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Electric boiler sizing for radiant floor

frenchdude | Posted in General Questions on

Hi all,

To make a long story short, I have demolished my existing basement slab and replaced it with a radiant slab.

1000 square feet. 8 foot basement with 6 feet being below grade.

I have installed vapor barrier and 2.5″ rigid insulation underneath the slab.
I downloaded a free trial of LOOPCAD to plan out my pex runs. I have 4 runs all being between 220 and 240 feet long. 1/2 inch pex spaced at 12″.

I plan on heating my basement evenly using 1 thermostat and eventually will put divisions to create bedroom/bathroom/mechanical room/living room. I planned my pex run layout in consequence and having a SS manifold will give me the option of installing actuators on the valves independently.

My current situation:

My slab was poured 2 weeks ago, it was mechanically polished and I am currently wet curing it. I will finish it with an epoxy when it is done curing.

My exterior basement walls will be spray foamed with 2.5″ closed cell. The house is located in Montreal, Quebec, Canada. 

Now since I have tackled this project pretty much DIY, I was looking for some expertise on the size of the electric boiler I need. I am currently looking at THERMO 2000 mini BTH / mini ULTRA.

Any advice would be greatly appreciated. Let me know if you need more information concerning my installation to make a complete calculation.


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  1. Expert Member
    NICK KEENAN | | #1

    I hate to say this, but the time to size the boiler was before you put down the loops. Hopefully it will work out for you.

    For a radiant floor heat system, there are three sizing calculations:

    1. How much heat does the space need? This is a basic Manual J, calculate the exterior area and multiply by the heat loss per square foot for each construction type for your climate.

    2. How much heat can the floor provide? This is based on the pipe spacing, the floor construction and the maximum acceptable floor temperature.

    3. How much heat can the piping provide? This is based on flow rate, loop lengths, pump capacity and temperature delta.

    The lowest of those three numbers will be the actual output of the floor and you can size the boiler to that. Hopefully it's #1, if it's not then you're not going to be able to adequately heat the space.

  2. Expert Member
    Dana Dorsett | | #2

    With only 2' of the foundation wall being above grade, and insulated to better than R10 or better the heat load doesn't change dramatically with daily swings outdoor temperature, even when it's extremely cold outside.

    Unless the basement leaks a huge of outdoor air, or has extremely high mechanical ventilation rates, or is a walk-out basement with a whole wall of glass sliders a fully insulated 1000' unlikely to have a design heat load of more than 1.5kw (~5000 BTU/hr) and may even be less than 1kw ( ~3000 BTU/hr.) But run a or Manual-J(-ish) load calculation on it as a sanity check.

    A really crude but still reasonable approach is to run an I=B=R load calc for just the above-grade portion, adding another foot of height as a fudge-factor to account for the below-grade losses. eg. If it's a 25' x 40' foundation the perimeter is 130', and lets call it 3' above grade (instead of the actual 2') for a total square footage of ~400 square feet. With 2.5" of EPS the U-factor is a bit less than 0.1 BTU/hr per square foot per degree F difference. With 2.5 closed cell spray foam it would be about U0.06, but for ease of calculation let's just call it U-0.1. The 99% outside design temp in Montreal is about -10F/-23C at the airport (see: ), and assume an indoor temp of 70F/21C, for an 80F temperature difference.

    The wall losses (with R10-ish board foam) would then be:

    U0.1 x 400' x 80F = 3200 BTU/hr (a hair less than 1 kw)

    Assuming 4 U-0.35 basement hopper type windows at 5 square feet each for 20 square feet, the window losses would be:

    U0.35 x 20' x 80F= 560 BTU/hr.

    Add it together and you're looking at 3760 BTU/hr, which would be (3760/3412 = ) 1.1kw, not more.

    Run that type of calculation on your actual foundation perimeter and window sizes. The real load might be slightly higher than that if it's a leaky basement, but it won't be twice that, or even 1.5x. It's highly unlikely you'd have enough window or above grade wall area to hit 2kw in that sort of crude calculation unless it's a walk-out basement with a lot of glass.

    Looking at their flyer, at 12kw even the smallest BTH Ultra series boiler would be LUDICROUSLY oversized. It would be like swatting a mosquito with a sledgehammer. The very smallest of the mini-Ultra series is 3kw, which is only RIDICULOUSLY oversized (are you anticipating outdoor temps in -100C to -150C range any time soon?)

    Run some load calculations and you'll see. To convert BTU/hr load numbers to kw divide the BTU numbers by 3412 (BTU/hr per kw).

    1. frenchdude | | #3

      Thank you so much for your detailed reponse, you really put in some leg work.

      All your assumptions are bang on. My basement has 5 of those hopper type windows. There is no other openings. My exterior/end joists are sprayfoamed as well.

      My house has been completely stripped. I have new rockwool insulation on the ground floor. R14 in the walls with a 1/2 inch rigid foam with a reflective laminate. My cathedral ceilings have R24 batts with a 2 inch rigid foam with a reflective laminate as well.

      So if i understand your math, a 3kw electric boiler is more than sufficient? My local dealer who sells the thermo 2000 use a basic calculation of new houses (new building codes) require 22btu/ft2 and old houses 35btu/ft2. I was looking at 30,000 btu in my situation = 9kw.

      That would be overkill.

      Thanks again.

      1. Expert Member
        NICK KEENAN | | #4

        The rules of thumb of certain btu/square foot are not terribly useful because houses lose heat through their exterior walls and it's the exterior surface area that matters. Different types of exterior walls have dramatically different heat losses. The kind of analysis that Dana did is the way to go.

      2. Expert Member
        NICK KEENAN | | #5

        If you can't find a 3kw electric boiler, some water heaters are rated for space heating, that would be a rather compact water heater.

  3. Expert Member
    NICK KEENAN | | #6

    If you can avoid using the closed-cell spray foam you should, it has a terrible global warming impact. You can get similar r-value -- at lower cost -- using polyiso sheets. If moisture is a problem use EPS foam sheets which are more moisture resistant than polyiso but have a lower R-value. . My personal prejudice is not to use any kind of fluffy insulation below the finished floor level of the first floor when there is a basement.

    The only time I would recommend spray foam for the entire wall is when the wall is rough and irregular and sheet foam won't fit to it.

    It's good that you have foamed the rim joists, that's the trickiest part of insulating a basement. If you're insulating the interior the insulation changes planes as you go to the first floor. You have to be extra careful to make sure that the insulation in contiguous through this area.

    1. frenchdude | | #7

      I'm fully aware of the spray foam being bad. The only reason I am using it, is because of irregular and rough finish on the concrete foundation.

      The price difference between a 3kw and 6kw is roughly $100. I am still in awe with Dana's calculations and I am wondering what are the consequences of having an oversized boiler?

      1. Expert Member
        NICK KEENAN | | #8

        Oversized boiler should not be a problem.

        1. jason_v | | #9

          minus the short cycling and whatnot, of course

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