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Community and Q&A

Back-Up System for Minisplits

AtTheBeach | Posted in General Questions on

For those in cold climates with minisplits, do you have a backup system and if so, what? We are considering switching over (from oil-fueled boiler) and are thinking thru whether we’ll need supplemental heat. The garage & utility room are directly under the master bedroom – they each have hydronic baseboard heat – and we’re worried it’ll be too cold without heating those spaces.

-New-to-us 20 year old, energy-efficient Canadian modular house
-Zone 6
-Failing oil-fueled boiler with no chimney and side vent that makes outdoors smell like a truck stop. Also noisy and right under master bedroom.
-We’ll heat to 55 for most of winter, use a lot in shoulder seasons and summer.
-1200 sq ft mostly open layout with 2 small rooms at walkout basement level so minisplits would probably do well
-Propane also an option with tank onsite already.


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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Ruth,

    Great question. This information is sure to be of interest to you: Backing Up a Minisplit Heating System.

  2. AtTheBeach | | #2

    Thanks. Since the house is filled with existing hydronic baseboard heaters, we were also wondering about a small system that would kick that on when it's 20 below, or possibly only the downstairs (currently on a separate thermostat) which includes the utility and garage. But then it seems like we'd be buying two systems rather than one. Is there such a thing as a small propane boiler that might accomplish this?

  3. Expert Member
    Dana Dorsett | | #3

    >"We’ll heat to 55 for most of winter, use a lot in shoulder seasons and summer."

    At 55F indoor temps mini-splits have a lot more capacity than you might think. The capacity at -20C would be pretty much the same as their AHRI tested maximum numbers @ +17F (-8C). Not all mini-splits can be set to 55F, but Mitsubishis can be set as low as 50F (+10C) when in "SMART SET" mode. Fujitsu's have a "minimum heat" mode which automatically sets it to a (non-tweakable) +50F. With other vendors you'll have to read the manual.

    A reasonably tight 2x6/R20 type 1200' with low-E windows over a full but not fully h heated basement that has at least R10 basement wall insulation would come in at about 18,000 BTU/hr at an indoor to outdoor temperature delta of 40C (-20 outside, +20 in). Of course there is quite a bit of variation from that. A pretty-good cold climate 1.25 tonner would cover that much load, but with a walk-out basement and air-leaky garage it'll likely be closer to 30KBTU/hr.

  4. CollieGuy | | #4

    Hi Ruth,

    I believe we're in Zone 5 (Halifax, NS) and our home is reasonably energy efficient. Most of our heat is provided by two ductless mini-splits, but last winter we still used
    eighty litres of fuel oil for backup purposes, and in some years upwards of two hundred.

    A couple things I'll throw-out for consideration: There have been times when our outdoor units have been buried under as much as two meters of snow, and it can be a day or two before we can shovel them out. If you're in an area where snow drifts could be an issue, a backup heat source is a nice to have. They've also been sidelined on two separate occasions -- one by a leaking evaporator coil and the other a failed control board.

    Lastly, our oil-fired boiler is tied to our backup generator, and so this provides us with an alternate source of heat and DHW in the event of an extended power cut (combined draw of the burner head and circulation pumps is about 200-watts).

  5. AtTheBeach | | #5

    CollieGuy, is your oil boiler from pre-mini split times or did you install it knowing it would be backup? We're trying not to have two whopping new system bills at once and the boiler we have does need to be replaced. Hence my thought that a smaller propane one might be cheaper and avoid the noisy smelly power vent situation.

    1. CollieGuy | | #11

      That's correct. We had installed our oil-fired boiler, indirect water heater and Tekmar Control system when we purchased this home in 2002, as the original system was past its prime. Our first ductless mini-split followed in 2005, the second for the basement level in 2008, and then the replacement to the first in 2011.

      By way of background, the previous owners in the year prior to our purchase consumed some 5,700 litres of fuel oil (and that happened to be a comparatively mild winter), and through various measures we got that down to 2,000 a year (fifty-two year old, 2,700 sq. ft. Cape Cod). Subsequent to that, with our two mini-splits and Nyle Geyser RO heat pump water heater, we're now down to as little as 80 litres/year.

      The one other thing I'll quickly add is that nothing beats the comfort of hot water baseboard heat, especially on a blustery winter's day. Our heat pumps are more economical to operate and they outshine the alternatives in just about every other metric, but not in terms of overall comfort.

