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Combustion air

ICE

ICE

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Joined
Jun 23, 2011
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Location
California
The white pipe closest to the camera is the combustion air intake and the pipe behind is the vent. Several furnace makers installation instructions require that the vent and combustion air intake terminate in the same atmospheric pressure zone. My take on that is they must both be outside located on the same roof plane. If the wind is blowing, the pressure can be different outside than it is inside and different on different roof planes.

Contractors say that no matter what, we are in the same pressure zone. Check the weather channel. They don't have two numbers for you, one for inside and one for outside. I tell them that the maker wasn't referring to the difference between Long Beach and Denver and they wouldn't mention it if it wasn't possible to get it wrong.

I have called two manufacturers and both times the technical advisor had no clue. They both took my phone # and didn't call me back.

Does anybody have an answer?

DSCN0453.jpg
 
You're right. There can be a significant difference when the wind is blowing. I think there is a maximum distance of a foot ot so between intake and exhaust. This looks like they're a lot farther apart than that.
 
Paul is right. Same pressure zone=same side of house. In fact, there are only a couple models of furnace that I know of that allow combustion air pipe to terminate in the attic at all. There are also a couple that allow the combustion air and vent pipes to terminate out different sides of the house, but both situations are specifically addressed in their manuals. It is possible to measure pressures on opposite sides of a building. There will be a pressure difference on opposite sides of a house if the wind is blowing. A perfect illustration of this is a south dakota blizzard. The ground on one side of the house is almost blown clean, and the opposite side will have a 10' rock hard snowdrift.
 
Last edited by a moderator:
Due to stack effect and other forces the pressure in an attic can be several pascals higher than outside. Consequently the basement would be equally as low as i=the attic is high. 5 pa in the attic and a -5 in the basement relative to the outside.
 
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