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Including Hydronic Heat Slab for comfort

idahobuild | Posted in General Questions on
Hey All,
We have a 2062 sq ft house with slab-on-grade which will be using an Air Source Heat Pump (Mitsubishi) as the primary heat source.  The finished floors will be polished concrete and we’d like the floors to be subjectively ‘comfortable’ to walk on.  Since we’re dealing with concrete and won’t be able to get a do-over after the fact, I want to install the pex tubing for the floors – just in case.  At about $100 for 400 ft it seems like a good cost for potential future benefit. 
The house is full electric. So, the heat for the comfortable floors would likely come from tankless heaters and I need to add that load to the electrical service calcs.  For this future system I need to know BTUs/h and GPM type info.
The manual J has the house’s Heat Loss at 20,000 BTUs. I know that the pros in the area are slammed, so I want to see if it is even feasible for me to do.
So, I was thinking (I know…not a good idea) that 20,000 BTU/h would be plenty to keep the concrete at a comfortable level. There will be R10 XPS under the slab. 
I am thinking that I need to know how much energy is needed to raise the temp of the slab.
Slab Volume:
2062 sq ft x .333 ft (4″) deep = 686 cu. ft
686 cu. ft x 133 density conc. = 91,323
Heat Capacity Conc (0.2 BTU/h) = 91,323 x 0.2 = 18,264 BTU/ per 1 degree F.
The formula doesn’t seem to account for the insulation. Is there a better formula to use to calculate raising the temp of the conc. by 5 – 10 degrees F?
For GPM I found:
GPM = BTU/h ÷ (DeltaT x 500)
18,264 ÷ (20 x 500)
18,264 ÷ 10,000
1.8 GPM of flow.
Can of worms now open.

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  1. plumb_bob | | #1

    Have you lived on a concrete floor before? I find them the farthest thing from comfortable because of the hardness.

  2. Expert Member
    BILL WICHERS | | #2

    Looks like you're working with specific heat. Specific heat is the energy required to heat the mass of a given material by a specific number of degrees, and since it is ONLY to heat the MATERIAL, insulation and heat loss don't come into play. Heat loss has to do with how much energy it will take to MAINTAIN the target temperature.

    Think of a rechargeable battery. The specific heat is like the number of watt hours it takes to fully charge the battery IN THE CHARGER. Heat loss is like putting the battery into a flashlight and shining the light -- that light takes energy from the battery, similar to the heat loss from the slab. The energy used by the light reduces the voltage of the battery, similar to how the heat loss from the slab reduces the temperature of the slab. If you're trying to raise the temperature of the slab AND overcome heat loss, that's like charging the battery in the flashlight while the flashlight is on.

    If you put in 18k BTU to raise the slab by one degree from whatever temperature it started at, you need to do that for each degree (so to raise it 10 degrees would take about 183k BTU, for example). You only need to put that energy in to RAISE the temperature of the material. Once you get to the target temperature, you only have to put in enough energy to overcome losses, which is likely to be less than what you put in initially, but there is a time component to this too, so it gets more complex. You might only need to put in 5,000 BTU to maintain the temperature, but that involves heat loss from the home, the outdoor ambient temperature, etc. etc. The colder it is, the more heat loss you have, so the more heat you have to put into the slab to maintain the setpoint.

    All that fun stuff said, you'll find a lot of postings on here about the energy it takes to make a heated floor seem warm is enough to overheat the home. To make something feel "warm" to the touch, it generally needs to be somewhat above your body temperature. Some quick Google mojo says that things start to feel "warm" when they get up over about 86*F. If you maintain the slab at 86*F, it will, over time, increase the temperature of your home to reach some sort of equilibrium between the slab temperature (which I assume is going to be thermostatically controlled), and the heat loss of the home. That probably means your home will get up around 80*F or so, and you won't be able to directly control that if you are keeping the slab temperature constant. If you try to keep the temperature of the home constant instead, the slab won't feel warm. The basic consensus on radiant floors is that they tend to overhead the homes they're in if you want to keep the floor feeling "warm".

    I would strongly advise against electric resistance on-demand heaters here. You are NOT saving energy with those over using natural gas heaters, you're actually likely to be using more fuel than if you burned the fuel directly to heat the water with a natural gas fired heater. "All electric" is only really reducing energy consumption when you use a heat pump, which may be an option for you too, but I have no experience with heat pump boilers -- I don't think they are very common in the US.

    One last thing, from someone (me) who works mostly commercial projects which tend to be concrete floors and steel structures: walking around on concrete all day, which has no give at all, is brutal on your bones. I use extra padding in my shoes (gel inserts), and change them out periodically as they degrade over time. I've had issues with aching feet before doing that, and it was my doc who recommended the inserts. I've heard other people talk about concrete floors and bone-related issues too for many people. I would not consider concrete floors "comfortable" to walk on for extended periods of time for that reason. Wood structures have a little bit of give to them, so you get less impact when walking around, letting the wood floors absorb some of that impact instead of your bones.


