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strix99 05-14-2011 01:27 AM

Combustible air high and low vents
This is as general of a question as I can ask, what is the purpose of having both high and low combustible air supply vents. I've done a lot of searching and haven't seen anything describing why they're required.

Here is my situation the house I'm purchasing had high and low supply vents to the basement before it was finished, in the process of finishing the basement the ducting was removed the vents on the supply side now go nowhere. The inspector noted that there was insufficient combustible air supply and suggested either the high/low vents be reinstalled or a louvered door replacing the door leading to the basement.

Thank you for any assistance

strix99 05-21-2011 09:26 AM

bump bump

Marty1Mc 05-21-2011 10:09 AM

Are you sure one isn't intake and one exhaust? My system when it was installed had two vents, both PVC. But one was for exhaust since a normal flue can't be used.

ETA: My system was a high efficiency furnace.

DannyT 05-21-2011 10:27 AM

i would make the seller definitely fix the problem and put a combustion air vent back in. is the exhaust vent still there? i would inspect the house really well. if some idiot took out the air intake for the furnace they might do anything just to finish the basement.
when you say ducting do you mean metal duct or pvc piping?

Artco 05-21-2011 04:41 PM


strix99 05-21-2011 10:31 PM

Let me make sure I'm being clear. The original high/low vents attached the basement to the living room of the house. I assume that the furnace has it's own air supply intake (again we haven't closed on the home so I don't have regular access to check these things). So I assume the high/low vents would have supplied additional combustible air to the basement for the water heater.

My plan at the moment is to simply punch through the drywall in the basement where the vent is in the living room, this vent will be near the floor of the living room and the ceiling of the basement. So my question is why would this not be adequate and why would high/low vents be required?

Marty S. 05-21-2011 11:02 PM

Perhaps beenthere knows the reason for one high and one low. I know they're required to be within 12" of the ceiling and 12" of the floor but nobody ever explained why two are needed instead of one.

beenthere 05-22-2011 06:23 AM


Originally Posted by Marty S. (Post 652352)
Perhaps beenthere knows the reason for one high and one low. I know they're required to be within 12" of the ceiling and 12" of the floor but nobody ever explained why two are needed instead of one.

Its been a while since I had to remember why they want 2. So I could be wrong.

Its a left over from the old days. When the theory was that the high vent/intake would provide the dilution air for the old furnaces and boilers, and the low vent/intake would provide the air for the actual combustion. It was thought that a low only, would cause an updraft outside of the burners and cause a flame roll back. Of course, it never actually works that way. And 2 opening aren't required with every type of combustion air intake method.

strix99 05-22-2011 06:00 PM

Thank you beenthere very informative.

That is the first reasonable answer I've seen. The only other explanation I saw was that that the vents were there if combustible gases were to accumulate the high vent would allow them to vent and keep from building up if they were warmer than the air in the room and the low vent would do the same if they were cooler. Which has nothing to do with combustible air supply at all.

So I'm going to assume that my approach of a single vent while not technically up to code would be adequate.

beenthere 05-22-2011 07:31 PM

Louvered doors work also.

strix99 05-22-2011 08:28 PM

Right, that is the other option we've been considering, but there are reasons we don't want to go that direction.

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