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Old 02-05-2010, 11:44 AM   #1
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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?

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Old 02-05-2010, 12:26 PM   #2
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Gas Furnace Vent Size


What size furnace?

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

It can not be vented into a chimney.

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Old 02-05-2010, 12:44 PM   #3
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Gas Furnace Vent Size


I believe it is a 105,000 BTU furnace. You are correct, 3".
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Old 02-05-2010, 02:04 PM   #4
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Gas Furnace Vent Size


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.
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Old 02-05-2010, 02:19 PM   #5
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Gas Furnace Vent Size


Yes - the transition is just before the elbow that turns horizontal.
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Old 02-05-2010, 02:56 PM   #6
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Gas Furnace Vent Size


I do some work for a local Rheem dealer.
He vents all his furnaces that way.
I believe it's a Rheem requirement.
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Old 02-05-2010, 03:45 PM   #7
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Gas Furnace Vent Size


Quote:
Originally Posted by tjeieio View Post
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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