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Hydronic Heated Floors for Small Cabin

drod94 | Posted in General Questions on

I am designing a small house (750 sqft single-level) for Vancouver Island coastal climate.

The client would like to heat with hydronic in-floor – It will be an 1.5″ heated slab over a crawlspace.

I am wondering the best way to set up a system like this – whether to have an air to water heat-pump,  an electric boiler, or just connect to the domestic hot water tank? 

As I understand, heat pump is the most efficient, but perhaps overkill for a space this small?

Any advice would be greatly appreciated!

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Drod94,

    A small minisplit would be more affordable and give you AC for the occasional heatwave. If building to the "pretty good" standard (good air sealing and sufficient insulation), this approach will deliver more comfort and bang for the buck than an in-slab system.

    1. drod94 | | #4

      Hey Steve,

      what exactly is a "mini-split"

      We rarely, if ever, require AC in our climate. Maybe 4 days per year!

      1. Expert Member
        Malcolm Taylor | | #6

        drod94,

        Mini-splits are better called air to air heat pumps. You will see them on the balconies of almost every new condo or apartment building being built on the Island now. Their big advantage, especially with our electric rates, is they deliver somewhere around three times the heat per unit of energy used, unlike electric resistance which yields one to one.

  2. DCContrarian | | #2

    The size of a building has surprisingly little affect on how much energy it takes to heat and cool it. What matters is how well sealed and insulated it is. Even though this is just a cabin you should do a Manual J and check the impact of various insulation strategies.

    Concrete slabs are terrible residential floors. If you want to do in-floor heating there are better ways. The concrete really doesn't do you any favors. It's also incredibly un-green.

    There's two factors in designing a heating system: equipment cost and energy cost. Generally, equipment cost from lowest to highest is:
    Resistance electrical
    Propane
    Heating oil
    Heat pump

    Energy cost from lowest to highest is:
    Heat pump
    Propane
    Heating oil
    Resistance electrical
    (although this can vary regionally quite a bit)

    Anything hydronic is going to at least double your equipment cost.

    When I hear "cabin" I think seasonal or occasional use. Buildings like that often have just resistance electrical because they're not used enough to justify anything more expensive to install. Propane is also popular because it's cheaper to run than resistance and not that much more to install.

    If you want to go resistance electrical you can get electric mats that go under a finished floor to heat that way. That might be all you need for a well-insulated cabin that's not used in the coldest part of winter.

    1. drod94 | | #3

      Hi DC,

      I've said "cabin" because it is an accessory building next to a larger house that will be built at the same time. They are doing hydronic heat in the main-house, which is why they want to include it in the cabin aswell - can be some savings since they are being built at the same time.

      The "cabin" will be lived in year-round.

      I'm not sure what you mean by concrete slabs are "terrible" residential floors? There is going to be finished flooring put over top... and as I understand, concrete slabs can act as a great thermal mass.

      The cabin is will be well insulated and air sealed, but there are large sections of floor to ceiling glazing.

      1. DCContrarian | | #5

        You don't want "thermal mass." You want your heating system to respond quickly to changes in temperature, that gives the best comfort.

        Concrete is incredibly un-green, its production is a major source of greenhouse gases.

      2. Charlie Sullivan | | #14

        How close it it to that main building and how is the water for hydronic heat heated in the main building? There might be an opportunity to use one larger air-to-water heat pump serving both.

    2. Stephen Sheehy | | #20

      DC. Is there any place where a btu of output from propane is cheaper than a btu from oil? In Maine, a gallon of propane is usually around the same price, or a bit more, as a gallon of fuel oil. The week of November 15, average oil price was $3.16 and propane was $3.26.
      Burning a gallon of oil produces around 135,000 btus. Propane only around 95,000.

  3. Jon R | | #7

    The small amount of concrete in your proposed slab will create about $50 of CO2 - don't spend much time worrying about it.

    On the other hand, with a crawlspace below it, it would be easy to over-heat the crawlspace, creating a radiant floor without concrete.

    1. DCContrarian | | #8

      Either way you should have insulation below it or you're losing half your heat.