  6. Expert Member
    Dana Dorsett | | #6

    >"We're trying not to have two whopping new system bills at once and the boiler we have does need to be replaced."

    Unless you're actually going to heat the place with propane it would be silly to install a propane boiler. An ELECTRIC boiler sized correctly for the design heat load (good for backup in even in the case your minisplits outright failed) would be a LOT cheaper and easier to rationalize than a rarely-used fossil burner. At any reasonable size electric boilers are dirt cheap compared to any cast iron or condensing propane solution.

    A 10-12 kw (34,000- 41,000 BTU/hr) electric boiler would probably be overkill, even though that's probably only half the output of the existing oil boiler. Even at full MSRP the boiler itself would cost only USD$1500, a few hundred cheaper than a cheap propane boiler, but the cost of the breakers & wiring would likely come in less than the cost of propane fuel line + venting. The plumbing is also going to be simpler than a propane boiler (either cast iron or condensing), for an overall cheaper solution. (Local emissions are also going be zero, with no CO risk.)

  7. jberks | | #7

    Dana, For the sake of discussion, what about using a propane tankless water heater instead of a boiler?

    A small tagaki or rinnai tankless water heater is a lot cheaper than a boiler, and to my understanding, the classic 180 degree temps for radiant systems are coming out style for 120 degree now. Venting and propane is already there and accessible. And essentially it'll only run as necessary when the T-stat calls for it (or you plug it in when you're cold)

    Still maybe about a $800 CAD + install for a backup system, so not great if you're trying keep costs down So you could still be right Dana that an electric could be cheaper overall as a backup system. (never doubt the number crunching of Dana). But I don't know the costs of an electric water heater since we don't really use them in Canada as our natural gas prices are quite cheap (at the moment), gas systems are the standard here. Ruth I'm sorry, I'm a city boy, I don't know what delivered propane costs.

    Another option is a heat exchanger running off your existing hot water heater. The heat exchanger is 200 cad , $120 for an extra pump + install. This is what I did at my last build for supplementary radiant heat (note not backup heat, it doesn't heat the house, just feet warming and snow melting when required).



    1. Expert Member
      Dana Dorsett | | #9

      It's likely that the existing radiation is undersized for the full output of the oil boiler, but the system is saved from short-cycling by the thermal mass of the oil boiler. But whether it would be enough radiation to not short cycle a tankless or mod-con or even heat the place at domestic hot water temps has to be determined with a bit of analysis. SOME of that basic napkin math for analyzing it lives here:

      There is also the matter of specifying a pump that won't over or under pump the tankless without having to design in a hydraulic separator (which would add another pump and more plumbing to the assembly.) I've designed heating systems around a tankless (including my own), but it's not trivial, way more than just an exercise in plumbing. With most tankless water heaters the warranty is voided when used strictly as a heating boiler, and for good reason. Most systems people end up building around them are over-pumped, short-cycle like crazy, putting a lot of wear & tear on the flow detectors (or even eroding the internals of water-tube heat exchangers!) And the tiny screen filters built into tankless water heaters clog easily with any sort of free floating system scale or grit making it an annual or even more frequent maintenance issue (ask me how I know! :-) )

      By comparison an electric boiler is designed to handle the flow requirements of the application, and in most cases could be simply dropped in place where the oil boiler gets snipped out, and can be operated at whatever temp the system really needs.

      1. jberks | | #17

        Very good.

        Could we discuss more on short cycling?

        I think a lot about short cycling, especially with all the times you've written about it. I get that boilers shouldn't short cycle. But have you seen tankless heaters operate? They are constantly short cycled as people wash their hands or use the kitchen sink. And I've seen people own tankless units for many years.


        1. Expert Member
          Dana Dorsett | | #19

          >"But have you seen tankless heaters operate? They are constantly short cycled as people wash their hands or use the kitchen sink. And I've seen people own tankless units for many years."

          I get that most draws of potable hot water are short cycles, but the raw NUMBER of draws in a day is miniscule compared to a short-cycling boiler/tankless.

