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Community and Q&A

Heat source for radiant slab

ZdRsx4MyBR | Posted in Mechanicals on


I have a new, well insulated (walls R-40, Attic R-70) and well sealed ([email protected]) 3000sqft home in Northern Vermont. The max heat loss is roughly 20k/hr. I heat the home using two Mitsubishi Hyper-Heat mini splits that are located on the first and third floor. The home is all electric.

My problem is that the finished basement is cold. I have a GE geospring air to water heat pump in the basement that puts out air at roughly 55 degrees in the summer. I moved in this spring so I don’t know how it will perform this winter. In April it was very cold down there. There is R-10 under the slab and the basement walls are approx R-30. I installed radiant tubing in the slab with hopes to set up a solar thermal system to help heat the home, some day. I currently cannot afford to put in the solar system.

What are my options for installing a system to heat water to run through the slab for under $2000?

Can I use my existing water heater to do this?

Can I vent my air to water heat pump to the outside?

I leave the home unattended for long stretches throughout the winter, and I also want to rent it out to vacationers, so I need the basement to be at least 60 degrees. I’m not opposed to adding propane if it is my only option to heat the basment slab affordably. Any advice would be greatly appreciated. Thank you for your time.


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  1. jklingel | | #1

    There are discussions of using water heaters for radiant floor on this forum; do a search. I think the concern was having the horsepower to supply water when it gets real cold. Maybe that's not an issue since you have other heat sources, but R10 below the slab is a potential problem, in my DIYO. With your high R's elsewhere, why is the slab so naked?

  2. GBA Editor
    Martin Holladay | | #2

    It sounds like you bought the wrong kind of water heater. You are using energy to cool your basement, whether you want a cold basement or not. Now you have to use more energy to warm your basement back up.

    Obviously, you don't want to circulate water from your heat-pump water heater through PEX loops in your floor. That would be nuts. The water heater would be trying to heat your basement and cool your basement at the same time, and the only result would be a high electricity bill.

    You might want to sell your heat-pump water heater on eBay and install a conventional electric-resistance water heater. See how that switch affects your basement temperatures.

  3. wjrobinson | | #3

    Options Graham...

    Pellet stove in basement. Easy for renters to use.

    Dedicate a propane water heater to floor loops. The more you spend, the higher the efficiency.

    If temps at the highest point on your home are above room temp, move your HPWHeater their, hook in a heat exchanger loop to your basement. All it will do is pump your already bought and paid for home heat down where it is lacking. Crazy idea really but may be the lowest intial cost and a neat experiment to boot.

    Logically speaking, HPWHs are for use in rooms that can use the cooling that they provide while they warm the air.

    Edit, being that you are super insulated, there should be no real extra btus at the high point of your interior. If so scratch the crazy experiment idea from my options list.

    Let us know your progress. We all can learn from it.

  4. kevin_in_denver | | #4

    Here's your simplest, cheapest and most efficient solution to your current problem. Hopefully you have a big enough window to install this 26" mini pthp in the basement:

    Martin's solution would work, but you'd lose money on your old water heater, and you'd slip back to a COP of 1.00. The PTHP solution gets your overall COP to about 1.5-2.0

    Don't worry about putting heat in the slab until you get the solar system. A small air-to-water split heat pump will be available eventually, but will probably cost more than $2000.

  5. GBA Editor
    Martin Holladay | | #5

    You proposed PTHP isn't a great fit for northern Vermont, where cold outdoor temperatures mean that the PTHP will be heating with an electric resistance element for most of the winter.

    Graham may not need any heating at all in the basement if he gets rid of his cooling unit (I mean water heater).

  6. kevin_in_denver | | #6

    Actually, the Geospring has a "standard electric" operating mode, which means he doesn't need to replace it. If he still needs heat for the basement, baseboard resistance could be under $100. A well-insulated basement kept only to 60F might not need any heat, just the heat radiating down from the upper floor may be enough.

  7. ZdRsx4MyBR | | #7

    Can I vent the HPWH directly to the outside?

    I agree that I should have put more sub slab insulation, but at the time the REMRate model said it would be a minimal return on the $1000+ investment. If I had known the slab was going to be as cold as it is I would have doubled it. Oh well. I do have a baseboard electric heater hooked up in the basement, but it is expensive to run and I am looking for a more efficient option.

    Thanks for all your suggestions!

  8. dickrussell | | #8

    Graham, indeed your baseboard electric heater may be expensive to run, having COP=1. But consider the overall heat balance of the house structure. Unless you burn a fuel inside (oil, propane, wood, pellets), bring in solar heat, or pump heat in from outside the thermal envelope (heat pump), then your only heat input to the envelope is in the form of electricity, and your overall COP still is just 1, regardless of what devices are used inside to move heat around within the envelope.

    As Martin points out, you have largely created that cold area in the basement by pumping some of the heat from the air down there into the hot water tank. Turning that off is your first step and perhaps all you have to do.

  9. kevin_in_denver | | #9


    If you can run a small duct system with a blower to and from your mechanical room, that would be like venting to the outside. But in the winter, all the water pipes in that room could freeze. To prevent that, you start getting into control systems that get expensive and have bad failure modes. It's much easier to run the water heater in "standard electric mode", which means electric resistance.

    And it's not the sub-slab's insulation that is making the slab cold, it's the HPWH. So don't kick yourself about that.

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