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Correct size flue for Furnace and water Heater?

Posted by mundo (My Page) on
Fri, Sep 24, 10 at 12:48


I have a Gas furnace and a Gas water heater that each have a 4" flue pipe from them. These two are then connected into a single 8" pipe that goes up through my chimney. I was told that 8" is too large and it has probably been there for years and no one bothered chnaging it when they installed the new furnace.
Can anyone confirm what size flue I should use to cater for two 4" pipes?

Follow-Up Postings:

RE: Correct size flue for Furnace and water Heater?

Look in the flue sizing tables in the gas code. Depends on your total BTU output.

RE: Correct size flue for Furnace and water Heater?

And since your furnace is 4" it sounds like a fan assist so you should be looking at the NAT + Fan total BTU chart with correct vent connector length and chimney height.

Heres a link why there needed and sizing charts, assuming its gas appliances

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