Thread: Venting gas furnace in attic

I need to vent a gas furnace 83% non condensing through pitched roof. Furnace connection is 4" dia with draft inducer motor. Furnace is located in attic with connection 30" below and 18" offset from roof peak. A) Must I use B vent? B) How high should the penetration through roof be" C) Is B vent rain cap suitable? Appreciate any input. Thank You Brian

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B-vent though the roof with termination 1 foot above the peak.

Originally Posted by David Bell

B-vent though the roof with termination 1 foot above the peak.

Is it allowed to use B vent with a draft inducer motor? I thought B vent was not allowed to be used with a pressurized venting system, and doesn't the draft inducer motor pressurize the venting system to some extent?

I'm asking because I do not know, I have not seen that done before (mostly electric around here).

The venting of an 83% draft induced furnace is typically done with single wall pipe to a b-vent thru the roof. No different than natural draft.

Use B-Vent with listed B-Vent cap. The termination height depends on the roof pitch, and is at least 12" and increases as roof pitch increases. The B-Vent is acceptable because Category I fan-assisted appliances deliver flue products to the vent with neutral or negative pressure. Check apliance installation manual for other requirements, especially minimum vent height since the outlet is fairly close to the roof.

An 83% fan assisted furnace is CAT I, which is approved for B-vent. In fact, since it is an attic, you must use B-vent. However, you also need to refer to the appliance's listed instructions for minimum vent height, which usually starts at 5 ft. If the appliance collar is only 30" under the roof deck, you'll need to extend the vent above the roof at least 18" plus any requirement for vent height per the roof pitch.

if the vent extends 5ft or more above the roof, you would need roof brackets to stabilize and support it. You also would need to figure how you are going to support and stabilize the vent so it doesn't wobble around.

Not my best hour of the day...or rather night (!); but this is a gas furnace, in the attic, right?

I'm thinking there is also prescribed rule regarding minimum developed height/rise that also requires a minimum of 10 feet from the bottom of the furnace appliance to the vent outlet? (not just a five foot rule?) or more as the manufacturer's instructions and listing may require further.

I'm with Bob H. that, you may not use a single wall vent connector to an appliance's flue collar or draft hood, which is located in the attic.

What's the make up, combustion & dilution air source, is the attic an open one? is it conditioned? finished?

I might have missed something. Up late and detoured as the bucket of halloween candy called out to me as I was heading back to bed.

Hmmm...there's some pretty informed inspectors here, but this is an inspection forum. You may want to try a gas fitter qualified HVAC contractor.