makeup Air

Makeup air will maintain the air pressure system in the space containing the furnace. If you are replacing the air that is being removed using a makeup air unit or other IRC-approved sources, the furnace’s combustion air doesn’t have to do it.

Combustion air will likely be unable to compensate for the amount of exhausted air since it is intended for a combustion process and not to supply air to a bigger space. Besides, the amount of combustion air available is the amount needed for only the furnace.

Providing makeup air will prevent compromising the combustion process, and, thus, avoid the formation of dangerous byproducts. It will also avoid backdrafting in your furnace.


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