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Cobra1964 12-03-2011 11:41 AM

Lining 6" metal flue with 4" Class B vent
I have an 80% gas furnace with a 4" exhaust attached to a 40 year old 6" double wall gas flue approximately 15 feet straight from basement to cap. It runs in a chase up the center of the house. A 4 " pipe runs approx. 5 feet from the furnace to the bottom of the flue. I have condensation running down the flue interior and from the reducer joint. The flue is clear. I could seal the joint, but then the water just ends up in the heat exchanger I would assume.
I am guessing the velocity of the gasses slows enough entering the 6" pipe to allow the condensation to form + interior surface area is too large to get warm enough to prevent it.
Can I ("within code") slide a 4" class b pipe inside the existing pipe? It will fit, just don't know if it is legal AND should that resolve the issue? OR is the only solution a condensation drain?
Thank you!!

yuri 12-03-2011 01:39 PM

It is legal. Our Pro chimney company slides smaller liners into 6 inch single wall liners all the time. Should have 4 inch B vent from the furnace to the chimney also. Loses a LOT of its heat on the way to the chimney. without it the moisture will run back into your furnace and SERIOUSLY damage it.

REP 12-04-2011 02:14 AM

Using the old chimney as a chase is perfectly allright unless your hot water heater uses that old chimney.

yuri 12-04-2011 09:16 AM

depending on the combined BTUs of furnace and H2o heater 4" may be big enough to do both and be OK

Cobra1964 12-05-2011 10:15 PM

Appreciate the input very much. Looks like a project for a weekend.


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