Combustion Air Duct Connected to Return

I’ve seen combustion air ducts connected directly to return plenums on conventional forced air furnaces many times. Just curious of other’s views on this installation. I suggest to clients to drop this duct to the basement floor to also supply combustion air for the other gas fired appliances in the house. It seams this would also be not be an energy efficient installation considering this duct is pulling in cool outside air (I’m in MN) rather than recycling indoor air.

I see this installation enough to wonder why it’s so common and the logic behind it.


Jeff Manders
Gadget Home Inspections

Negative pressure can cause backdrafting and prolonged spillage from
fireplaces, gas-fired water heaters, furnaces, boilers, or any other device
that uses house air for combustion. It can also cause flame roll-out from
the bottom of residential water heaters and increased carbon monoxide
production in both water heaters and furnaces.


Very common and usually not wrong.

The cool outside air gets heated eventually either way.

I agree that the air would be heated eventually.

Would this installation lend to more cool/cold air be heated vs. a duct exhausting near the floor?

Would this impact pressure within house? Possible positive pressure from the direct connection?

I have no scientific facts to support these theories. Just curious.

I tell clients one way or the other the combustion air will come from outside.

Either through every bypass in the building envelope or when you enter the home and the door knob gets sucked out of your had because of negative pressure.

I prefer the outside vent :wink:

Your using the wrong term in your explanation. The duct from the exterior into the return air plenum basically has nothing to do with combustion air as they are two different sources. Just because outside air is being introduced into the return air duct does not mean that there is sufficient air for combustion of the burner gas. You need to think make-up air for the total home and secondly combustion air for the burner.

Agree with Charlie, up here we call it ‘fresh air’ and ‘combustion air’ and code requires both for furnaces with chimneys (not sold in Canada since 2010), newer homes with direct vented furnaces have only a fresh air intake which goes into the return air duct somewhere, there is also a ‘whole house ventilation’ ventilation switch (code required) which operates at minimum a centrally located exhaust fan and furnace fan. Better homes have an HRV which does the same thing, exchange inside air for outside air.

There was a time in my installing days when the local building inspectors allowed one external air intake for combustion and fresh air, and we would make a small hole in the furnace plenum and cover the hole with a 3" top take off, which I always thought was pretty mickey mouse, but do what the boss says. It did what it was supposed to do, supply air at the furnace for combustion, even if the furnace was enclosed in a furnace room.

Your probably seeing a direct vent /closed system where newer high effeiciency furnaces have these. Normally 2 pvc like pipes about 6" diameter.

There would still be interior air return to cycle the already heated/cool air throughout the house.