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Community and Q&A

Water heater pad

Norman Bunn | Posted in Energy Efficiency and Durability on

My HPWH is going in a tall crawlspace and I want to protect the base. Since it is going in before the crawlspoace is conditioned, I was thinking about using a condenser pad, but most of those top out at less than 400 lbs or load and my water heated will weigh roughly twice that when filled.

I found the Insulfoam water heater pad, but it is not available online from what I can tell.

What alternatives are there?

Also, since this is going in before the conditioning of the crawlspace, I was thinking of putting some thick plastic sheeting between this and the drain pan to allow the contractors something to better seal to. Is this a good idea? I am concerned that a seal to the pad will not be as good as a plastic to plastic one.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    I'm not sure what you mean by a "pad."

    Are you saying, "I'm installing a heavy water heater on a dirt floor"?

    If that's what you are saying, why not pour a small concrete slab?

  2. User avatar
    Dana Dorsett | | #2

    The Insulfoam water heater pad is a disk of EPS insulation designed to cleanly fit under a water heater, to limit the rate of heat transfer between the bottom of the water heater and the floor or slab.

    A condenser pad is a thin (typically 2") pre-formed reinforced concrete structural support, designed for supporting the outdoor half of a split AC or heat pump system in locations where there isn't a pre-existing paved area or slab to support that weight.

    They aren't even remotely the same thing.

    I suspect what you're looking for is a small structural concrete slab with sufficient capacity to handle the weight of the water heater.

    In climate zone 3 up to 1.5" of EPS on the floor of a crawlspace can be financially rational on a lifecycle energy use basis. A roughly 4' x 4' x 4" wire-reinforced concrete slab with 1.5" of EPS under it should be fine, even if the rest of the crawlspace will only have a 2" non-reinforced rat slab.

    It's a lot more work than just wrestling a big condenser pad in there but...

    Compact the soil well and install a few inches of washed gravel, compacting the gravel. A single sheet of 3/4" EPS cut in half gets stacled on the gravel, followed by a sheet of EPDM or 6-mil or heavier polyethylene at least 6' x 6' to have plenty of overlap with the eventual ground vapor barrier. Set up your concrete forms and install a layer of #21 reinforcement mesh propped up 1.5-2" off the vapor barrier with a few chunks of cheap concrete paver of the appropriate thickness tied to the mesh with steel wire prior to pouring. (Box stores typically have flat pieces of 3.5' x 7' mesh for under $10- cutting it to 3.5' x 3.5' and centering it in the forms will be fine.

    Making the forms out of pair of 8 foot 2x6 cut in half would make a nearly 4' square pad (46.5" nominally) , and would be the right depth for 1.5" of foam + 4" of concrete if resting on the gravel. Alternatively you can cheat the thickness a bit and go fully 4' square on the EPS and use 2x4s for forms resting on top of the EPS, but it might be hard to keep the forms on top of your foam unless you firmly stake them. With the 2x6 approach you'd have to trim the EPS into 46.5" squares that fit inside the forms.

    That will take about six 80lb bags of typical pre-mix "high strength" concrete sold at box stores. It's a fair amount of hand-mixing to do in a wheelbarrow or 5 gallon bucket with a shovel, but do-able.

  3. Calum Wilde | | #3

    After mixing concrete by hand for a small basement project, I've come to realize that the price to rent a small mixer from a box store is pretty compelling.

  4. Nick Welch | | #4

    It sure sounds like the crawlspace floor will be rock with a vapor barrier over it. Not concrete.

    I would probably just make sure the rock is level, put a bit of rigid foam over it, and put the water heater on top.

    An 800lb water heater, assuming a 22" diameter, only exerts 2.1PSI. There's really no need for special foam or concrete.

    A scrap of vapor barrier under or over the foam, to seal the rest to, seems reasonable.

