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I'm about to replace the furnace in our house. The existing furnace uses a 6" stack through the roof. I know most new furnace installations just use PVC and poke out of the wall at about ground level. Question is, can I reuse the existing stack since it's already there? Or are we better off sealing it up and forgetting about it. I'd rather not go through the wall if possible just because we have a deck right on the other side of the wall where the furnace is located.

  • A couple of years ago when I had a repair on my furnace, my HVAC guy told me he can repairing it for pretty much anything but a cracked heat exchanger and that it likely makes sense to keep making repairs because replacing it means HE, which means new exhaust venting (as you have found out) which means hot water tank probably end up getting replaced at the same time etc. ===> $$$$$ – manassehkatz-Reinstate Monica Mar 27 at 14:43
  • @manassehkatz you don't have to replace the water heater - I kept the original exhaust stack that was shared between the furnace and the heater when I replaced the furnace to a HE model. The non-HE water heater is still using the original exhaust (with the old furnace entry point capped), and the furnace has a new PVC exhaust – Eli Iser Mar 27 at 15:33
  • @EliIser I think my HVAC guy was just warning what might be ahead. He actually was NOT trying to sell me "let's replace it all now" as many others would have done. – manassehkatz-Reinstate Monica Mar 27 at 15:36
  • @EliIser often when removing the furnace the chimney liner (sized for both furnace and water heater) will need to be downsized to accommodate just the water heater. Your HVAC guy didn't do you any favours by leaving the original chimney in place. Too large a diameter means too much draft causing inefficient operation and possibly condensate in the flue leading to a severely corroded chimney. It's pretty common but that doesn't mean it's okay. – Joe Fala Mar 27 at 17:52
  • Luckily (depending on how you look at it), my water heater is electric and was never gas as far as I can tell. So no combined stack issues for me. – Dave Mar 28 at 17:17
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The problem with reusing non-HE (High Efficiency) exhaust for a HE appliance (furnace or water heater) is that HE exhaust gases are much more acidic and humid than non-HE exhaust.

This means that for typical galvanized steel exhaust, a HE appliance will cause corrosion in short order, eventually perforating the stack, causing leakage of exhaust into the house.

The use of PVC in HE appliances is simply cost - PVC pipes are cheap, and the temperature of HE exhaust is low enough to allow for PVC to be used without melting (although there is some controversy around that, especially if the PVC pipes pass behind finished or through otherwise confined spaces). The other alternative is stainless steel, which is much more expensive.

If your existing exhaust stack is stainless steel, you should be able to reuse it for a HE furnace. If it's the more typical galvanized steel, your best bet would be to abandon it and run a new PVC exhaust stack. Going through the wall is typically the shortest path (and is allowed for HE exhaust), but depending on the installation you can also run the exhaust through the roof, perhaps even re-using the existing exhaust path.

When I installed a HE furnace (which replaced a non-HE one), the contractor ran about 12 feet of PVC pipe to move the exhaust on the exterior wall away from the power and gas meters, so in your case you could probably ask them to place the exhaust away from your deck. It all depends on the nature of the installation (finished/un-finished area, location of furnace in relation to the walls/roof, etc.).

  • I didn't realize there was such a difference in the exhaust gases, I knew they were lower temperature but that was about it. The existing stack is galvanized, so based on what you said, it's probably not a good idea to try to use it at all. I was also thinking about running the new exhaust line up the center of the old, but that would probably still lead to corrosion issues. Just because of the layout of the basement, if we aren't going to use the existing stack, it'll probably take almost 30' of PVC to get the exhaust to a good location. – Dave Mar 27 at 14:16
  • HE appliances typically use two pipes - one for exhaust and one for intake. I'm not sure if you could run two PVC pipes through the existing 6" exhaust stack. On the other hand, 30' of PVC shouldn't be too expensive. Ask the contractor about his suggestions, asking about both options (running through the existing stack and running to a remote corner of the house). – Eli Iser Mar 27 at 15:27
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You cannot use the flue in it's current state. High efficiency furnaces exhaust less heat out of your house. The exhaust gas is cool enough that it condensates that's why it needs a drain and also why the exhaust pipe is PVC The condensate is slightly acidic and will corrode even some types of stainless steel overtime. You can however, use the old flue as a chase to run your PVC pipe through. It's not easy but if your house has the furnace located in the middle in a finished basement, it's sometimes the only option.

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