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Community and Q&A

Backup heat for mini-splits? Electric cove heaters?

DarkNova | Posted in Mechanicals on

I’m planning a house build and am seriously considering using mini-splits. The house will be in climate zone 7 with a -13F 99% design temperature, so I want to make sure I have a source of backup heat when the mini-splits turn off during really cold nights (-25F happens several nights a year), and also to make sure the bedrooms stay warm enough if the doors are closed.

I’m considering using electric cove heaters. I would put one in each bedroom and a few in the common spaces. It seems like it would be pretty cheap to put enough in to meet the entire heat load, if necessary, although I would hope that they would not have to run very often.

I’m wondering if anyone can share their experience with these type of heaters. Are they quiet or do they make cracking noises as heat is applied like some baseboards do? Are they are comfortable heat, especially when sleeping?

Or is there another option that someone would suggest? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, all evidence suggests that most cold-climate ductless minisplits will still be putting out heat when it is -20F or -25F.

    Second, it usually doesn't stay that cold for many hours in a row. With a decent thermal envelope, your house can easily coast for 12 hours if it has to.

    Third, you don't really need an electric resistance heater in every room. One or at most two electric resistance heaters, almost anywhere in the house, should get you through most cold snaps.

  2. Svig | | #2

    Martin, I am also building in zone 7, planning on mini splits. I assume I need to do a manual J or something to please the inspector. And that might mean putting in more backup heat than I hopefully will need. Is that thinking wrong?

  3. GBA Editor
    Martin Holladay | | #3

    You should direct your questions to your local building department. Local codes vary.

    Most codes require that a Manual J load calculation (or an approved equivalent method for performing a heating and cooling load calculation) be performed, but many jurisdictions don't check whether this is done.

    I don't see why backup heat would be required, as long as your ductless minisplits are sized to meet your load.

  4. charlie_sullivan | | #4

    There are some electric cove heaters that have design features meant to eliminate noise. I'm afraid I don't remember the brand.

    Even though not much is needed to get through cold nights, it's not very expensive to install, and it is not a bad idea to have a backup in case a mini-split fails, which might not happen for decades but it's nice to be comfortable while you wait for repairs. On the other hand, a wood stove is a backup that works even if the failure is the grid, rather than your mini-split.

  5. Expert Member
    Dana Dorsett | | #5

    Cove heaters are primarily convectors, but still give a comfort-edge over baseboard convectors due to the direct radiation. Due to the physics of the buoyancy of air they don't really distribute the heat into the air as well as finned baseboard convectors or wall panel-radiator type convectors. For fairly low heat loads they're fine- the comfort comes on before the temperature of the air rises much. But in a high heat load rooms temperature stratification issues may make them less than ideal. Electric radiant floor would be a more comfortable (but more expensive) option for very cold climates.

    The Mitsubishi FH and FE mini-splits have published capacity numbers at -13F, so it's possible to size them adequately for code purposes, but they will actually turn off at some temperature below -18F, and don't auto-start until the temperature rises to -13F. Second and third hand reports are that the published -18F turn-off number is really an upper bound, and that most will still work in to the -20s, but it's something that has to be considered.

    The Fujitsu RLS2H and RLS3H series have published capacity down to -15F, and are probably a better choice if you regularly see temps into the -20s, since they don't have that shut-down issue, and will keep on chugging away no matter how cold it gets, at some unspecified output capacity. Second hand reports are that the output air is noticeably cooler when the output temps hit -30F, but it's still putting out quite a bit of heat.

  6. Expert Member

    I specced radiant Cove heaters instead of baseboards in a house I designed last year and the owners are very happy with them. However the increase in cost was quite high. As Dana said, the heat they produce isn't entirely radiant, but you do feel the difference from the purely convective baseboard heaters.

  7. brp_nh | | #7

    The setup in our house and experience this past winter may be helpful for you. We are in a cold/snowy area of climate zone 6 (White Mountains of NH). I don't have exact measurements, but we had several nights around -20F and colder.

    We are happy with our Mitsubishi FH12NA mini split that heats our entire two story house (total conditioned space 1320 sq ft). Assuming you're going to put in one or two ductless units to heat an open floor plan, I do think you need to have a "doors open" policy in the house, at least for the majority of the day/night.

    We do have backup heat and turned it on a few nights, mostly out of caution and curiosity.....but I don't think we really needed it and I'm not sure if the mini split ever shut down. Regardless, a simple/inexpensive backup setup would probably be a good call in your climate.

    We have Stiebel Eltron wall mount in each bedroom and bathroom (500w)...two in the open downstairs area (one 500w and 1000w). They are inexpensive and look nice:
    Stiebel Eltron CNS-50-E

    The basic Convectair units are more expensive, but they have a digital thermostat and available in 120v:
    Convectair Apero 500w

    The digital thermostat would be nice compared to the Stiebel's imprecise dial and if you wired them for 120v, those plugs could be used for other purposes and the units taken off the wall outside of the winter season. Overall though, putting in a couple/few of the Stiebel units would be the most affordable adequate setup.

  8. Expert Member

    Brian wrote:

    "...those plugs could be used for other purposes and the units taken off the wall outside of the winter season."

    What a good solution.

  9. DarkNova | | #9

    Thanks for the responses so far. I do understand that theoretically the minisplits probably won't turn off at -13F, and if it goes lower the house can coast, but I'm looking to avoid complaints that a bedroom gets too cold on a super cold night, and if I can do that for like $100 per room or whatever a cove heater costs, that's worth it to me even if it hardly ever gets used.

    Some of the other units that have been posted are interesting. The main criteria I'm looking for is:

    - Quiet. I'd like to avoid fans if at all possible, unless I could be confident that it was really quiet, but there seem to be fanless options.

    - Safe for small children. For this reason I'd like to avoid baseboard, and I'm hesitant about any heater mounted low enough for crawling children to touch.

    - Comfortable. By this I mean basically I'd like to avoid the scenario where the unit turns on and you feel like you are right in front of a campfire, and then it cycles off and then you feel cold. This is one concern I have about the radiant cove heaters, although perhaps it is unfounded if you don't oversize the unit? And it would appear like some thermostats such as the Honeywell RLV3100 have triac controls which can apply only partial power to the unit, so it's not just on or off, so I would imagine that would help. But I haven't really read very much from people discussing this issue.

  10. Dana1 | | #10

    A $50 1500W oil-filled finned radiator type electric space heater would more than cover the load in almost any room to deal with comfort complaints. The radiator types are bulkier than some alternatives, but they don't have blowers or high-temperature focused radiation, making them more comfortable & fire safe in cramped quarters despite the bulk.

    For the visual on what I'm referring to, one such option among many:

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