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Indoor air intake into high-efficiency tankless water heater

rocketscientist51 | Posted in Building Code Questions on

I have the option to let our new tankless water heater intake indoor air (it will be installed in a semi-finished basement), or intake it from the outside. The manufacturer seems to be OK with either approach.

What code would/does apply in this case?

PS. Our furnace/boiler intakes indoor air as currently installed, so logic tells me I could do the same with the tankless water too!?

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  1. user-2890856 | | #1

    While there may have been enough free air for your old atmospheric water heater and your boiler , The likelihood that there will still be enough is almost nil . Take the combustion air from outside as you are supposed to . The manufacturer is not the one who will die in your home . Maybe you should read the disclaimers they also wrote about using indoor air also .

    This is a bad idea . Do it the right way for your safety

  2. GBA Editor
    Martin Holladay | | #2

    In decades past, all combustion appliances (for example, gas-fired water heaters, furnaces, and boilers) took their combustion air from inside the house. These appliances were called "atmospherically vented appliances."

    The industry is now making the transition to sealed combustion appliances which take their combustion air from an outdoor air duct. These sealed combustion appliances are preferred to the old atmospherically vented appliances, since sealed combustion appliances are much less likely to backdraft when an exhaust fan (for example, the range hood exhaust fan) is operating.

    Code requirements on atmospherically vented appliances vary from jurisdiction to jurisdiction, but most codes allow atmospherically vented appliances in older homes. Check with your local code enforcement officer for more information.

    All of that said, sealed combustion is preferred. If you have a choice, definitely take Richard McGrath's advice. Install the outdoor air duct for combustion air.

  3. walta100 | | #3

    Using indoor air for combustion will work if your home is leaky enough and almost homes are.

    Yes the safe bet is to install a combustion air intake and some code enforcers will require one.

    Only your local code enforcement can tell you what they want but in general if you follow the manufactures printed instructions they will be happy.

    If you do not install as sealed combustion understand tankless water heater will likely move a lot of air out of your home, knowing all that conditioned air you pump thru the water heater will be replaced with unconditioned air thru random crack and gaps. This means all winter long you are pumping warm moist air and pulling in cold dry air and all summer long you are pumping cool dry air out and pulling in warm moist air.

    I recommend you install as sealed combustion.


  4. rocketscientist51 | | #4

    Thank you Richard, Martin, and Walter for your counsel.

    I am seriously space constrained in the corner of the basement where I must install my new tankless; which is the best location for cold and hot water connections, but not so much for the vent, the air intake, and the condensate drain. Hence my indoor air intake consideration — which, as I have indicated, is well within the range of options offered by the manufacturer, and fits their suggested calculations.

    Nevertheless, you have persuaded me to reconsider my preferance. I plan to install a concentric vent termination to the side of the house.

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