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A contractor just finished installation of new HVAC system with a category 1 furnace (model: GMES800805CNAA).

He originally used a reducer to connect 4 inch exhaust to a 2 inch PVC pipe. He claimed that he has done so in the past and it worked well. After we started the new unit, we ran into some issues and eventually ended up with a completely melted PVC pipe. He replaced first few ft of the exhaust with a metal 4 inch pipe, but terminated it with a PVC 4 inch to 2 inch reducer and let it vent through the wall.

His claims that PVC melted because it is summer, and during winter pipes will be completely fine.

Is it safe to use a PVC 4 to 2 inch reducer and PVC pipe venting to the side of my house?

replacing melted pvc: enter image description here

finished product: enter image description here

Dimi
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2 Answers2

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HELL NO

Whichever joker is installing an 80% furnace with a PVC pipe for the exhaust needs their license revoked pronto, because that's a patent CO hazard. Non-condensing appliances operate at flue temperatures well above the safe working temperatures of PVC or any other plastic, and thus need to be vented using a metal B-vent. (Conversely, condensing appliances generate corrosive condensate that will eat ordinary B-vent alive. Stainless steel double-wall can be used as a "universal vent", and is what you're supposed to use on condensing appliances as per UL listings, but the condensing gas appliance manufacturers haven't quite gotten the memo incorporated into their install manuals yet.)

ThreePhaseEel
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Are you sure that contractor is not secretly a hitman? I give him props for creativity, that's for sure!

On a serious note, furnaces with efficiency of less than 90% must be vented via metal pipe above your roof line. The exhaust is hot and extremely gaseous. You don't want the gases creeping into your house through your eaves/soffit nor an open window/door. You also don't want someone standing next to it because it can burn them.

His claims that PVC melted because it is summer, and during winter pipes will be completely fine.

Why are you running the furnace during the summer? Regardless, PVC should not exist anywhere in your furnace's exhaust system, period.

MonkeyZeus
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