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Electric-resistance backup heaters for high-performance homes

user-3813901 | Posted in Mechanicals on

Hey There Fellow Tradespeople,

I’m looking for advice on sizing and locating electric resistance back-up heaters for high performance homes that use mini-splits for primary heat.

Specifically wondering:

1. Do old rules of thumb, like placing them underneath windows, still apply?

2. If I know the building’s heat loss in Btu/hr, can I simply aim for that number to establish a count and sizing of heaters, and disperse them as evenly as possible?

3. What considerations should I make for a two story cape style home? Should I aim for 2/3 of my heating load on the first floor and make up the remaining 1/3 on the second floor?

4. I’m interested in using a combination of standard electric baseboard heaters, and a few “Envi Wall Heaters” for smaller rooms to meet the demand. Anyone out there have any experience with incorporating “Envi Wall Heaters” into their designs, as back-up heat? ( see link)

Thanks so much!

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  1. STEPHEN SHEEHY | | #1

    Why Do you need backup heaters at all? Minisplits are pretty reliable. If you really worry, buy a couple of cheap portable heaters.
    In a two story house, you'll probably use two minisplits. They won't both break at the same time. And a well insulated tight house will manage fine with one minisplit out of commission.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Stephen.

    That said, if this is a new high-performance home, location of the heaters is more flexible than it would be in an old, leaky home.

    No matter whether the home is old or new, anyone who is designing a heating system should perform a room-by-room heating load calculation. This is a code requirement. So if you are following this requirement, you should know the design heat load of each individual room.

    If you want to design a heating system capable of heating the house when both minisplits break, use your room-by-room heating load calculation as your guide.

  3. brp_nh | | #3

    I've chimed in on questions like this a few times because we installed a few hard wired Stiebel Eltron wall mount electric resistance heaters in our house (zone 6 in the NH mountains, two story, one mini-split first floor, high performance building). We probably use the wall heaters under 10 times per winter and only if the overnight lows are expected to approach -15f or -20f. But they aren't necessary, more just used for extra comfort on the coldest nights.

    As mentioned by others, backup to a mini-split(s) may not be necessary, but I can understand considering it. The answer to the overall question really depends on preference and budget. If you or your client want to wing it and keep costs down, then just wire some extra outlets in possible wall heater locations or plan on cheap portable heaters mentioned by Stephen.

    1. Since they won't be used much or at all, I'd just plan the location so it doesn't interfere with furniture, people, and movement.

    2 & 3. Matching the house's full heat load seems excessive. I'd just place them in some key locations like bathrooms/bedrooms/living and just enough for worse case backup and/or extra comfort.

    4. I personally don't like the aesthetics of electric baseboard heat and it would collect dust and dog hair. I'd skip the baseboard entirely and plan for Envi or Convectair wall heaters.

    If building (high performance home w/ mini-split) again, I'd just go the route of extra wall outlets (and only buy/install the wall mount heaters if needed) or have a couple cheap space heaters stored in a closet.

  4. Expert Member
    Michael Maines | | #4

    One additional point I don't see mentioned above is that some insurance companies require a heater in every room that can automatically keep the room at 68°F. Installing cheap electric baseboard heaters does not cost much. A 24" to 36" run is more than enough in most rooms in a high-performance or Pretty Good House. Envi heaters are nice (I have one) and look good but they don't have many advantages over electric baseboard heaters, which cost less.

  5. Trevor_Lambert | | #5

    1. Do old rules of thumb, like placing them underneath windows, still apply?
    Depends on the windows. If you have PHI certified windows, I'd say no. In either case, one under every window in a super insulated house is probably going to be overkill.

    2. If I know the building's heat loss in Btu/hr, can I simply aim for that number to establish a count and sizing of heaters, and disperse them as evenly as possible?

    3. What considerations should I make for a two story cape style home? Should I aim for 2/3 of my heating load on the first floor and make up the remaining 1/3 on the second floor?

    4. I'm interested in using a combination of standard electric baseboard heaters, and a few "Envi Wall Heaters" for smaller rooms to meet the demand. Anyone out there have any experience with incorporating "Envi Wall Heaters" into their designs, as back-up heat? ( see link)
    1. Do old rules of thumb, like placing them underneath windows, still apply?

    Not too sure about the other questions. I can say that in my unfinished home, through the winter, the second floor was generally as warm as the main floor with no heat whatsoever. I expect that to change somewhat once the drywall is up, but it's hard to imagine how it would require a 50:50 split.

    We considered a mini split. Even with huge rebates right now, it just didn't make economic sense, when we considered how infrequently we expect to run the electric heaters (as little as zero, but I'm putting them in there just in case). Just one mini split would cost three times what we paid for 6 cove heaters. How useful is just one mini split, and how do you distribute that heat around? We have 4.5kW in-floor heating, plus 5.4kW backup heat, split 2:1 main floor to 2nd floor. We opted for radiant "cove" heaters. That way we don't have to worry about keeping furniture and body parts clear of them.