          The tankless I'm using for a heating boiler at my house (since the 2008/2009 heating season) is serving a 48 gallon buffer tank (~400lbs of water mass just in the tank) as it's only "zone" at a storage temp of 130F (at turn-off) with five heating zones sipping from the tank. Some of the zones are cast iron rads with significant water + iron thermal mass to work with too. The minimum burn time as-operated is in the 3-4 minute range (with no zones calling for heat) but even so it's running about 2-3x the annual numbers of burns as a tankless that's only serving hot water in a residential application. With an order of magnitude less thermal mass to work with it would have been toast a decade ago.

          A 1200' house with a couple of zones heated with fin-tube baseboard probably has no more than 100' of baseboard, probably even less than 80' - it could be less than 100' of plumbing + baseboard for the whole thing. But for the sake of argument let's assume it's 200' of 3/4" plumbing + baseboard on the system, and the tankless has a min-fire output of 15,000 BTU/hr (a typical minimum fire for a tankless) Even with the water in the tankless only about 40-45lbs of water in the entire system. Call it 50lbs split between two identical zones (25lbs each), with 40' of fin-tube per zone. At 130F average water temp (135- 140F max out of a condensing tankless) fin-tube emits about 250 BTU/hr per foot or about 10,000 BTU/hr. That's 5000 BTU/hr (83 BTU/minute) more heat going in than is being emitted.

          The system temperature would then be slewing 83 BTU/25lbs= 3.3 F per minute. The temperature regulation on tankless systems are pretty tight- call it 2F, so it will cut the burner and re-start when it cools back into the right range (or worse, some would cut the burner and halt completely spitting out an error code) delivering MANY burns/hour shorter than a minute in duration during calls for heat on just that zone, on the order of a dozen or more burns before the thermostat is satisfied.

          How many distinct domestic hot water draws do you think most homes make in an hour, on average?

          Bottom line, you have to do at least this level of napkin-math analysis. If you screw it up the tankless won't make it more than a couple of heating seasons, with easily more than a decade's worth of ignition cycles each year that it would experience in residential water heating application.

  8. BillDietze | | #8


    I'm in a "warm" zone 7. Design temp here is -6F. It's Zone 7 because the shoulder seasons are quite cool. In new construction, I placed two heads in open areas (open floor plan), one on each floor. The heat pumps were designed for 105% of my worst case load. I placed electric cove heaters in the corner bedrooms and one other above grade corner of the house not part of the open floor plan. No propane in the house.

    So far, I've used the bedroom cove heaters when the outside temperatures drop below about 3 F and it hasn't been sunny (we heat the house to ~ 69 F and the door-open bedroom temps drop below 65 F). The heat pumps are two separate units and the cove heaters were sized to cover the rooms they were in and the full -6 F heat load when one heat pump fails. ...and I have a small 24,000 btu woodstove.

    So far it's all worked well with two winters in the house (lowest temp has been -5 F so far, for maybe 4 hours). If I had to do it again, I'd design the heat pumps at 125% of the max load because it takes a long time to heat up a cold house in winter. Setting the house temp to 60 F (55 F in the bedrooms) and coming back after a week took a surprising amount of time warm up when 10 F outside.

    There was an all electric house designed and built in Minnesota and they used a simple 7 KW boiler (I recall) for hydronic heating for all their heating needs. See . Since you already have the hydronic heating in place, you could just install and electric boiler and only use it when the heat pumps can't keep up - when it's real cold outside.

    I hope that helps!

  9. AtTheBeach | | #10

    Wow, thanks everybody. We had uttered the words "electric boiler" early on and at over 16 cents per kilowatt we just got blank stares from the plumbers. Ha! But as a backup, could work. Minisplit guy is coming back tomorrow - we'd been asking him about the odd warm spell and dehumidifying needs but he has renewed energy now and I feel better understanding the options a bit more. Will report back.

    1. CollieGuy | | #12

      I'd be inclined to go with a small electric boiler as well. With respect to operating cost, Nova Scotia Power currently charges 15.805-cents per kWh and that will reportedly increase to 16.215-cents come January. At present, a litre of fuel oil retails locally for 61-cents and nets you approximately nine kWh(e) of heat at an AFUE of 84 per cent; that puts its cost per kWh(e) at 6.8-cents.

      Our propane tank was filled on 15 August, and at that time I paid $1.2894 per litre, plus an additional $9.22 HazMat Fee and a $17.00 delivery charge (Superior Propane). At a 90 per cent AFUE, you'd net a little over six kWh(e) of heat, so your cost in this case is closer to 21-cents per kWh(e).

