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Community and Q&A

How to Calculate Water Heater Size for Slab Heat

emma_vt | Posted in Mechanicals on

I’m aware that in-floor radiant heating is a bit frowned upon in these parts…but we’re doing it anyway. Not as our primary heat, which will be a ducted ASHP, but to take the edge off the polished concrete floors we will have in parts of the Zone 6 house we’re building. We plan do this with some variety of DHW water heater rather than a boiler or ATW HP.

Can anyone tell me how to calculate the amount of water heater capacity we’ll need?

We’ve spec’d a 60 gallon Rheem HPWH for our DHW, plus a drain heat recovery system. I’m trying to figure out how I can calculate whether bumping it up to an 80 gallon unit will suffice to run low temp water through the floor. If not we’ll either do a second HPWH, or an on-demand ER unit. 

If anyone else is looking for similar info, I’ve found this presentation by Travis Smith really helpful.

Thank you!

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  1. Expert Member
    NICK KEENAN | | #1

    Not to be facetious, but how much heat do you want?

    Ok, let's say you want the slab to be at 80F so it doesn't feel cold. At a room temperature of 72F that's a temperature difference of 8F. Siegenthaler says you get 0.7 BTU /hr per square foot per degree so that's 5.6 BTU/hr/sf. How many square feet are you talking about?

    1. emma_vt | | #2

      I didn't include the variables because I was hoping for an equation to do the work myself, but if you're offering...

      Zone 1 (basement) - 950sf
      Zone 2 (half of main floor) - 400sf

      Room temp of 68

      1. paul_wiedefeld | | #4

        I think the coefficient from Caleffi is 2 Btu/hr/ft2/ degree F (floor temp - room temp). So for a 34k Btu heat loss for the entire house , you'd need 74 degree floors on the coldest day of the year. Since the majority of the radiant is in the basement, you'd probably have even cooler floors down there. I think your linked presentation is correct when it says "Radiant concrete floors are not ideal for a low energy home." That said, if the floor is maintained at a constant temperature (say 70?) and supplemented by the ASHP, I don't think sizing will be much of an issue and if one day the floor's circulator broke I doubt anymore would notice.

        1. emma_vt | | #6

          Thanks for the calculation, Paul.

          I'm not sure where you're getting that quote though - I just searched the whole presentation for it and it doesn't exist. It does say, "HEATED CONCRETE FLOORS CAN WORK IN LOW ENERGY HOMES IF DESIGNED PROPERLY" on slide 35. (His all caps, not mine)

          1. paul_wiedefeld | | #7

            I should have said paraphrased. On slide 24, he says "NOTICE THE THICKNESS OF THIS SLAB, THIS WOULD TAKE 36 HOURS TO RESPOND TO TEMPERATURE CHANGE! NOT IDEAL FOR A LOW ENERGY HOME!" (Also his all caps, not mine). Important to note that his 4 degree over setpoint guideline (8 btu/sqft) was with the floor as the sole heat source.

  2. Trevor_Lambert | | #3

    An 80 gallon water heater has the same heating capacity as a 60 gallon, so that is going to have minimal effect in your scenario. Not quite zero, because you will have a larger initial draw capacity for DHW, but recovery rate is going to be the same.

    I'm not sure your goal of "taking the edge off" is actually possible. Any heat you add to the slab is going to raise the air temperature. So if you have your heat pump set to maintain the air temp at 68F, if you put heat into the floor that just means the heat pump will do less work. In order to get the slab up high enough to be noticeably different on your feet, it probably means the heat pump won't be doing anything at all. Unless one of those zones is naturally cooler, in which case the floor heat may work ok.

    If this is a new build, and you're planning on decent insulation and good air sealing, the in floor heat is almost certainly not going to do what you're hoping. You'll just end up overheating the space. My house has in floor heat, but the coverage is just the outer perimeter, about 18 inches out from the wall. Even with that minimal floor space heated, in order to maintain room temperature the heated floor area was barely perceptible even in the dead of winter.

    If you have areas that aren't served by your heat pump, then in floor heat in those rooms would make sense. Note that you're still unlikely to get those floors warm enough to feel warm.

    For just about any amount of in floor heat, you probably want a second water heater. I would hook the two up in series, so that you can use all the capacity for either in floor heat or DHW.

  3. emma_vt | | #5

    Ok, ok - I was hoping to not have to explain/justify our system plans but I can see that inquiring minds are going to want to know. Zone 6, roughly 30k btu heat loss for the whole house.

    When we started building we were planning on doing an ATW heat pump and so put tubes in the basement slab accordingly. We moved away from that plan because it took six months to get anyone to give us an estimate and that estimate was astronomical.

