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Community and Q&A

Ductwork for furnace and water heater tank

Greenpeace | Posted in Energy Efficiency and Durability on

Planning to install a 60000 or 80000 btu high efficiency furnace for an old house. I am going to replace for a new return duct as I will get rid of the humidifier. Wondering if I change the size of the return duct from 8″ x 24″ to 10″ x 24″, will it help improve the performance of a 2- stage new furnace with multi-speed ECM motor? No changes will be made to other ductwork.

After installation of the new furnace, the metal chimney will only be used by the water tank. The furnace supplier said the venting duct that connects the water tank with the metal chimney is too small. should be 4″ instead of 3″ for the size of my water tank (150 liter) according to Code. He said I must change the duct to comply with code before they can install the new condensing furnace?  My rental water tank was installed in 2005. The fitter did not change the vent at that time. who should be responsible now?

Would appreciate someone who can verify the size of venting duct for water heater tank? Are there any risks of not having the right size duct?

is it okay to have an orphan chimney?

Thank you.

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  1. Expert Member
    Dana Dorsett | | #1

    The size of the venting has to do with the size of the burner, not the volume of the tank. If the prior metal chimney was sized for the combined output of the water heater + old furnace the main chimney is probably going to be too large, even if the connecting branch is smaller than code.

    When the flue is oversized for the output of the burner there is a greater risk of backdrafting, and much greater amount of flue-gas condensation inside the flue, which shortens it's life. Rather than replacing just the branch, you may need to slip a right-sized liner for the lower output of just the hot water load to improve stack velocity and reduce backdrafting risk.

    Which code authority are you subject to (province/state/ country/city)?

  2. walta100 | | #2

    In my opinion if you tend to leave your doors open any change to the return ductwork will unnoticeable unless your return grill are noisily and whistle.

    Can we assume all your ductwork is in your basement and not in the attic?

    If you have a brick chimney sized for a furnace and water heated and only use it for a water heater the combustion gases will condense on the bricks. This condensation is acidic and will attack the mortar after a few years the bricks will literally start falling down. I agree a stainless steel liner would work well.


    1. Expert Member
      Dana Dorsett | | #3

      The liner doesn't have to be stainless for an ~80% combustion efficiency burner like a water heater- it just has to be the right size for the burner.

      A typical 150L/40gal. water heater has a ~35-40,000 BTU/hr burner, and can be vented with 3" B-vent or a 3" sleeve inside a larger flue. It's a bit suspicious that the contractor is insisting on 4".

      With as little as 15' of vertical rise from the draft hood to the top of the stack, even with 10' of (properly sloped) lateral feed before turning vertical a 3-incher would still accomodate a 40,000 BTU/hr burner. With even more stack height it has even better draw. See table 1:

  3. Greenpeace | | #4

    Thank you all for your sincere help. I am actually located in Toronto. Questions:

    1. is metal liner also required for the metal chimney which is now used for the combination of both furnace and water heater?
    2. when the water heater was changed in 2005 for a bigger size, the installer did not change the vent duct of the heater.
    3. who can install a metal liner in the chimney?
    4. the dealer said if I also change the water heater to a tankless unit, we don't need to bother about the 3" duct any more? Chimney problem also solved. I heard tankless water heater is not so reliable and is not going to save much monies, so I am reluctant to change.


  4. Expert Member
    Dana Dorsett | | #5

    1: It depends on the burner size & flue size. If the flue is oversized it'll backdraft and condense. If the chimney is stainless it won't destroy the chimney, but there could be liquid condensate finding it's way back to the water heater during cold weather, or dripping out the seams.

    2: When the water heater size was changed the burner size probably didn't change much , if at all.

    3: That's a local code compliance issue. Some juridictions allow DIY changes to venting, others require licensed certified (and bonded) techs to do the work. I don't know what's required in Toronto.

    4: A tankless water heater would be a ridiculously expensive "solution" to a venting size problem. In most cases it would even require upgrading the gas plumbing inside the house.

    I'm not convinced that the dealer is giving you good information on the code requirements. Not saying that they'er lying, but they might be.

  5. AlanB4 | | #6

    A 13 year old water tank is nearing its last legs. They have a 15-20 year expected lifetime.
    I would replace it with a powervent in the near future (maybe next year at the latest).

    That would get you a new tank partially paid with the liner money and would eliminate the chimney issue. If you get the liner expect a couple years before the tank leaks then you would have to pay for a new tank anyways.

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