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Old 07-27-2011, 11:55 AM   #1
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Venting a new furnace?


I need to vent a new furnace from my basement to the roof of a two story house. Its a power vented furnace but is not a condensing furnace. The house is old and has an 8 inch inside diameter vent pipe which currently runs from just above the basement ceiling up through the roof. The pipe is a cementitous/asbestos material with thick walls and is in good condition with no blockage. Can I just run B vent from the furnace to the just inside the lower opening in the large pipe as opposed to running the B vent completely through to the roof (inside the large pipe or not)? The pipe bottom would be sealed shut with a suitable material. I could probably fabricate a sheet metal transition from the B vent to the pipe and seal any remaining openings that might leak with a fire rated foam or other appropriate sealent. What about venting a second appliance into the same pipe? Either a second power vented tankless non-condensing water heater or my existing natural draft tank water heater? Being able to do so would reduce venting costs and complexity considerably especially for a tankless water heater which used very expensive 3/5 double wall stainless material from Metal Fab Inc. Is any of the above allowed? I am aware of the issues with asbestos but except for the lower access to the pipe it is sealed inside walls and is not accessible except in the attic and I intend to wrap that to seal it soon. Thanks.

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Old 07-27-2011, 05:02 PM   #2
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Venting a new furnace?


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Originally Posted by Mike in Arkansas View Post
I need to vent a new furnace from my basement to the roof of a two story house. It’s a power vented furnace but is not a condensing furnace. The house is old and has an 8 inch inside diameter vent pipe which currently runs from just above the basement ceiling up through the roof. The pipe is a cementitous/asbestos material with thick walls and is in good condition with no blockage. Can I just run B vent from the furnace to the just inside the lower opening in the large pipe as opposed to running the B vent completely through to the roof (inside the large pipe or not)?NO. Code requires the B vent to be sized for the BTUs of the appliances to be used plus the height/rise of the chimney and MUST exit above the roof line according to pitch of roof and code. Fail to do so and you could get pressure switch tripping problems/flame rollout/downdrafts/fire/CO poisoning/sued by the next owner if something goes wrong etc etc/voided house ins. The pipe bottom would be sealed shut with a suitable material. I could probably fabricate a sheet metal transition from the B vent to the pipe and seal any remaining openings that might leak with a fire rated foam or other appropriate sealent. What about venting a second appliance into the same pipe? Either a second power vented tankless non-condensing water heater Probably not as they use a VERY specific type and size of venting and method and are standalone units or my existing natural draft tank water heaterYES if you add the BTUs to the furnace and size it properly? Being able to do so would reduce venting costs and complexity considerably especially for a tankless water heater which used very expensive 3”/5” double wall stainless material from Metal Fab Inc. Is any of the above allowed? I am aware of the issues with asbestos but except for the lower access to the pipe it is sealed inside walls and is not accessible except in the attic and I intend to wrap that to seal it soon. Thanks.


How old is the furnace? if it is on it's last legs you may want to buy a high efficiency condensing type, abandon the chimney and vent it and a power vent water heater thru a side wall. Some people like to reclaim the floor space/real estate by removing the old chimney if it runs thru the house. makes Momma happy with more closet space or a nice chase to run ducts/wires thru.

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Last edited by yuri; 07-27-2011 at 05:11 PM.
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Old 07-27-2011, 07:47 PM   #3
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How old is the furnace? if it is on it's last legs you may want to buy a high efficiency condensing type, abandon the chimney and vent it and a power vent water heater thru a side wall. Some people like to reclaim the floor space/real estate by removing the old chimney if it runs thru the house. makes Momma happy with more closet space or a nice chase to run ducts/wires thru.
Thanks Yuri. It is a new furnace. It is 80 percent efficiency and it is a new installation also. That's why I wanted to use the existing vent and not have to run new stuff all the way up stairs into the attic. Just a lot simpler and cheaper if allowable. it seems to me there shouldn't be a problem as the vents would be terminated at atmospheric pressure although at the bottom of the chimney. But like you point out that would not exit above the roof line although the chimney itself does.

Last edited by Mike in Arkansas; 07-27-2011 at 07:50 PM.
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Old 07-27-2011, 08:03 PM   #4
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Venting a new furnace?


What size furnace are we talking about venting? My guess is that the 8" vent would be way to big.
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Old 07-27-2011, 08:10 PM   #5
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Absolutely not!!!! You cannot do what you want to do.Its not only against any code I ever heard about,but its against code because its a serious saftey issue.
You can use the " exhisting" chimney as a chase where you could run properly sized B vent up through it.You would still need to seal off the top and the bottom of the chase.
You can use a natural vent water heater and your new furnace in a single B vent if sized correctly and most places the code is b vent starting from the water heater and furnace draft hood to the single b vent riser.
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Old 07-27-2011, 08:32 PM   #6
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Thanks Yuri. It is a new furnace. It is 80 percent efficiency and it is a new installation also. That's why I wanted to use the existing vent and not have to run new stuff all the way up stairs into the attic. Just a lot simpler and cheaper if allowable. it seems to me there shouldn't be a problem as the vents would be terminated at atmospheric pressure although at the bottom of the chimney the problem is the chimney stack effect.hot air rises and cold air settles. on a normal old school draft diverter open type furnace you have a continuos natural draft as the heat rises, that can fail when the barometric pressure changes or you get negative pressures in the house from exhaust fans etc but most of the time you have an updraft. power vented furnaces cannot handle that quantity of COLD dense/heavy air in the off cycle to get a draft going when they start so they trip out on pressure switches and have flame rollout/rollback and other serious problems. NO way around it. But like you point out that would not exit above the roof line although the chimney itself does.

Return the furnace/sell it on Craigslist and buy a Goodman high efficiency online and install it yourself if you are capable. May need to be a licensed gas fitter and take out a permit in some States and all of Canada. Need to do a LOT of research and read an install manual for a high efficiency furnace as venting can be tricky/where it vents etc etc. That is why we have Pros doing the work. DIY is fine but later you can have warranty problems and no help to make it work properly if the venting is not sloped properly or the burner setup properly etc.
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Last edited by yuri; 07-27-2011 at 08:51 PM.
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Old 07-27-2011, 10:40 PM   #7
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Venting a new furnace?


Thanks to all for your help. Look like I'll be looking for another venting option

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