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

  • 8
    Come on. Is this a joke post?
    – jay613
    Commented Sep 9, 2021 at 14:41
  • 13
    Take pictures and contact your state licensing board. And don't let this person in your house EVER again, not even to "correct" things. Get someone competent to do it.
    – Doktor J
    Commented Sep 9, 2021 at 15:04
  • 7
    And make sure the "contractor" pays the costs - a lawsuit would be a turkey shoot with this degree of shocking negligence. You'll need a real chimney installed and that will add to the costs. Why are you buying an 80% furnace in 2021 anyway? The cost of installing the chimney will probably eat up the difference between a 95% furnace (which you could use PVC for), and you'll end up with lower heating bills and a lower carbon footprint.
    – J...
    Commented Sep 9, 2021 at 16:50
  • For anyone that hasn't looked into newer high-efficiency furnaces, they have additional heat exchangers and actually do vent their exhaust gas via 2-3" PVC pipe, but the air temperature tops out around 120F and is perfectly safe. As was mentioned in the post, older style furnaces vent the hot exhaust directly without cooling, and it's easily hot enough to melt plastic.
    – JPhi1618
    Commented Sep 10, 2021 at 20:59

2 Answers 2



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.)

  • 5
    pvc-NO..1. A reducer is a no-no. Creates back pressure and is not allowed 2. You cannot run the exhaust horizontal it must slope upwards.( quarter-inch per foot) 3.Safety Controls i) A fan proving switch that prevents the heater from operating if the power vent used to operate the side wall vent is not working ii) A timer control that keeps the power vent operating for a period after the burner has stopped, typically for 45 seconds, to purge remaining combustion gases from the heating appliance NB Safety controls and power vents for gas fired heating appliances are certified by the AGA,
    – Brad
    Commented Sep 9, 2021 at 11:02
  • 2
    @Dimi -- I'd be seriously considering ditching gas heat altogether, especially if you can tighten up the house (air seal/insulate/weatherize) while you're at it Commented Sep 9, 2021 at 11:33
  • 6
    If you are unable to install a traditional metal flue in the way it's supposed to be installed, if you HAVE to use long horizontal runs or PVC, then you HAVE to install a suitable boiler, not the one you have. You should get rid of this contractor. He should be in jail. "It melted because it's summer" really sounds like something from a Monty Python plumber. I'm hoping to hear this is all a joke. But if not, don't let this person back in your house. (I have seen power vented high temp flues, but they are rare and you really have to know what you're doing. And they are not plastic.)
    – jay613
    Commented Sep 9, 2021 at 14:46
  • 13
    Update: Not sure if one of the agencies that I reported him to reached out to him, or if one of his 'colleagues' was an actual certified HVAC technician, but contractor completely changed his tune and went from "I already did all the work. Contract is done. I cannot return unit. If you want new one you have to pay me for new unit and full re-installation costs" to "Just pay me the difference for high efficiency furnace and one that you have, I will replace it for free.". Thanks everyone for giving me courage and information I needed. Y'all might have just saved my life. Will post updates.
    – Dimi
    Commented Sep 9, 2021 at 19:54
  • 4
    @JacobKrall I have hired a local HVAC professional to advise me on this matter. He said that the best course of action for my wallet is to have that 'contractor' replace 80% unit with 96% one (I will only pay the difference in cost of those units) and have an HVAC/Code professional thoroughly examine his work to make sure everything is good. Furnace is being replaced right now. Gas valve will remain in OFF position until all work is inspected by a 3rd party.
    – Dimi
    Commented Sep 10, 2021 at 17:05

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.

  • 3
    We powered furnace on to make sure it is working. Flame was going out after 15 seconds and entire house was smelling like gas. Fun all around. While it is still summer, temperatures do get somewhat cold at night and require occasional 15-20 minute bursts of heating to keep temperature comfortable.
    – Dimi
    Commented Sep 9, 2021 at 13:03
  • 8
    @Dimi Wow, I'm sorry that you're in this fiasco. That contractor does not sound qualified to install furnaces and it could kill your family in the middle of winter. I suggest getting a real HVAC contractor but I do understand that money might be tight.
    – MonkeyZeus
    Commented Sep 9, 2021 at 13:15
  • 1
    @MonkeyZeus The lawsuit against PVC guy might pay for it. Commented Sep 9, 2021 at 15:06
  • @chrylis-cautiouslyoptimistic- lawsuit will cost more than cost of current 'ticking timebomb' and new HVAC system together.
    – Dimi
    Commented Sep 9, 2021 at 15:12
  • @chrylis-cautiouslyoptimistic- Unless you're a lawyer or can get one to work pro-bono for you then there isn't much of a case here. No one was hurt and there is still time to do things right. If OP lets the install proceed with PVC at this point then this question can be used against them in court to counter-sue with a fraud case.
    – MonkeyZeus
    Commented Sep 9, 2021 at 15:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.