GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Hello everyone, I was hoping for some advice on the size of electric boiler that I am currently arguing over with my plumber.

Adam Smerchanski | Posted in Mechanicals on

I am building my house in Kenora, ON which is zone 7A. Its a two story with a footprint of 768sq’. The second floor is open to below in about 1/3 of the footprint. It is a heated slab on grade with R15 under the slab except under the footing (which our engineer adamantly wouldn’t approve and is a whole other topic?)
The walls are R22 + two staggered layers of 1.5″ xps. The attic will be about 2′ of blowin-in celulose for at least R70 ish. I taped the osb and final layer of foam with Siga Wigluv for our air and wrb. I am hoping to achieve 1.0 ach. The windows and doors are fiberglass frame triple panes from Duxton Windows. About 35% of the south wall is glazing with low solar gain, the other walls have minimal glazing in comparison.
We are going with all electrical mechanicals (except the wood stove) as we will put at least 6kW, for now, of PV on the roof.
Our plumber is telling us that he wants to install a 20kW boiler which puts out 68,243 BTU’s. He said he can also disconnect half of the elements so it will technically be downsized. I am trying to tell him a 10 – 12kW boiler should be more appropriate for this kind of a build based on my research.
Any thoughts or opinions? Thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Charlie Sullivan | | #1

    In an electric boiler, the penalty for oversizing is small to zero, other than the cost, so it might not be worth arguing with him.

  2. Tim Brown | | #2

    Adam.... I think someone else will chime in about the plan to heat a very well insulated building with a heated slab. I believe your heat Load will be too low to use that scheme as the floor will be only Be slightly warmer than the room air.. that means almost room temperature concrete. I have read here numerous times times that a heated slab is most appropriate for a BIG enough heat load to maintain a floor temp well above room temp.

    Have you considered an ASHP.... I'm just north of K town and plan on two ASHP units with baseboard heaters as backup in case we aren't there to feed the wood stove.

    PS. Have you priced the PV system in Kenora? I am looking for a 10 kW ground mount system and got quoted $40000 Ouch!

  3. Adam Smerchanski | | #3

    Ya I was aware that a ASHP would of been the way to go but there is just something about a heated polished concrete slab we couldn't resist! And having a low heat load is why i'm concerned about the 20kW boiler being so oversized. Will it short cycle and lower its life expectancy or will it waste to much electricity? It just seems like overkill. I'm going to contact the boiler manufacturer tomorrow and get their take on this. For the PV, a friend of mine is in the industry in Manitoba and we are getting our gear from him. I thought prices here were a little high too.

  4. Expert Member
    Dana Dorsett | | #4

    Oversizing an electric boiler won't use more electricity, and most have pre-settable off times between active cycles to prevent true short cycling of the type that would shorten it's life.

    Yes, cushy warm floors are nice, but you could just set the slab temp with a floor thermostat, and manage the room temperature with heat pumps. In a place as cool as Kenora there will be time where you may need to have the slab deliver the majority of the heat, which allows you to size the heat pumps for the average rather than the peak heat load, which improves their shoulder-season efficiency.

    Unless you've actually run the heat load numbers you don't really know how oversized the 20kw boiler is, but without much better than a WAG based on location an crude construction description I'd say 20kw at least 50% oversized. If you installed a couple of 0.75- 1.25 ton cold climate heat pumps (one per floor) you can easily rationalize dropping the electric boiler to 8-10kw for keeping the under-foot temps in the "aaaaahhh" range and still keep the place up to temp when it's -30C outside (Kenora's 99% outside design temp, at least a the airport: )

  5. Expert Member
    Malcolm Taylor | | #5

    In a well insulated house that size wouldn't there always be a conflict between the floor temp. slab thermostat and the room temp. heat pumps? For a slab to feel heated it would have to be what - somewhere around 90 degrees? Would the heat pumps ever come on?

  6. Charlie Sullivan | | #6

    Malcolm, yes, you wouldn't be able to heat the slab very warm without overheating the room, I imagine Dana is thinking about having it at say 76 F which might feel slightly nicer than having it at 66 F.

    Adam, as Dana says, the boiler can take care of itself as far as avoiding really short short cycling, and in any case, there's no efficiency penalty for short cycling, and the worst case would be wearing out a relay in 5 years instead of 20, and that's only a $10 part.

