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Thread: foam block under water heater?

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    DIY Member charles2's Avatar
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    Question foam block under water heater?

    Does it cause any problems to put a block of insulating foam under a 50-gallon water heater before installing it? If not, what density is required?

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    Moderator & Master Plumber hj's Avatar
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    Default heater

    1. It would depend on the heater because some draw their intake air from the bottom, and WHEN the heater settled into the foam, and it would, the combustion air would be cut off.
    2. ANY heater will compress it because of the weight
    3. WHY would you want to anyway? There is little, if any, heat loss downward from the heater.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by charles2 View Post
    Does it cause any problems to put a block of insulating foam under a 50-gallon water heater before installing it? If not, what density is required?
    I doubt the answer will differ, but what type of water heater, electric or gas?

    For gas, I don't see that it would serve much if any purpose and depending on what is selected there could be major problems. There are two standard designs/manufacturers of traditional atmospheric vent gas water heater. One has its combustion air draw on the bottom. Therefore you can't cover that. You could rest the legs on some sort of insulating isolation pads, but the area under the water heater should not have combustible materials. (There is non-foam insulation surrounding the walls of the combustion chamber of the one I have, but this does not appear to be the case of the base itself.)

    Even if you thermally isolate the feet of a gas water heater from contact with the ground, they still will behave as "fins" from a heat transfer perspective so I doubt the overall losses from them would change much unless you actually made an insulated boot for each foot.

    For electric I suspect the bottom head of the water heater is already well insulated, I can't think of any reason it shouldn't be, whereas gas is more challenging. Since electric water heaters have thicker insulation on average (R16 seems to be the starting point vs. R8 for the starting point for gas) there is much less potential for savings for adding insulation to them. The thick insulation's impact is reflected in the very high efficiency factors for electric: for example, State's 50 gal electrics in last year's product guide were 0.91 for R16 (2"), 0.93 for R20 (2.5"), 0.95 for R24 (3").

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    Forum Admin, Expert Plumber Terry's Avatar
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    In the Seattle area, we install electric water heaters on foam pads.
    We buy them from the supplier that supplies the water heaters.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Terry View Post
    In the Seattle area, we install electric water heaters on foam pads.
    We buy them from the supplier that supplies the water heaters.
    Is that a carryover from years past (days of thinner insulation), or is it relatively recent? If the manufacturers don't insulate the bottom head as much as the walls and top, then this could make some difference, particularly since this end is actually contacting a solid surface such as concrete and would have a higher conductivity.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Is that a carryover from years past (days of thinner insulation), or is it relatively recent?
    At least ten years.
    It prevents the tank from rusting on the bottom too.

    If you have ever slept on snow, and used a closed cell sleeping pad, you would know how it works.

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    Last edited by Terry; 07-18-2010 at 07:36 PM.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Terry View Post
    At least ten years.
    It prevents the tank from rusting on the bottom too.

    If you have ever slept on snow, and used a closed cell sleeping pad, you would know how it works.
    I actually wondered if it might help prevent rusting of the metal insulation jacket like that. Having that insulating layer next to it should keep the surface much warmer and drier even for today's thicker insulation.

    Haven't slept on snow, but use my Therm-a-rest camping a few times a year...not often enough. With kids' sporting events/coaching etc. it seems like we can never get away in prime camping season.

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    Apprentice Plumber D'Brie's Avatar
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    Quote Originally Posted by Terry View Post
    In the Seattle area, we install electric water heaters on foam pads.
    We buy them from the supplier that supplies the water heaters.
    Same here in Whatcom Co. Washington.

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    DIY Junior Member bobwilli's Avatar
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    Default foam block under water heater?

    Quote Originally Posted by Terry View Post
    In the Seattle area, we install electric water heaters on foam pads.
    We buy them from the supplier that supplies the water heaters.
    Do you install the heater directly on the pad, which in turn is in a drain pan?

    I have a GE GeoSpring 50 gal heat pump hybrid water heater to install.

    Thanks.

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