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Removing furnace from gas vent

ChuckChuck Member Posts: 88
I am removing a furnace from a gas vent.

What will be left is a residential water heater (typical 40MBH with 3" (I think) vent connecter) and 8" BVent up inside a house probably about 25 feet.

I am not clear about the rules. Is the 8" now too big for just the water heater?

But not too big if the vent connecter is 4"?


  • Bob HarperBob Harper Member Posts: 732
    orphanned water heater venting

    According to the 7x rule, you can leave an appliance with a 3" vent collar vented into a 7" but not 8" vent. However, if it is a 4" collar, you can vent that into a 10.5" vent. Doesn't make it a great idea and certainly doesn't guarantee performance but under the gas code, it is acceptable if all else is.

    Now, venting a 40mbh WH into a cold exterior 8" vent even with 4" vent connector is a recipe for condensation and/ or backdrafting.

    Consider replacing it with a power vented WH out the side and be done with it.
  • ChuckChuck Member Posts: 88
    Water heater vent connection

    The b-vent is inside the house.

    I am not sure if the vent connecter is 3" or 4". For typical 40 gal/40MBh water heater is it optional? (looking at typical cuts, both are listed)
  • Tim McElwainTim McElwain Member Posts: 3,893
    Try using a 4" "B" vent connector to the 8 " vertical vent

    and look for any condensation at the draft hood if there is no condensing and the water heater is drafting (homeowner test with some smoke at the draft hood skirt and see if it is sucking the smoke in or blowing it out, sucking in is good) If you are a pro do you have a draft gauge or combustion analyzer?

    How long is the vent connector?
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