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tjeieio 02-05-2010 12:44 PM

Gas Furnace Vent Size
I have a 90+ Rheem gas furnace. The contractor came off the unit with a 2" pvc vent pipe, but one foot later upped it to 4" for a 12 foot horizontal run to the outside of my house. I am confused as to why they would go from the furnace's 2" discharge to a 4" pipe. Why couldn't they just run with 2" all the way, since that is what comes off of the unit.

If it is related to the pitch of the pipe, could I keep with 2" if I went directly into the nearby fireplace flue and kept it mostly vertical?

beenthere 02-05-2010 01:26 PM

What size furnace?

Is it a 4" pipe, or is it a 3" ID pipe?

It can not be vented into a chimney.

tjeieio 02-05-2010 01:44 PM

Furnace Vent
I believe it is a 105,000 BTU furnace. You are correct, 3".

hvac benny 02-05-2010 03:04 PM

3" is most likely the right size for a 105mbtu furnace. make sure the pipe increases on a vertical section, not horizontal or it will trap condensate.

tjeieio 02-05-2010 03:19 PM

Yes - the transition is just before the elbow that turns horizontal.

tinmanrob 02-05-2010 03:56 PM

I do some work for a local Rheem dealer.
He vents all his furnaces that way.
I believe it's a Rheem requirement.

beenthere 02-05-2010 04:45 PM


Originally Posted by tjeieio (Post 395265)
I believe it is a 105,000 BTU furnace. You are correct, 3".

Its vented properly then.

Your installer did his due diligence.

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