    It's a passive solar house that will also have a three inch concrete slab poured on the main floor. The south facing rooms will have engineered wood over that, the north facing rooms will be polished concrete (laundry room, entry, master bath). On that floor it is only the north facing rooms that would have the radiant.

    We will have a 12k btu ducted heat pump for the main and second floors, and a separate 6k wall mount unit in the basement.

    I have zero expectation of warm to the touch floors as a result of this plan - just shooting for something warmer than actively frigid. I like the idea of running it off the DHW tank if possible to keep the additional infrastructure to a minimum. It may well turn out that we end up not needing it, but I have enough anxiety about ice cold floors that I want to at least future proof ourselves with options - especially since we already have the sunk cost of tubes in the basement.

    1. Expert Member
      NICK KEENAN | | #10

      Sounds like you've already poured the basement slab but not the main floor. If you haven't, just don't. Concrete has no business as a finished surface in a residential setting.

      Most of the talk about "thermal mass" in passive solar is pseudo-science. Concrete is terrible for the environment and won't make your house more comfortable.

    2. Expert Member
      MALCOLM TAYLOR | | #16

      "I have zero expectation of warm to the touch floors as a result of this plan - just shooting for something warmer than actively frigid."

      There's the rub: A properly insulated slab will track the indoor air temperatures within a couple of degrees, and with some solar gain may exceed them. With in-floor heat you won't have much appreciable gain over that. What makes a concrete floor feel cold is its conductivity, and it takes heat from your body any time it is below body temperature.

      I can sense the frustration you have at receiving advice beyond the narrow scope of your question, but it doesn't make much sense for posters to simply ignore what they see as flaws in the thinking behind it.

    3. chicagofarbs | | #23

      What underslab insulation do you have in the already poured basement?

      In CZ6 and air to water heat pump isn’t going to work well for you (yet).

      If you really want to keep it in and hope to rately use it, would an electric boiler be that bad of a compromise?

      Or you could have the system “ready” and go with a more suitable cold weather heat pump in the next few years when the technology progresses.

      Or you can scrap radiant idea entirely. How significant is the “sunk” cost ?

  4. Expert Member
    Akos | | #8

    Most HPWH have a 4000BTU heat pump unit. Generally hot water use is about 2000BTU average, so that leaves you 2000BTU.

    Assuming the 68F room, if you want to bring the slab up to 72 to take the edge off, that is:


    So your 2000BTU of extra capacity can heat about 250sqft of floor. Also keep in mind that at that point your HPWH is running continuously, so it might not last.

    If you really want a heat pump for the floor heat, you can look at a SANCO2 which is good for up to 8000BTU of space heat. Not cheap though.

    Probably the simplest is to go with a electric resistance tank or a small resistance boiler for the floor heat. As long as you keep the floor temperature low, the operating cost would be reasonable.

    1. emma_vt | | #9

      Thanks, Akos - appreciate it. Any particular reason why you think an ER tank would be a better choice than an ER tankless?

      I should maybe mention that we have no shortage of electric capacity. We're installing two 200 amp panels.

      1. Expert Member
        NICK KEENAN | | #11

        Not to complicate things, but if you're going with resistance heat, wiring under the floor is a lot cheaper and simpler.

      2. Expert Member
        Akos | | #17

        You only use a tankless unit when you are tight on space. Besides being a power pig, they are not zero maintenance because of lime scale. Putting more flow through the resistance tankless by using for space heat is also poking the tiger in the eye.

        As for resistance tank or resistance boiler, you'll have to do a bit of digging. Maybe out there, but I don't think there is a resistance tank rated for combi application. It certainly can work but without proper rating it would be a code violation.

        The simplest is really a dedicated small resistance boiler. This saves you the cost of your isolation heat exchanger and stainless pump. The cost of these two is probably near the cost the boiler. Some even come with outdoor reset which can simplify your install as you can most likely skip a thermostat if you dial in the reset curve just right.

        This still feels like an expense for basically doing nothing. Your dollars are better spent covering the exposed concrete flooring, even with a budget laminate. This would "feel" much warmer and since it is much more compliant, it would be way easier on your feet.

        For your tiled bathroom and kitchen area go for one of the better resistance wire heat setups (ie Ditra heat). Also make sure to install the floor heat sensor in a small pex pipe (5/16 pex is great for this) so it can be easily pulled out and replaced when it fails.

        1. emma_vt | | #19

          Thanks, Akos - That's helpful. I'll keep that in mind.

          Since running it off the HPWH we're putting in for DHW won't work we'll likely hold off on hooking the tubes up to anything for a year and see how it goes. We are going to go ahead and put pex in the main floor when we pour that slab to future proof ourselves for the day when the mini-splits reach the end of their useful life in hopes that ATW HP's will have become more of a thing.