    All of this is probably over-kill. We originally only had the in-floor heat in the design. We freaked out a bit when we fired it up in the middle of winter and it took 10 days to get the slab temperature from 13C up to 20C. Hence the back-up heat plans. However, since then the temperature doesn't vary much. I haven't had the heat hooked up in over a week, and the temperature only swung from 19C to 17C, and in that time the outside temp dipped to below freezing several times, as low as -5C; and most of that heat loss can probably be attributed to having the doors open for several hours on a couple cool, windy days.

  6. user-3813901 | | #6


    Normally we provide some type of backup/ supplemental heat in these types of buildings, for subzero snaps that challenge mini-splits, ( which are rare and short lived), and for rooms that are far from the primary wall unit. Electric resistance heaters are good cheap insurance against call backs.

    The building I'm working on now is a small footprint, open floor plan, two story cape, using a single zone Mitsubishi Hyper Heat, with only electric resistance heat on the second floor.


    I suppose I won't agonize over placement, but should follow your advice and do a room by room calc., as you suggest.

    Thank you both!

  7. Expert Member
    Dana Dorsett | | #7

    Radiators/baseboard/registers under windows was the old-school way of preventing copious window condensation on single pane windows, and to counteract /mix with the window ventilation air with the window cracked open (a safety precaution in the days of coal heating, with it's high carbon monoxide potential.) There's no good reason to put them under high performance windows.

    The whole-house load number isn't as relevant as the room's load at the code required minimum interior design temperature (typically 68F, at 3 feet from the floor, per the IRC). There's no need to install more resistance heating than what's needed to fully heat the room, but may be necessary for code compliance to provide that much if the 99% outside design temperature is below the extended temperature capacity specs for the mini-split in question (even if it's likely that the unspecified output of the mini-split would still cover the load.)

    Most individual room loads in a high performance house will have loads well below that of an individual mini-split, and oversizing by a large factor will negatively impact the as-used efficiency. Installing mini-split heads in the large open spaces that can fully cover the load of that space with some margin that would cover the adjacent space loads, with only the doored-off spaces set up with the minimum resistance heating necessary to cover their individual room loads would be about right. The spaces with the properly sized mini-split heads don't need any sort of back up as long as the capacity specs indicate that it can cover the load.

  8. Jon_R | | #8

    Depending on weather, height, etc, even good modern windows can create excessive drafts. There is sometimes still a good reason for registers under windows.

  9. user-3813901 | | #9


    Thanks for your detailed response. I expect to use the electric heaters about as much as you have in your building. Another thought was that during a sustained power outage, which are rare, I could use the gasoline generator I already own, to run the baseboard heaters for bit, and not subject my mini-split to "junky" power output that the generator creates. It was suggested to me by a colleague that running a mini-split on a generator might not be a good idea, for that reason. I need to check with Mitsubishi on that. Might not be an issue.

  10. user-3813901 | | #10


    You're right about insurance companies wanting a back-up heat source. They are not particularly fastidious about verifying those details, in my area, but they do ask.
    At least the Envi Wall Heaters boast half the wattage consumed for a comparable sized electric baseboard unit. I have not verified this, but it sounds nice. : )

  11. user-3813901 | | #11


    Our windows are PH Certified; made by Klearwall. Agreed that under-window placement for heaters is probably pointless. ( see Dana's comment)
    On another project we recently completed, we used a single 8' strip of electric baseboard heat, located in the basement to heat 1200 SF of finished space, before the heat pumps were installed. Basement and main floor were toasty all winter during finish construction phase. Subs were complaining that it was too hot and kept opening the door; testament that R-20, 30, 40, 60, works. Comfortable in every room, no matter how far from the heat source we were.

    Thanks for sharing your set-up and experience with it.

  12. user-3813901 | | #12


    I assumed as much, on under-window placement. The approach you describe to sizing the single zone mini-split is close to what we've planned. Main floor is one big room, with two tiny side rooms.

    Generally, I have more experience building and working inside of the handful of single level high performance homes we've built, and less experience with two-story cape style buildings. I'm curious to see how heat stratification factors in, ( probably not much), and how cozy the far-away second floor rooms stay, with reference to the main floor.
    In the end, I'll likely have more "backup" heat than I'll ever use, but it's a cheap experiment that will inform future projects. Best to make dumb moves on my own house, and not repeat them for customers. : )

    Thanks for your input. I was hoping you'd chime in.

  13. STEPHEN SHEEHY | | #13

    Michael: We had to switch insurers when we built our new house, because the insurer we used for our old house claimed a heat pump could never keep the new house warm. Hanover Insurance was happy to cover us.
    Dylan: I suppose if you build in a place that regularly sees -30°F or colder, you might need supplemental heat. But where we live in Maine, occasional -15°F hasn't been an issue. We have two minisplits. One in the approximately 900 square foot great room and one in the master suite. The one in the great room basically heats the whole house. The guest room has a Convectair wall mounted heater, but it's only been on once. If I were doing it again, I might opt for a single cheap portable heater.

  14. user-3813901 | | #14


    Agreed. Sustained subzero (below - 20) events are rare here in the Northeast Kingdom of VT, and it usually warms to within cold climate heat pump range during the day. Often, it's sunny during those types of cold snaps, which helps of course.
    Nice to hear your Maine home is cruising along nicely without much need for supplemental heat. Thanks!

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