Furnace Combustion
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Furnace Combustion
Combustion Air for Wood Furnaces
COMBUSTION AIR FOR WOOD FURNACES
Combustion air means outdoors air that is brought into the furnace room. A common question for folks installing wood furnaces is, “why do I need it?
A wood furnace requires what is called a Class A chimney. This could be a brick chimney or a HT (high temperature) insulated Stainless Steel chimney. Wood stoves, wood heaters and wood furnaces require from a 6” to 8” chimney, depending on the size of the unit. When you start a wood fire, the smoke and flue gases vent out through this chimney. As the fire gets hotter, more air is used to vent the unit.
Where does this air come from that is going out the chimney? The chimney requires what is referred to as make-up air or combustion air.
Some folks may say, I don’t need this extra supply of air because my basement is leaky under or around the doors. It does not address the need for a wood furnace or even a conventional gas or oil furnace. Other reasons you need outside air are to replace the air removed from the home by gas or oil water heaters, bathroom or kitchen fans, cloths dryers.
If you don’t have outside air installed, you will often notice cold drafts around your windows and doors. Another thing you will notice is that when you open a door to the outside, cold air will rush in. That’s because the chimney requires air to work, causing a vacuum in the home.
The proper and most effective way to deal with this requirement is to install a combustion air intake similar to installing a clothes dryer vent. The out of doors air should be taken in about 6-7 feet above the basement floor. A vent cap similar to a clothes dryer vent can be used. It should be at least 5 inches in diameter, but can be as large as 8 inch. A metal or plastic pipe should be connected to the cap. It should end about one foot above the basement floor and as close to the furnace as possible but at least in the same room. Only enough air will come into the home as what the chimney is taking out. Some folks put a 180-degree elbow or a 5-gallon pail at the bottom of the intake to dispel any draft.
Another acceptable method of make-up air is to install the vent from the roof or attic to one foot above the floor the same as above. The clothes dryer style is the easiest and least expensive to install.
If you burn wood and follow the above suggestion, you can expect less creosote buildup in your wood heating appliance and chimney. Your wood fire will also burn cleaner.
OTHER CONSIDERATIONS
*Air-to-Air Heat Exchanger. This is a device that exchanges outside air with inside air. They have become popular over the last several years, especially in new homes with thick walls and heavy insulation. These homes don’t breathe. In other words, outside air does not naturally penetrate through your walls and windows. We recommend these and in fact are required in some states to avoid oxygen depletion in the living area.
They do not take the place of combustion air for either a wood, gas or oil furnace that have a conventional chimney.
If you have doubts on the above theory, consider the following: Years ago, folks had an open top trash burning barrel, usually a used 55-gallon drum. Unless you cut some holes for air near the bottom of the barrel, the trash would not burn or if it did it would be very Smokey. The reason is there is not enough oxygen at the bottom of the barrel for proper combustion for whatever you are burning.
If your outdoor charcoal grille has an opening in the bottom, the fire will get hot faster because of a fresh oxygen supply at or under the flame level.
If there is a fire in your home, it is always recommended that you get as close to the floor as possible until help arrives. That’s because the air closest to the floor has the most oxygen. The same theory holds true for wood stoves and furnaces.
About the Author
David Tjosvold is the President of Alpha American Co. Manufacturers of Yukon-Eagle Multi-fuel and wood furnaces. David started out with his first furnace distributing company in 1965. In 1976 he developed the Yukon multi-fuel warm air furnaces that were to become the first UL Listed wood/coal and multi-fuel warm air furnaces on the market and still are today. Check out the products at Wood Furnace Company
Secondary Air Adequacy for Furnace?
My home inspector just got done with his inspection on my pending house purchase. He mentioned that the secondary air for combustion and flue draft appears to be inadequate. He said that not having enough secondary air can cause carbon monoxide buildup. He suggested something about putting in two vents, one 12" from the floor and one 12" from the celing. I'm wondering if I just have to add these vents to a wall that goes around the furnace room or if it is something more than that?
Vents throught the wall will be fine. It may be easier to install them in the door, or replace the door with a louvered door. Either way, not a real big problem to fix, but it does need to be done.
Furnace Closet Combustion Air Demonstration