Quick B-vent question

[CapnCrunch]

Does anyone know if you can use a larger diameter B-vent than required by a manufacturer?

Eg. A garage heater requires a 4" diameter B-vent but you use a 6" diameter instead.

I'm assuming going lower is a big no no but does going bigger causer any issues?

I'm asking because I'm replacing my old heater and there is currently a 6" hole in the roof already that would be really handy to re-use.

The vent will come out about 1 foot horizontally from the heater, then 90 degree straight up through the roof about another 4-6 ft.

[Ca_Silvia13]

You need to maintain the air velocity within that pipe so the combustion gases do not "stall" within the venting. Sometimes bigger is not better. If your heater is newer you should be able to bring up the manual on it online. Only the manufacturer would know for sure if 4" or 6" venting can be used. Otherwise you're rolling the dice with having CO fill your garage.

[blitz]

I think you could run a 4" inside the 6" pipe to maintain velocity.

[Maxt]

Originally posted by blitz
I think you could run a 4" inside the 6" pipe to maintain velocity.
Yeah, common practice when doing retrofits. On a fan powered heater, the biggest problem isn't Flue gas spillage, but rather the water condensing out of the gases and rotting everything out if the vent is oversized.