Estimated reading time: 3 minutes, 52 seconds

Air for Your Gas Appliances

By May 28, 2018 Appliances

Estimated reading time: 3 minutes, 52 seconds

The fire triangle

There are two forms of air that are needed to make gas appliances work properly. Without them things go haywire and it could kill you and your family. There is makeup air and there is combustion air. Although they both do the same thing they are though of in different ways. First off, fire needs the triangle to work. Then the products of combustion need to leave the home through a flu pipe. (High performance appliances are now extracting so much heat out of the gas that they can be vented through PVC pipe)

Combustion air: The air needed to complete the fire tangle and allow the fire to burn

Makeup air: This is the air that allows the hot exhaust gasses to leave your home through the exhaust pipe.

Simple experiments

  1. If you live on a sealed up home and try to light a log fire in the fireplace and no doors or windows are opened it will quickly go out. Open a window and watch it jump to life. This is because you are starving the fire of combustion air and likely makeup air too if your house smells smoky.
  2. Close up your home and turn on the exhaust fan over the stove. Walk into the living room and smell the chimney exhaust filling your living room. This is because the range hood is using the least resistance flu to get the exhaust air. Open a window near the hood and it should stop.


example of a Carbon Monoxide alarm

Imagine a furnace in the center of your home with the chimney and everything hooked up properly. If there is no air allowed into the room then the lame may go our for lack of combustion air. Now put a small hole in the door and allow just enough air to let the fire burn but not enough to let the exhaust out the flu. I have seen soot all over a water heater top because it could not get out of the normal path.

The build up of products of combustion including CO (Carbon Monoxide) is deadly. Now allow a free flow of air from the outside into the furnace room and everything works properly. Please have at least one of these in your home. I have seen what happens when you don’t.

Contaminant danger

The only concern we have come across is when the furnace room is given makeup air from an attic which should be sealed from the home or the crawl space which should also be sealed from the living space. Both of these spaces are ripe for allowing bad things into the home be it mold, fiberglass, radon, the list goes on.

Makeup air and the Code

Current code calls for makeup air to be introduced mechanically or naturally through your HVAC system in the event your exhaust hood is more than 400 CFM.

Consult your local HVAC contractor if you feel that you home is not breathing properly. They have ways of testing if your home is balanced. I have used a primitive test fro years in the restaurant business where makeup air is essential due to swamp coolers used in the kitchen and air conditioning in the dining area.

The primitive pressurization test

Tear a section if toilet paper or a napkin into a 1/4 inch strip. HINT: (Most paper has a grain so it tears one direction easily and the other not so much) with nothing on, hold the paper dangling in the crack of the door as you open it. If the strip of paper blows to the outside your home is pressurized, if is sucks in it is depressurized and is causing a vacuum. if it hangs limp you are balanced.

Now try it with different fans on in the home. Each fan is designed to pull a certain number of CFM (Cubic Feet Per Minute). If you are in a depressurized state, they will not do their job correctly or steal air from the darnedest places.. Simple as that.

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