    1. idahobuild | | #3

      Great analogy Bill. I get what you are saying. My uncle's house has heated floors but only in the master bathroom. The floors are warm to the touch and that room gets hot. I think that they are experiencing exactly what you are describing -- floors are too warm and they are making the space HOT.

      We have considered the wood structure base. Growing up in a slab-on-grade house, I have found it difficult to get used to the walls moving and house creaking with every foot step or door closure. Taking that into consideration we've accounted for the ability to add padding and carpet later -- when my bones can't take it any more.

      1. Expert Member
        BILL WICHERS | | #4

        Walls shouldn't move and floors shouldn't creak. If you want more solid-feeling walls (and also a quieter home), use 5/8" drywall everywhere instead of the 1/2" more commonly used on residential projects. The material cost difference is around a dollar a sheet, and the labor and materials to do the finishing is exactly the same.

        Creaking floors are usually loose nails in the subfloor or sill plates on walls. Ring shank nails will help, screws will pretty much eliminate the problem. I have found some spots in my own home where the creak was from a nail in the sill plate, and I fixed those issues with a screw driven in at an angle behind the baseboard. In new construction, it's easy to use adhesive and/or screws to prevent future squeaks.

        If you want to build on a slab, you can put down a cushioned pad under wood flooring to provide a bit of give. You may even find out it's not that expensive when compared to polished concrete, since the polishing process is surprising expensive (I've had a few 60,000+ square foot projects over the past two years that needed polished concrete, and those guys get a lot of money!).


        1. Expert Member
          DCcontrarian | | #6

          With 5/8" drywall you can go to 24" joist spacing and it ends up saving money.

      2. Expert Member
        Deleted | | #7


  3. Expert Member
    DCcontrarian | | #5

    The rule of thumb is 2 BTU/hr/square foot for every degree of difference between the floor temperature and the air temperature.

    First, congratulations on the 20K BTU heating load, that's an accomplishment.

    With ~2,000 square feet and a 20,000 BTU/hr heating load, you need a 5F difference between floor and air. So floor at 75F and air at 70F for example. That's not going to be very noticeable.

    Here's the problem you're going to run into:

    I think the specific heat of concrete is more like 0.5, and you're slab is going to have a heat capacity of more like 50,000 BTU/degree F. Let's say it's a cold but sunny winter day, your slab is putting out maybe 15K BTU and you're getting another 2K BTU from solar gain. Then the sun goes behind a cloud, and you need to make up that 2K. In order to do that you have to raise the temperature of the slab by half a degree, which takes 25,000 BTU. But your whole system does 20,000 Btu per hour, so it takes 1.25 hours of output just to get to the point where you're meeting demand. Then the sun comes back out, and it takes another 1.25 hours to cool the slab off again. What you find when your radiative surface has a lot of heat capacity is that it's hard to maintain a setpoint. This is particularly acute in the morning and evening when the sun rises and sets.

    It used to be believed that having a lot of "thermal mass" in the radiative surface was a good thing, we've since learned that the opposite is true. Systems like WarmBoard advertise that they have low heat capacity and can respond quickly to changes in demand.

    As noted above, concrete makes for a lousy residential floor.

    Here's what I did in my own home: heated floors only in the bathrooms. This allows them to run warmer than they would if I did the whole house. Minisplits to make up the difference between what the floors put out and the heating load. I have a two-stage thermostat, with the floors as the first stage and the minisplits as the second stage, so that the minisplits only run when the floors are unable to keep up. In the winter in the evening, when the sun goes down the minisplits kick on for a while until the floor heats up. In the morning we get a little overshooting when the sun comes up but it's not unpleasant.

    In the summer the minisplits provide cooling.

  4. paul_wiedefeld | | #8

    If your heat loss is 20kbtu and you have an ASHP, I’d keep the electric boiler under 20kbtu - so about 6kw max. If there’s ever a need for fast response, you have the heat pump. The floor will never be responsive and won’t be all that warm either - the 20kbtu heat loss happens only a few days per year. Probably 80% of the time your floor will be 72-73 degrees.

    Also, get a boiler, not a tankless heater. They’re cheap and designed for heating. It’s a subtle difference.

    The 20 degree delta T isn’t for floors - I’d switch that to 10 degrees, thereby doubling the flow needed. 20 is fine for radiators, baseboard, etc because you won’t care if the bottom of a radiator is 20 degrees less than the top but you might notice if a floor is cooler at the end of a loop.

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