          1. chicagofarbs | | #24

            Not a bad idea if you can justify the up front cost.

            Personally, I am in favor of hydronic systems as they typically reduce the overall amount of refrigerant in your building (you currently are running 3 split systems vs 1 to make hot water). Someone mentioned concrete being terrible for the environment, refrigerant (depending on the type) typically has a ridiculously huge carbon footprint.

        2. Expert Member
          NICK KEENAN | | #25

          "This still feels like an expense for basically doing nothing. "

          And given the environmental cost of concrete -- and the fact that it's a lousy interior surface -- I'd argue less than nothing.

          I realize I probably come across as having a bit of a bee in my bonnet about passive solar, but I just feel there's an awful lot of self-delusion that goes on with people who are trying to do passive solar systems.

  5. walta100 | | #12

    Consider that your idea of using the potable hot water to heat the building is a risky adventure at best and a code violation in many if not most locations.

    The risk is that when the heating system may not operate for months in that time the water in parts of the system maybe stagnant bacteria could grow in that water and people could be sickened.

    Legionnaire’s disease had been known to kill.

    My guess is that if you have built your house to a modern set code requirements the heated slab will never be more than 15° warmer that the air in the rooms as the slab will warm the air. So the water heater will provide 95% of the BTUs to heat your house at 4 times the cost quadrupling your electric bill.


    1. emma_vt | | #13

      If we used the same hot water heater for both purposes they would be hydraulically separated with a heat exchanger, not an open system.

    2. Trevor_Lambert | | #14

      An open system is no more risky than simply having a DHW tank. Like anything, it has to be properly designed, but that is trivial. You just plumb it so that the cold water intake to the water heater is in series with the heating loop. Every time hot water is used, the loop is flushed. No part of the system is stagnant any longer than any of the supply pipes in any house. It's not prohibited by the IRC, which is the basis for most local codes. They would have to specifically add the prohibition into the local code. So while there may be places where it's a code violation, it's almost certainly not most locations.

      I'm pretty much against any type of in floor heating as a primary heat source (unless it was heat pump powered), but if you're going to do it on a limited scale, the open system is the simplest, most economical way and I've never seen a valid argument against it.

  6. Expert Member
    NICK KEENAN | | #15

    I'm not getting the plumbing. So the floor is filled with cold water every time hot water is used? So the inlet is on the hot side with a check valve? Seems like if someone took a long shower during heating season the floor would get cold. And in summer the floor would be cold all the time, possibly to the point of condensation.

    1. Expert Member
      Akos | | #18

      You can read more about it here:

      It works quite well. It takes a lot of time and energy to budge the temperature of a slab, taking a shower will do almost nothing. In the summer time, you get a bit of cooling but nowhere near enough to cause condensation issues.

      The nice benefit of this type of setup is the slab works as a preheat for the incoming water in the winter time. This means you can get much more output out of a water heater for a fair bit of time, no issues with running 3 showers off a 200kBTU tankless unit.

  7. Jon_R | | #20

    Regarding "open direct water/space heating"

    There is a fundamental difference between systems that have warm piping sections where legionella will grow that feed back into the tank and typical DHW systems that don't. From what I've seen, it's an open question as to how much risk this adds.

    Electric (vs gas) tank water heaters significantly increase risk:

  8. Expert Member
    Akos | | #21

    Make sure somebody runs the math on the resonance of your floor system with the 3" of concrete added.

    Generally concrete is a good thing as it increases stiffness but it adds a lot of mass which can reduce your floor natural frequency bellow what people find comfortable (rule of thumb is to be above 10HZ).

    You don't want the floor to feel like you are waking on a water bed.

    There is also a general misconception that there is a benefit to a heated slab in a passive solar setup.

    My place has a light weight version of this (reasonable south facing windows with 1.5" heated slab) and even there the slow reaction time is an issue. After a sunny day, the heat doesn't tend to start up until the early hours of the morning, by the time the sun is out the slab is already warm, so the slab is not really helping things.

    I could program the thermostat to keep the heat off in the morning but who wants to wake up in a cold house in the morning? Spiking the heat in the morning does not work as the slab takes too long to heat up.

    Concrete is great for sound but doesn't do much else. Overall, the heated slab was not worth it, definitely going to skip it next time.

  9. chicagofarbs | | #22

    We looked at this approach for a similar high performance home in Chicago. Radiant in floor in a few select spaces just for comfort, seperate ASHP providing primary heating.

    We spoke with AO Smith and they advised against this with their residential units due to similar concerns someone else raised above. They are not designed to run at the frequency required to provide hot water for a radiant system. The compressor will go out way sooner than expected. They said their commercial unit would work much better in this configuration.

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