    2. Bas Sommer | | #9

      a splendid idea.
      the (unvented) crawlspace needs insulation anyway, so just overdo it a bit and put that miniplit into the (open) CS and set it to the floor temperature you want. Just make sure that the floor is airtight beside some vent channels for air exchange to the main.
      That way you can use a rather simple floor system, the "noise" of the minisplit is reduced once more and possible draft risks are gone.
      Unless - if the heated water would not be created in that ADU but in the main building - then maybe a fan coil in the basement (or that hydronic slab) would still make sense.

  4. Keith Gustafson | | #10

    Unlike some here I am a fan of radiant.
    However such a small space the cost seems high

    Also by cabin it reads to my like intermittent use.
    Radiant is only really useful in a 24/7 space.

    1. DCContrarian | | #11

      I'm a fan of radiant too, I'm putting it into a house I'm building for myself. It's a lot more expensive, starting with the fact it needs a much higher level of engineering. Hydronics is ideally suited to burning gas or oil in a boiler, but we need to start thinking all-electric and the thermodynamics works against you with a heat pump.

      1. Charlie Sullivan | | #12

        I agree with all your comments on this thread except the very last point about thermodynamics. If you have an air-to-water heat pump and a radiant floor with a good envelope, you can run the water temperature extremely low, maybe 85 F. That low temperature combined with the high heat capacity of water makes the heat pump more efficient than one delivering 95 F air to the conditioned space, and if the plumbing is done well, the pump uses less energy than the fan in the minisplit head would. The minisplit is already efficient enough that the difference doesn't matter much, and the cost of the hydronic system is much higher, so it's rarely a good design, but it can be super efficient if done well.

        1. DCContrarian | | #17

          You're right, air-to-water and radiant floor are made for each other. But one of the nice things about conventional hydronics is you can mix and match. So if don't want to spring for in-floor in the whole house, or if you have a room where there isn't enough floor area for the heat load, you can just drop in a radiator, convector or baseboard. With 180F water it doesn't have to be big at all -- 18 feet of baseboard and Bam! there's 20K BTU/hr of heating. Of course for the floors you have to temper the water down which adds complexity.

          With a heat pump delivering water in the bathwater range that option goes away. Although Chiltrix does sell a line of convectors that deliver a lot of heat in not much space with lukewarm water. I think this approach shows a lot of promise. While in-floor will always be expensive, I see this technology as being fundamentally simpler, more versatile and more efficient than mini-splits. The problem right now is the cost, because it's a niche product it's several times more expensive than mini-splits.

  5. drod94 | | #13

    The crawlspace will be unconditioned and vented, as we are in a flood zone. We cannot run any mechanical in the crawl, again because it's below the flood level.

    The floor system will be well insulated below the slab with batts, and additional foil-faced poly-iso foamboard continuous and airtight on the underside of the I-Joists (to stop thermal bridging).

    The cabin will be lived in 24/7, 365.

    1. Steve Knapp CZ 3A Georgia | | #15

      Not sure I would want a cold floor with batts covered by rigid foam in a flood zone. Why not go with a pier construction? Would this assembly be easier to access if (when) the floor system is submerged?

      1. DCContrarian | | #16

        If it's above the flood line this sound fine.

  6. Walter Ahlgrim | | #18

    I have to ask why a hydronic slab over a crawlspace?

    What is the real goals and is a hydronic slab the only way to meet the real goals?

    I seem silly to build an elevated concrete just so you can have a heated slab. If you have quality windows in a well insulated building your heated floors are not going to feel warm.

    With so few square feet it seems silly to devote floor space to a boiler.

    If the client is demanding heated tile floors then electric resistance over a wood base may be you low cost option.

    Walta

  7. DCContrarian | | #19

    To the original question: what I'm doing in my house is air-to-water heat pump, radiant heat in the "showpiece" rooms, convectors in the rest of the house, convectors also sized to provide cooling. It's expensive and you may have difficulty finding someone to install it.

  8. William Hullsiek | | #21

    If you go down the concrete slab route, be sure to pull the PEX tubing up in the slab.

    We use Warmboard in our house, which is connected to the Water heater via a heat exchanger. Warmboard recommends R20 below the subfloor so the heat will have less resistance going into the house. In terms of a heat source, I would use a cold climate heat pump that provides heat and DHW. Both Artic and Nordic are Canadian heat pumps.

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