  7. Expert Member
    Malcolm Taylor | | #7

    I just took a wild stab at how warm someone would want a slab to be for comfort. Do you think it has to be over body temperature, or is there some point (say your 76 F) where it is appreciably nicer than an unheated but well insulated one?

  8. Stephen Sheehy | | #8

    Malcolm: We have unheated concrete floors and in winter, they are cool, but acceptable. We have a heated bathroom floor and at 73° it is noticeably warmer than the rest of the floors. I don't really care, but my wife likes a warm floor when showering. We keep the floor heated all winter.
    We have 4" of foam under the floor, so the energy used is not excessive.

  9. Tim Brown | | #9

    I'm thrilled to see this being discussed. I was sure (based on previous reading) that a heated slab was out of the question in a case like Adam's.
    In my build which is similar (in insulation values and size) I plan on heating with mini splits and thought that baseboards were the only logical choice for back up.
    I would MUCH prefer the heated slab (even if only warmed to the mid 70s F) over baseboard backup heat. Having the slab a constant mid 70's would be wonderful.
    I run the electrically heated bathroom tile in my daughter’s condo at about 78f and that plenty for bare feet to be comfy.
    I have related questions that are performance related...
    #1, During most of the year (shoulder seasons) when the ASHP can supply the majority of the houses heat requirements... I assume that the slab would be idling at or near room temperature (heated as part of the building envelope by the ASHP) it would only need to be “boosted” from room temp (say 72f) to the barefoot tolerable 78f or so. Since the temp delta is small in this case how do the systems “live with each other?”
    #2, During the coldest part of the year when the ASHP can’t supply the heat requirements but may supply some.. I was wondering how the systems “live with each other?” In this scenario I would assume the boiler would take over the load (like baseboards would).. How are these two scenarios dealt with planning wise?
    Thanks for letting me pile on to this thread, I think my situation is related “enough” to justify not starting my own..

  10. Expert Member
    Dana Dorsett | | #10

    A floor that's 90F is uncomfortably hot and delivering a HUGE amount of heat per square foot into the room.

    A 75F floor feels great, and is delivering on the order of 10 BTU/hr per square foot into a 70F room.

    A 72-73F floor also feels nice, at just 5 BTU/hr .

    Assuming just the 738 square feet of slab is getting the radiant and not the 250 square foot loft/second floor, the surface temp of the slab would be around 80F, delivering ~15,000 BTU/hr into a 70F room with a 4.5kw electric boiler running full-on. Much beyond that the comfort level will be going down, not up.

    A single 3/4 ton Fujitsu 9RLS3H delivers about 9000 BTU/hr running flat out when it's -25C outside, and is still delivering almost that much when it's -30C out (Kenora's 99% temperature bin.) A pair of them would deliver over 15,000 BTU/hr

    It's highly unlikely that your 99% heat load is more than 30,000 BTU/hr, but it's probably more than 15,000 BTU/hr. It wouldn't be insane to put 10kw of boiler in there to have backup capacity in the case of one or both mini-splits failing. But from a comfort perspective running the radiant slab with a floor thermostat set to 23-24C (74-76F) delivering 3000-5000 BTU/hr into the room and letting a mini-split (or two) manage the ambient air temperature puts most of the heating load on the high efficiency heat pump(s), while maintaining barefoot comfort (at any room temp.) If the heat pumps crap out, with a 10-12kw boiler you can still crank it to where you can practically fry an egg the slab to keep the upper level rooms warm.

    If you ran a 20kw boiler flat-out into 738' of slab, that's 92 BTU/hr per square foot. To emit that much heat the surface temperature would need be a dangerously high 115F to deliver the 68,240 BTU/hr of heat into a 70F room. If the thermostat or boiler controls ever failed-on, that really COULD happen, and is reason enough NOT to install a 20kw boiler.

  11. Adam Smerchanski | | #11

    Thanks a lot everyone! This site truely is amazing in the wealth of knowledge and people's willingness to share it. Love it. We had planned to complement the boiler with a ductless minisplit down the road, especially if we find it getting too warm in the summer, unfortunately we just didn't leave room for it in the budget at the moment. When we do get to installing one, I will now make sure I rig something up so the boiler thermostat so it is right on the slab. Great idea. Also I will make sure they don't install anything over 12kW now as i never even considered a situation where the controls failed and it was on full bore for a length of time. Although pretty sure my wife would be happy with 115F, especially if it happened in the winter. Thanks again everyone!

  12. Jon R | | #12

    If you want the efficiency of an ASHP and a radiant floor, consider this:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |