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I am currently buying a house with a 80% efficiency furnace that wasn't properly vented. When it was installed they didn't put a liner in the chimney. To solve the problem the sellers have aggreed to fix it. They want to install pvc pipe and vent it out the basement. Is this standard code and will it poss a safetly hazard
 

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You can NOT vent a 80% furnace with pvc. The flue gasses are much too hot and will melt the pipe and possibly catch the place on fire. They'll either need to spring for a liner installed to keep the 80% or change the furnace to a 90% and vent with pvc.
 

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Can't vent an 80% with PVC piping. has to be 90% plus for that. They could put a liner in the existing chimney and keep the 80%.

What ever they do-make sure it is inspected by an authorized person when completed.
 

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I'm Your Huckleberry
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From what I understand 80% gas furnace vent pipe must be dual wall metal flue pipe. It has to do with the temperature of the exhausting fumes. Only on high efficieney (something like 95%) can pvc legally be used. I might be wrong but I think I'm right. We install a lot of 80% and it's always 4" double wall Metal Fab vent, like below.

 

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I'm Your Huckleberry
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It's unanimous, I was right.

The less efficient a gas furnace is (80 being less than 95) the hotter the exhaust fumes.
 

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Wire Chewer
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I would try to talk them into just putting a 90%+ high efficiency furnace that takes 2 pvc pipes (intake and exhaust). Considering the cost of adding the proper liner, may as well pay a bit more and have a more efficient furnace.
 

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I'm Your Huckleberry
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Ask about roof run and rise (pitch) which determines the height above roof the metal vent pipe must achieve to co-incide with the btu of your furnace which also determines the size in diameter of the flue pipe needed.

An 80% furnace with a draft induced exhaust does not pressurize the exhaust through the vent pipe. From the time the gas makes it past the draft inducer motor it is natural draft meaning hot air rises on it's own so the angle of the vent pipe, length and heigth, all needs to be determined so the gas can freely escape.

If the angle is not correct, well, all gas products burned produce moisture and that moisture (and dirt in the air) can accumulate in the pipe and trickle back down and eventually block the pressure switch and then you won't have any heat and you'll be spending money on a service tech to so nip this in the bud correctly the first time.

Not too mention if the gas collects in the exhaust pipe as it isn't allowed to flow freely you can have it travel back and flow into your system. Flame roll out or possible CO poisoning can then occur. Two very harmful, potentially fatal, situations.
 

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From what I understand 80% gas furnace vent pipe must be dual wall metal flue pipe. It has to do with the temperature of the exhausting fumes. Only on high efficieney (something like 95%) can pvc legally be used. I might be wrong but I think I'm right. We install a lot of 80% and it's always 4" double wall Metal Fab vent, like below.

I see a code violation in that photo: flex gas line is not allowed to go inside the furnace case. :yes:
 

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I'm Your Huckleberry
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I see a code violation in that photo: flex gas line is not allowed to go inside the furnace case. :yes:
Not my install. I just googled image something for presentation of the metal flue pipe.

Not sealed between the coil and furnace in the front and the electrical is kind of hack as well. Drain line looks like a blind guy installed it.
 
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