  5. User avatar
    Dana Dorsett | | #5

    Nick- how are you inferring that the crawlspace floor will be rock w/vapor barrier? It could just as easily be uncompacted soil with vapor barrier, closed cell foam sprayed directly on soil, or any number of possibilities that would need something to stabilize the dead weight of the water heater. The foam doesn't need to be anything special, but the underlying soil has to be able to take the weight and not move too much over time too.

    A 4' square concrete pad should still have steel mesh reinforcement to limit the cracking issues. With a half ton of dead weight load centered on it cracks may still develop in a poured concrete pad (almost certain in even a small earthquake scenario). But with wire mesh reinforcement any cracks that develop will not grow over time, or at least not quickly enough to matter within the lifecycle of a water heater.

  6. Norman Bunn | | #6

    The crawlspace floor is red clay with some rock thrown in. Head room is at a premium and it is 50+ feet from the crawlspace door. What about a gravel base with a generator pad? I can get one of those for $200-300 and they will hold a ton of weight (literally). Or, this pad by Diversitech, can hold 1000+ lbs per their website () and is around $50.

  7. Calum Wilde | | #7

    Norman, here's a shot of my water heater on a sheet of dricore and then 3 inches of basic rigid foam insulation. Even a 10psi rigid foam with that much surface area is going to be fine as long as the base is flat. That's where the effort is going to come in for you, making the base flat. Dana's idea of poring a small pad after compacting the soil is a great one and exactly what I'd do if it was even remotely possible to compact the soil a little.

    Edit: I'm guessing it was Martin that attached the file for me? Thanks.

  8. User avatar
    Dana Dorsett | | #8

    Clay soil is fairly plastic (varying in plasticity with moisture content)- it deforms and flows over time when under constant pressure. Fully compacted or undisturbed clay soils are often good for 1500lbs per square foot or better dead weight (under footings for walls, etc.). Uncompacted it will be a lot less. Even if the pat can take it, the pad has to be quite a bit bigger than the ~25" diameter footprint of the water heater and reasonably rigid (to distribute the weight) for this to work, since the compression ratings of the soil are unknown, and usually pretty lousy for clay soils. With just the footprint of the water heater to work with the clay would be seeing something between 200-250lbs per square foot of constant pressure.

    The DiversiTech pads come in sizes big enough to float an 800lb load on clay soil, but since they're just Type-I EPS core for a polypropylene fabric reinforced cement slurry monocoque they may not be rigid enough for softer soil. Maybe with a sheet of 5/8" fiber cement board (eg 5/8" Durock or Certainteed GlasRoc tile backer) between the water heater and pad it might work with the 36" x 36" DiversiTech pad (rated for~1500 lbs max) or bigger. Without something stiffer to distribute the water heater's weight over a greater area of pad I would expect the pad to eventually buckle near the water heater if the soil isn't sufficeintly compacted. With a rigid (3' x 3'=) 9 square feet or bigger it will be under 100 lbs per square foot dead weight applied to the soil and even fairly soft clay soil won't move a heluva lot. With a more rigid sheet between the water heater and pad the corners of the pad are less likely to buckle if it happens that the soil does compress a bit. Whether the extra rigidity is necessary depends on the actual soil properties.

  9. Calum Wilde | | #9

    Could there be a way to suspend the water heater from the joists above with drop links, or cables, something along those lines?

  10. User avatar
    Dana Dorsett | | #10

    Anything is possible, but a poured concrete pad would be simpler and more reliable, with less design work required than any sort of suspended tank approach. The acoustic coupling might be an issue with a suspended heat pump water heater too.

  11. Norman Bunn | | #11

    Thanks for the good feedback. Could I get a few more thoughts on my second question?

    "Also, since this is going in before the conditioning of the crawlspace, I was thinking of putting some thick plastic sheeting between this and the drain pan to allow the contractors something to better seal to. Is this a good idea? I am concerned that a seal to the pad will not be as good as a plastic to plastic one."

  12. User avatar GBA Editor
    Martin Holladay | | #12

    Putting down a rectangle of polyethylene under the water heater is fine.

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