Our house has an electric AC and natural gas furnace heater setup. The AC is getting old and having issues, so we want to replace it, and are thinking of upgrading to an air source heat pump. We talked to a few contractors and one option they offered is to replace the AC and furnace with a heat pump and air handler. However, one contractor stated that this would not be the most cost efficient option, because the gas furnace is in a common vent configuration with the natural gas water heater next to it, and if we disconnect the gas furnace, we would have to shrink the common vent as it will be too large (diameter-wise) for just the water heater.

This has me a bit confused. The other contractors did not mention this as a problem. What would the problem be that an oversized vent could cause? Also, if it is a problem, then in the summer months when the furnace is off and the water heater is still running, wouldn't it already be a problem now?

Edit: We're looking at air source heat pumps.

  • ignore the first contractor
    – Traveler
    Apr 11, 2023 at 3:30
  • what kind of heat pump ?
    – Traveler
    Apr 11, 2023 at 3:32
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    Depending on where you are you may need secondary/emergency backup heat. If so, and if your furnace is in reasonable shape, it may make sense to keep it and use the existing air handler with the heat pump. Apr 11, 2023 at 4:08
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    @Ruskes my air source heat pump does just fine down in to the teens, only using the resistance heat for defrost. The tech has come a long ways, provided you buy good hardware. You even see mini splits with no backup in places like Vermont nowadays.
    – KMJ
    Apr 11, 2023 at 6:26
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    Perhaps the vent sizing issue is a drafting issue. Modern gas appliance exhaust is less hot than in the old days so it doesn't tend to rise as much/quickly. If you spread out cooler exhaust in a larger diameter vent it may not draft well enough. This is why gas water heater makers often specify that at least the first foot of venting should run vertically up from the heater before any bends.
    – Armand
    Apr 11, 2023 at 9:16


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