What size vent do I need?

[mtaccone]

I have a new furnace with 80,000btu input with a 4" vent pipe and a water heater with 32,000 input with a 3" pipe what is the smallest I should use to line my 8x8 broken chimney? Was thinking of running 4" bvent up but not sure if I NEED to go larger. Any help is appreciated.

PS in my old building the previous owner lined the shop chimney with galvinized stove pipe that seems to work fine, other than codes obviously what is wrong with using galvanized?

 

[PaulRicklefs]

I have a new furnace with 80,000btu input with a 4" vent pipe and a water heater with 32,000 input with a 3" pipe what is the smallest I should use to line my 8x8 broken chimney? Was thinking of running 4" bvent up but not sure if I NEED to go larger. Any help is appreciated.

PS in my old building the previous owner lined the shop chimney with galvinized stove pipe that seems to work fine, other than codes obviously what is wrong with using galvanized?

According to my Canadian codebook you are looking at 4" b-vent for the main vent and you can get away with c-vent (single wall) for the vent connectors (appliance to chimney) in most applications. I prefer to run b-vent for the vent connector in all cases to maintain higher vent temps and prevent any acidic condensation in the main vent. (I always run b-vent from appliances to the main vent to cover my ass and it's good practice)

Water heater should be vented above the fan-assisted furnace. Water heater must have a 3" to 4" adapter at appliance and run 4" to main vent.

Make sure there is a significant rise on the vent connector (1-2') before entering the main vent.

Galvanized pipe will corrode excessively with the acidic nature of gas exhaust but will not with oil venting. B-vent has an aluminum inner liner.

Remember the smaller the chimney diameter, the more draft. If you were to run a 5" chimney (vent) you would not have sufficient draft to keep the chimney warm through your water heater.

Also if your appliances are not supplied with sufficient combustion air they will not operate properly especially in a newer tighter home. Usually you provide a minimum 4" duct for fresh air in your utility room.

As usual I strongly suggest a professional gasfitter to perform any work.

 

[mtaccone]

What I have now is the 4" pipe from the furnace running into the 8x8 masonry chimney via the 6" flue thimble with the 3" water heater connected nearest to the flue with a T onto the 4" coming from the furnace the 8x8 flue tile has all gone to hell at the bottom of the chimney. I have looked into aluminum flex liner kits but lowes carries all the 4" bvent that I would need for about $100. since I am lining the wood side of the chimeny for my new stove I figure I might as well line the gas. Currently I have ALOT of draft in the 8x8 masonry. looks like you are saying 4" bvent rundown the chimney will work and have good draft so unless anyone else has any ideas I might as well go for it.

 

[PaulRicklefs]

What I have now is the 4" pipe from the furnace running into the 8x8 masonry chimney via the 6" flue thimble with the 3" water heater connected nearest to the flue with a T onto the 4" coming from the furnace the 8x8 flue tile has all gone to hell at the bottom of the chimney. I have looked into aluminum flex liner kits but lowes carries all the 4" bvent that I would need for about $100. since I am lining the wood side of the chimeny for my new stove I figure I might as well line the gas. Currently I have ALOT of draft in the 8x8 masonry. looks like you are saying 4" bvent rundown the chimney will work and have good draft so unless anyone else has any ideas I might as well go for it.

That sounds about right. Basically you are using a common vent connector which isn't ideal with a fan-assisted and gravity appliance combo but it will do in your situation. Using 4" b-vent as a liner is the best option for your setup as it will hold the heat in your flue gases better than the masonry chimney, besides it's minimum code in most places. Problem is the newer gas appliances don't have as high of a flue temp as the older ones so the gases condense in the masonry chimneys before exiting, becoming acidic and corroding the heck out of them. The b-vent will hold the heat in long enough to let the gases out before they condense.

Also when you line the old chimney make sure you are at least 12" above it.

 

[mtaccone]

Why the 12" above? What do you meen by fan assisted? Thanks for your help so far..