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Community and Q&A

Electric Boiler for Radiant

$#E$#$ | Posted in Green Products and Materials on

2400sqft home with radiant heated floors in Southern Ontario and wanting to go all electric for the “boiler”. I have looked at air to water heat pumps, but would prefer to wait for next generation. I also only have 200amp electric service and its time of use, at half the rate at night.

One potential supplier had an interesting suggestion, that at first I thought was a bit of a hack, but after some thought, wanted to ask others opinion.

The suggestion was use a stainless insulated hot waster tank that supports multiple electric heating elements (enough for the required btu for the floor heating) and keep that at floor heating temps (warm water). This tank would also store heat for higher priced, lower needed daylight hours.

Then use a separate instant hot water heater for hot water supply.  This could be connected inline to boost the warm water from the tank in winter when well water is really cold or run alone in summer more efficiently – with the tank heat turned off.

The elements can be replaced by a air to water heat pump and the tank used as storage later.


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  1. Expert Member
    Akos | | #1

    You were mostly good till you added a instant resistance tankless unit into the mix.

    Install instead an oversized standard tank put this on timer so it only runs during the night. If you are worried about running out in the morning, you can put the timer only on the lower element and crank the temperature only on the lower element. This way if there is high demand, the upper element can still kick in, at night time the lower element kicks in to get the whole tank extra hot.

    1. $#E$#$ | | #2

      I believe the idea for the instant hot water was to bump the temp up 20 degrees odd since the floor temp water would need to be lower. I guess if you use heat exchanges though the tank water could be kept hotter.

      So electric boiler or storage tank with heating elements. My concern would the efficiency in summer of keeping the tank heated. Thoughts?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        The problem with that is storing the domestic hot water in a tank at too cold a temperature to discourage bacterial growth.

        1. boxfactory | | #17

          What if the Rheem Marathon was used in this case? Could the insulated tank keep the water at a high enough temperature to inhibit bacteria?

          Part of my motivation for asking this, is I was considering doing this exact thing for domestic hot water. However, I’m not rushing into such a system due to the reason described above. Any info would be much appreciated.


          1. Expert Member
            DCcontrarian | | #18

            I didn't do an exhaustive search, but on the Rheem website it looks like they only recommend their gas water heaters for space heating.

          2. Expert Member
            MALCOLM TAYLOR | | #20


            I think the problem is the desired temperature for the floor heat, not the ability of any type of heater to maintain a set temperature over time. Depending on who you listen to, domestic water heaters need to keep their contents at between 120F and 140F to be safe.

      2. Expert Member
        Akos | | #7

        It makes no sense to use the floor heat tank for pre-heat on your DHW. Even with a heat pump, the cost of the coil or heat exchanger for this is more than the upgrade to a heat pump water heater. An open system is asking for trouble especially if you are talking about 100F water.

        As for the floor heat and buffer tank, you can keep the buffer tank much hotter than what your floors need. A standard mixing valve (or if you want better something with outdoor reset) is used to bring it down the temperature your slab needs. By keeping the tank hotter, it stores much more energy and since the slab temperature is decoupled form the tank temperature, you get better control and comfort.

  2. walta100 | | #3

    Have you done the math?

    My wild guess is the house is leaks air like crazy and has very little insulation. If my guess is right and your coldest day has a high temp under -20°F you will need to fill 25% of the home with 200°F water in order to be above 65° after 12 hours.

    Or you could hang a few mini split heat pumps on the walls and cut the number of kWh required by 75% and pay the higher rates and still have a lower bill.


  3. $#E$#$ | | #4

    No new build on plan still. It be tight house with huge thermal mass (on heavy slab). In short, the problem is the tank water temp ideally will be la lot lower than my tap water - requiring 2 temperatures of hot water. The thermal mass makes heating by air less attractive and less comfortable. It take more than 12 hours to heat that slab by air.

    1. paul_wiedefeld | | #8

      Two temps are needed but this is a clunky way of doing that. If you go the electric boiler route, keep the tank at DHW temperature and mix down to floor temperature. No need to get a tankless in the mix. There’s a couple of ways of doing this, but the gist is that there’s a single boiler.

      1. $#E$#$ | | #13

        This was my initial plan, and as I said, it sounded a bit like a hack. But then the keeping stored water at lower temps and only in winter nagged on me along with if demand heater really do saves energy use, using it only for the warmer half of the season, made me wonder if it really was.

        Looking at some electric boilers though, they seem to do the same thing anyhow , just in one device. So the hack is more cobbling it from different parts.

        1. paul_wiedefeld | | #14

          Yeah, tankless heaters don’t save significant energy that’s the hang up. What saves energy (this is for gas/propane) is condensing which is not something the manufacturers/contractors explain well.

          1. Expert Member
            DCcontrarian | | #16

            I would argue in this case there's no energy savings from tankless.

    2. Expert Member
      DCcontrarian | | #10

      The heavy slab isn't going to help your efficiency at all. Unless you have some weird structural requirement, do a normal slab.

      In general, concrete makes a lousy floor. It used to be believed that having lots of heat capacity was good for radiant heat. We've since learned the opposite is true, ideally you want zero heat capacity in your radiative elements so that your output responds quickly to changes in demand.

      1. $#E$#$ | | #12

        Yes structural engineered requirement due to soil, water table, grade and buildings weight. Its now a feature.

        1. Expert Member
          DCcontrarian | | #15

          In that case I would recommend a layer of insulation between the radiant heat and the concrete and a low heat-capacity system like WarmBoard. (Not specifically recommending WarmBoard, just giving them as an example of a low heat-capacity system).

  4. walta100 | | #6

    Time has proven that the 1970s pipe dream of thermal mass as heat storage are impracticable in reality.

    Very few places are left that most people will live without air conditioning (cooling) you would be crazy to build a house without the ductwork for cooling today in most locations. The fact that the ductwork exists makes you makes your radiant floor heat an very expensive luxury that is going to turn out to not be very luxurious as when install under the tight well insulated building you described makes the temp of the floor is going be much cooler than the skin on your feet so the floor will not even feel warm.


  5. BirchwoodBill | | #9

    I use a Taco RMB connected to my hot water tank, it works as an injection mixer to my Warmboard manifold. It will mix the water down to 95F water. One side is potable, the other side is glycol.

  6. Expert Member
    DCcontrarian | | #11

    Resistive electric tank water heaters are so cheap there's no reason not to have a separate one for hot water, it makes everything simpler.

    Is the floor going to be the only source of heat in the house? If you have any sort of conventional heat then a heat pump water heater makes a lot of sense. It makes no sense if you're heating with electrical resistance. They can only be used for domestic hot water, not space heating.

    It may be different in Canada, but I'm not aware of any resistive water heaters sold in the US that are listed for space heating.

  7. Expert Member
    DCcontrarian | | #19

    Here's the thing about this question: If you're doing resistive heat, there's no reason to do hydronic. It's simpler and cheaper to put the heat where you want it with resistive elements.

    Future-proofing is a gamble. Hydronic radiant floors have to be carefully engineered, how can you do that if you don't know what your water temperature is going to be? What tubing spacing should you use? At this point we don't know if the winning technology is going to be conventional heat pumps, which do best with relatively cool water, or something like the CO2 pumps that produce hotter water. Or maybe something completely different.

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