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Community and Q&A

Electric hydronic radiant heat

samueldnewman | Posted in Mechanicals on

Hi all,

I’m designing a cottage for my mother, to be built in central Vermont next summer. It’s 20′ x 34′ single story, probably shed roof (16′ high on south exposure, 12′ on north exposure). Wall assembly isn’t totally hammered down yet, but likely 2×6 walls with cellulose in cavity and rigid foam on exterior, lots of cellulose in the roof, and careful air sealing throughout (the envelope is very simple in order to make air sealing easier). Goal for R values will be 30-40 in walls and 60 in roof.

My question is about heating the building. I installed radiant floors with a gas tankless heater in my own house, and I really love it, and would like to do the same for her. But I’m also interested in keeping all the utilities electric, so that we can reach net zero by adding a small PV array. I know I could just go with a heat pump, but I really don’t like them, for several reasons – I don’t like blown air, and I don’t like having an ugly compressor outside the house. So, my question is: is it crazy to use an electric hot water heater to heat hydronic floors? My sense is that in a building this small, built to fairly high performance standards, it would probably work out ok.

Please note this question is about using an electric water heater for radiant, not so much about the pros and cons of radiant heat. I know it’s not for everyone, slow to respond, etc., but I really like it.

Thanks very much,

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  1. Reid Baldwin | | #1

    Electric radiant would probably be cheaper to install. The only advantage of hydronic with an electric heat source would be the option to change heat source in the future.

  2. user-1072251 | | #2

    radiant heat is nice, and would probably heat the cottage, but it's lousy at air conditioning and dehumidifying, which minisplits excell at, and which are both important to seniors. I've built several homes where my clients did not want ugly wall units (minisplits), but after seeing the units in action, changed their mind and have had no regrets. We had "hot air" heat for decades in our home, and upgraded to minisplits a few years ago - there is no comparison. The mini's use a very low fan setting; they are quiet and unobtrusive, and by far the finest heating source I've ever used. But if you are just concerned about heat and not comfort, your plan should work fine.

  3. Expert Member
    Dana Dorsett | | #3

    A shed roof pitch facing north means no rooftop solar. Is that "...small PV array..." going to be ground mounted?

    An electric water heater is a pretty crummy substitute for a purpose made electric boiler. Electric boilers come in a large range of sizes to suit just about any heat load. A sub-2kw boiler can be had for under a grand, and unlike an electric water heater it's designed for the sort of duty-cycle and flow you would need for a radiant floor.

    Choosing the heating system before running the heat load numbers is folly.

    The perfomance goals of "...R values will be 30-40 in walls and 60 in roof..." are extremely ill-defined, unless you're talking about "whole-assembly R", that calculates all there thermal bridging in the framing, the R-values of sheathing, siding, gypsum board, roofing, etc. Five inches of HFO blown 2lb foam in a 2x6 studwall 16" on center is about R35 at center-cavity, but would result in a whole-wall R of less than R20. A 2x4/R13 wall with 3" of continuous polyiso sheathing would be only R31, but would have a whole -wall R in the R28-R30 range.

    1. caliberger | | #5

      Hi Dana
      Just came across your comments from 2018. I have the same questions as the OP. Can you provide more info about electric boilers? Specific questions are typical longevity, maintenance, and would you couple the boiler with an indirect water heater for domestic hot water?

      Can you also comment on calculating heat loads? Is this something I can do myself? Are there online calculators you recommend?

      Zone 3C
      Thank you

      1. Expert Member
        Dana Dorsett | | #7

        >"Can you also comment on calculating heat loads? Is this something I can do myself? Are there online calculators you recommend?"

        If you lived in a colder climate and had a heating history you could get reasonably accurate results for load using energy use against heating degree-day data. But zone 3C it's so temperate the results would be skewed by solar gain, water use, and negative HDD days in any given billing period. Though it will probably deliver paradoxical results in your climate zone, that method looks like this:

        That leaves you with Manual-J or similar:

        ...or... an I=B=R method approach:

        Even when using the most aggressive assumptions on air tightness LoadCalc tends to overshoot by 20% or more, and will overshoot by a LOT if all of your inputs err to the conservative rather than aggressive/optimistic. CoolCalc is an ACCA registered Manual-J tool, but like all calculators garbage in= garbage out. The instructions in the ACCA Manual-J specify that all factors that could potentially reduce the load number need to be factored in for it to be reasonably accurate, so be aggressive/optimistic on all R-value, U-factor, and air tightness assumptions.

        >"Can you provide more info about electric boilers? Specific questions are typical longevity, maintenance, and would you couple the boiler with an indirect water heater for domestic hot water?"

        In zone 3C a heat pump water heater would be far more efficient than an indirect with an electric boiler. Even a plain old electric tank would be more efficient.

        Electric boilers are pretty rugged, and don't need much in the way of maintenance. As long as the systems water isn't too acid (check the pH every year or three) and it isn't being crazy over-pumped (or drastically underpumped) it should last for decades at the kind of duty cycle you'd see in zone 3C.

        1. caliberger | | #8

          Thank you!

  4. ethant | | #4

    if your load is relatively low, a Sanden HWH tied to radiant hot water is a nice efficient solution. I'm looking right now at Sanden heat+DHW plus a Minotair for cooling,dehumidification, ventilation, and supplemental heat.

  5. Jon_R | | #6

    A Chiltrix air to water heat pump is another option. You can use it with both a radiant floor (Winter) and fan coils (Summer AC).

    Anything with a water tank is going to allow some time shifting of electricity usage. This will become increasingly important (to both $ and the environment).

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