      1. AtTheBeach | | #13

        I still don't completely understand why minisplits are supposed to be so reasonable to run, as opposed to astronomical, in a Pretty Good House. Especially compared to oil.

        1. Expert Member
          Dana Dorsett | | #14

          >"I still don't completely understand why minisplits are supposed to be so reasonable to run, as opposed to astronomical, in a Pretty Good House. Especially compared to oil."

          Because averaged over the season any decent cold climate mini-split heat delivers 3x to 3.5x more heat per kwh used than resistance heating (like an electric boiler). During the shoulder seasons it's more like 4x as much heat per kwh.

          Take a look a the Zone IV HSPF numbers in the ARHI sheets for any model that's being considered. An electric boiler has an effective HSPF of 3.4, which means it delivers 3400 BTU/kwh as heat going into the system. (The purists would point out it's closer to 3412 BTU/kwh, but this math doesn't really have or need that many digits of precision . :-) ) Most single zone cold-climate mini-splits have an HSPF between 11-14 (11,000- 14,000 BTU/kwh).

          Say you end up installing a mini-split with an HSPF of 12.5. It's effectively delivering 12.5/3.4= 3.7 kwh of heat per kwh used. So even at 16 cents/kwh it's the equivalent of running an electric boiler with 16/3.7= 4.3 cent electricity, which substantially cheaper than the 6.8 cents/kwh-equivalent cost of oil heating CollieGuy's analysis in response #12.

          Minisplits are cheaper to heat with than #2 oil or propane in almost ALL markets, even cheaper than natural gas in some markets.

        2. CollieGuy | | #16

          On average, over the course of the heating season (which for us spans 01 October through 31 May), our ductless mini-splits supply us with three and a half kWh of heat for every one kWh they consume. Thus, our delivered cost per kWh(e) of heat is 4.5-cents, or about one-third less than fuel oil at its current price. As you can appreciate, oil prices can be highly volatile, and there have been times when oil heat was more than twice as costly as that furnished by our mini-splits.

  10. AtTheBeach | | #15

    Thank you!

  11. AtTheBeach | | #18

    I have another wrinkle - I may need to start a new thread. House layout is attached. Husband is worried about pipes freezing. Upstairs we'll have a cassette in living room and in bedroom. I would think leaving doors open for air flow will keep the upstairs bathroom pipes from freezing?

    Downstairs, one more cassette. It could be in the hallway but the doors to garage and utility room are typically shut, so no heated airflow to those spaces. There are hydronic heaters in the utility room and garage which wouldn't kick on unless the [electric boiler] backup came on in the house. BUT downstairs is really a walk-out basement (with the walk-out being the front entry); the bathroom and utility room are built into the side of a rocky slope. So doesn't that protect them from a pipe-shattering freeze? I don't care about the garage being freezing (no canned vegetables, no car stored there) - or should I since there is a hydronic baseboard heater?

    I think minisplits with a small electric boiler backup would work, but the minisplit guy isn't sure or husband is confusing him with questions... It should work, right?

    1. Expert Member
      Dana Dorsett | | #20

      Do yourself a favor and do some of the really basic analysis for designing a heating system. Design-by WAG is a truly lousy method, unlikely to yield optimal results.

      Start with a room-by-room Manual-J (-ish) heat load calculation using better than average freebie on line tools such as CoolCalc or LoadCalc , using 68F or 70F as the indoor design temp:

      Both of those tools are likely to overshoot reality by 15-30% even when being really aggressive on input assumptions, but it's at least a stake in the ground to work with, way better than a WAG.

      Also measure up how many feet of baseboard you have in each room, and total it up for each zone.

      Then calculate the BTU/hr load per foot of baseboard length there is in each room, and each zone.

      The room or zone with biggest load/foot will determine your water temp requirements.

      We can make adjustments to those to see what it would take to maintain only 55F rather than 70F at the 99% outside design temp, but in reality you can see temps drop 15F cooler than the 99% outside design temp during Polar Vortex disturbance cold snaps, so sizing the heat source (and adjusting the temperature) to meet the 70F indoor temp with the backup system still makes sense, but it doesn't need to be more than that.

  12. Deleted | | #21


  13. AtTheBeach | | #22

    Homework! Will do.

