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Community and Q&A

Underfloor heating in MD-DC-VA

bsawers | Posted in Mechanicals on

A neighbor has a cold floor. He believes it’s a floating wood over a concrete slab, but it looked and felt like wood directly on the slab to me.

He’s looking for ways to make the house more comfortable. I proposed underfloor heating. But, he does not have tall ceiling, so he is constrained on how thick an installation he can manage.

I’m most familiar with the hydronic systems, but an electric system is probably easier to install. If the electric system is thinner and thus allows for more insulation, does this do something to make up for the higher cost of electric resistance heating?

I’m concerned that either system installed directly on a slab (almost certainly uninsulated) will be expensive since a lot of heat will go towards heating the ground.

I tried to sell my neighbor on a air-to-water heat pump to feed the hydronic system, but I think the neighbor was more interested in the simplicity of a gas tankless water heater.

Lastly, any recommendations for someone in the MD-DC-VA area who might have experience installing these systems?


  1. paul_wiedefeld | | #1

    Tankless gas heaters are for showers, not floors. They make gas wall mounted boilers, but they aren’t cheap or simple either. Think $10k plus for a pro install. The floor will be extra.

  2. bsawers | | #2

    I had read that people were using tankless water heaters for infloor heating. Here's one description:

    1. paul_wiedefeld | | #3

      Plenty of people try that route. It’s not what those heaters are designed for - those same manufacturers make wall mounted boilers.

      1. Expert Member
        Akos | | #5

        Most tankless units are rated for combi heat, I've used them like this without issues. When used for space heat the warranty is reduced but since they are much cheaper than any modcon so it is still a pretty good deal. If your water is hard, the extra runtime will cause a lot more scale buildup which does mean more maintaince.

  3. Expert Member
    Akos | | #4

    Instead of ripping the flooring up, I think a much simpler solution is some exterior insulation. If you insulate the stem walls and add in some extra wing insulation it will bring up the temperature of the slab a fair bit. Won't be warm but will be much more comfortable than as is.

    When the flooring needs to be replaced, they can add even as little as 3/4" of rigid under the subfloor for even warmer floors. No need at all for the complication of floor heat especially anything hydronic.

    If they do want a bit more comfort in tiled areas, some of the resistance matts are a good option. Provided there is some insulation under, the operating cost of a couple hundred sqft of resistance floor heat for 10h-12h per day is not that much especially if something else is providing the bulk of the house heat.

    1. Expert Member
      DCcontrarian | | #7

      I want to comment upon "especially if something else is providing the bulk of the house heat." One of the challenges with heated floors is that you can pick the floor temperature, or you can pick the heat output, but you can't pick both -- they are chained together. If the walls are insulated basements tend to have low heating loads, so if you want to have the floors at a temperature where they are noticeably warm -- say 80F -- there's a good chance that's the entire heating load.

      1. Expert Member
        Akos | | #8

        At 80F you are correct.

        My suggestion works well if you only want to warm the floors up a bit. Raising a cold floor at 60F to say 70F/72F only puts a couple of extra BTUs into the house but makes a world of difference in comfort.

  4. Expert Member
    DCcontrarian | | #6

    Retrofitting floor heat into existing construction is not a small project, especially if you want to do it in an energy-conscious way. Before embarking, I'd consider first putting something like a throw rug on the floor, you can lose a lot of heat to direct conduction through the floor and a good insulator can block. My local Home Depot sells 2'x2' "project panels" of foam insulation, get one of those and stand on it in bare feet and see how much warmer your feet feel.

    Also try just directing more heat into the basement. It's all about comfort, I think there are easier ways to get equivalent amounts of comfort.

    An uninsulated basement loses heat in the winter, but it also contributes cooling in the summer. I'm in DC, my basement walls are insulated but not the floor, my Manual J indicated that the summer cooling would be greater than the winter heat loss. We have about 10% more heating degree days than cooling degree days, but when you add in solar gain in both summer and winter -- which degrees days don't take into account -- the annual cooling load is higher than the heating load.

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