  14. STEPHEN SHEEHY | | #23

    Fuel oil is really cheap right now. I did the math a while ago, and assuming a COP of 3 for a minisplit and .16/ kwh for electricity and $1.85/ gallon of no. 2 oil in a 90% efficient boiler or furnace, price per btu is about the same. Propane at around $2.50/ gallon is twice as expensive as a minisplit.

  15. greengalt | | #24

    We're in a similar situation and made the switch to cold weather mini-splits to heat the house. We got our first snow today and so far so good (coolcalc for room by room numbers for the win). We have our 30 year old boiler for backup hydronic heating, but we haven't decided what to do when we replace our 30 year old oil boiler. It's our hot water source as well. I've been considering a few options for hot water, but am also concerned about back-up heat in the case of a power outage. As I see it these are my options:

    Lead option:
    1. Install heat pump hot water heater AND direct vent gas fireplace...our chimney will not accommodate a wood stove or insert.

    2. Replace 140k BTU oil boiler with a smaller unit and continue to use for hot water and back-up heat
    3. Install smaller propane condensing boiler for hot water and emergency heat

    Are there any other good non electric back-up heat options? Ideally I'd like to remove our hydronic baseboards as they're starting to leak -- piping is PB rather than PEX/copper so a matter of when not if.

    1. STEPHEN SHEEHY | | #25

      Our Honda inverter generator runs our heatpumps. We needed a generator for power outages anyway.

      1. greengalt | | #26

        Ah of course. I don’t think that’s ideal for us as we’ve got a portable generator hooked up to a panel with critical circuits on it. When the previous owner wired it they obviously didn’t consider heat pumps. My understanding is full house standby is very expensive.

        1. STEPHEN SHEEHY | | #27

          Our generator is a 7000 watt portable that connects to our electrical panel. All circuits can be used, though we're careful not to run more than one high draw appliance at a time. Heat pumps don't use much power. Ours are on 15 amp, 240 volt circuits.

          1. greengalt | | #28

            Sounds very similar to our set-up except the installer of our mini-splits was adamant to not put them in the generator panel due to draw and clean electricity. Perhaps I'll have an electrician come out and see how we can move a few things around to make it work.

  16. onslow | | #29

    Ms. Remington,

    Per your comment #18. Do make an effort to track and identify all the pipes in your heating as well as water and waste. If you make the change over to mini-split for your main heating, the pipes feeding the back up radiators will cool well below what might have been typical in past winters.

    The former constant cycling of the boiler would leave hot water in the pipes, which would not likely cool anywhere near freezing between cycles. The warmth the pipes use to give off between cycles may well have kept other areas warm (like waste lines) that will no longer be warmed for long periods if you only keep a boiler for back up in extreme conditions. You will care if the garage freezes.

    This is not to say the mini's won't heat you well, just that you need to review the pipe layout for unintended consequences. I had to rescue a rental house on short notice one winter after the tenants took off for a ski weekend without remembering to close the garage door. The poorly placed half bath against the below grade back wall froze completely cracking the toilet apart. The previously unidentified path of the return leg for the baseboard heat in the bedroom over the garage revealed itself spectacularly.

    Three weeks of tear outs and repairs was the result. I reset the return line high in the joist space and made sure the maximum insulation was in place. The tenants made very sure to stay focused after that. Hopefully you won't leave your garage door open, but just the same be sure the pipes passing through or near are fully in the heated envelope.

  17. an123 | | #30

    Hi, I have a similar situation (1200 sq ft house with cast iron rads/oil boiler, wanting to switch to mini-split ductless heat pumps for heat, located in NS (Zone 7)). I like the idea of electric boiler back-up but it's not a cheap option for me as I would need to upgrade my 100 Amp electrical panel to 200 Amps first. Ruth (if you still have a membership), I'm wondering how your project turned out?

  18. ErikOlson | | #31

    I'm planning a new house in rural zone 6 (MN) and the electric co-op requires heat backup for power outages. They recommend a "Steffes" electric thermal storage unit. I think they are motivated more by adding off-peak usage than efficiency.

    Main heating source is planned as cold climate mini splits. Am thinking a direct vent type propane wall furnace as backup. These units are not terribly efficient, but can function without power and don't require a roof penetration. Worst case heat requirement is about 18k btu.

    Has anyone else used a direct vent wall furnace to back up mini-splits?

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