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Community and Q&A

Backup Heat Source for Minisplit System

kieran973 | Posted in General Questions on

Hi, I recently had two Fujitsu Halcyon ductless wall units installed on the first floor of of our two-story 1800 sq ft house (Climate Zone 5). It’s looking like these units will handle 95% of the space heating needs in the entire house — they keep the downstairs at 70F and the upstairs at around 65F (with the three upstairs bedroom doors left open). The only small snag in this plan is that the youngest member of our family sleeps in one of these upstairs bedrooms from 7 PM – 7 AM with the door closed and the temp in that room can drop to around 60F or so during this time. So long-term, we will still need a little supplemental heat in at least that bedroom (let’s say all three bedrooms to be safe) just to take the chill out and raise the temp 5F or so as needed. Our mini-splits also only work down to -5F, so a backup heat source would be wise for rare cold snaps.

Of course, we do have a central heating system, and this is a hydronic radiator system supplied by a natural gas boiler. Downstairs the radiators are cast-iron baseboards. Upstairs the radiators are severely oversized upright cast-irons (5 feet long by 2 feet tall radiators in each of the 100-150 sq ft bedrooms). The hydronic system has two zones, but one zone is just for the first-floor bedroom/bathroom, and the other zone is for the entire rest of the house — on the first-floor: kitchen, living room, dining room; on the second-floor: three bedrooms and one bathroom. The gas boiler is the last fossil fuel appliance in the house. I eventually want to replace it with something electric — my goal is to have no fossil fuel appliances inside. We currently have a 7.7 kW solar array that produces 10,000 kwh a year. We plan to add maybe 1-2 kW to this a few years down the road, since the total annual electric usage for our entirely electrified house would be in the 12,000-13,000 kwh range. Given all of this, my question is:

Should I replace our natural gas boiler with an electric boiler? Or should I get rid of the boiler and hydronic system entirely and just put in electric baseboard radiators, room by room, throughout the house? Or some hybrid combination of these two options?

The heat load for the whole house at our 99% design temp of 11F is around 30,000 btu/h. If I was going to go with an electric boiler, I was thinking of something in the 9-10 kW range (with an output of around 30,000 btu/h), like an Argo or Electro. Something like this:

Or this:

If I was going to rip out the boiler and old hydronic system entirely, I’d still prefer the new individually-zoned electric baseboards to themselves be hydronic, so I was thinking of something like Cadet Softheat hydronic baseboards:

But what are the pros and cons of each of these two strategies? Right now, if I want to get a 5F heat bump in the upstairs bedrooms, I have to turn the gas boiler on for 20-30 minutes. I imagine this is partly because the bedrooms are on the same zone as most of the first floor, so the boiler first has to send that 180F water through the dining room, kitchen, and living room before it winds its way upstairs. So would individually zoned baseboards like Cadets in the upstairs bedrooms be significantly more efficient? Since the only need for supplemental heat is in those upstairs bedrooms, is my more-or-less single-zone hydronic system just a hopelessly inefficient way of achieving this, regardless of boiler type?

At the same time, I imagine there are some risks and downsides to just ripping out the old radiators and abandoning a hydronic system in the walls that’s been there for almost a hundred years (the house was built in 1925). New mouse superhighways along those formerly 180F and now cold-to-the-touch hydronic pipelines? New moisture/mold issues in the walls now that we’re suddenly not heating and drying out their insides? Inherent problems with just abandoning a bunch of pipes in the walls and letting them rust there? I guess the more general question I’m asking here is does this 1925 house as a whole system need its hydronic heating system for better or worse in order to remain healthy as a whole system?

Any advice would be much appreciated. Thanks.

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  1. anonymoususer | | #1

    Sounds like you have some good ideas. Infrared ceiling panels such as ducoterra solaray might also be worth considering:

  2. Expert Member
    BILL WICHERS | | #2

    Note that if your goal is to reduce emissions, you're better off using the natural gas boiler instead of electric RESISTANCE heaters like baseboards. The reason is that most electricity in the country is sourced by a mix of coal and natural gas, and it's less efficient to use electricity produced from natural gas to heat than it is to just use the natural gas for heating directly. If you use electric resistance heaters, you will ultimately be using more fuel than if you'd just used natural gas for the heat. This is likely to be true regardless of any net zero solar efforts, since you'll still be using that fuel to run those heaters regardless of what happened with your solar system at others times of the year.

    What I would do is if you only have one room needing some extra warmth, see if you can use some circulation fans to even things out a little if you have some warm and some cold areas. It takes a lot less energy to move warm air around than it does to actually warm up cold air. Sometimes you can use small duct booster fans to act as small air handlers between rooms.

    The next option would be "spot heating". This is similar to how task lighting works: with task lighting, you put light where you need it (such as a desk lamp), which is more efficient than lighting an entire room just to get the desk bright enough for you to see. The same works for heating: use a small supplemental heater only in the room that really needs it. In this case, an electric baseboard might make sense, since you're only using it to add a little heat in one room, with the heat pumps doing most of the work. Since heat pumps are moving heat (pumping it in from outdoors, like an air conditioner in reverse), not making it directly, they are much more efficient in terms of energy use compared to electric resistance heaters.

    If your yougest is old enough to safely use an electric blanket, that's the most efficient way to go. I use electric blankets myself, and let the rest of the house drop down quite a bit at night. The electric blankets let us keep everyone at a comfy temperature for a minimum of energy use, while the rest of the house is only minimally heated while it's unoccupied.


    1. kieran973 | | #7

      Hi Bill,

      Thanks for your reply. In southern CT, the grid portfolio is roughly half natural gas, half nuclear and renewable (with most of that being nuclear). Coal is less than 1%. Given that, and given that the COP of resistance heat is 1 while the COP of my gas boiler is .8, could it really be true that using electric resistance for space heat is more carbon intensive than using the gas boiler? And if that is true, wouldn't the same be true of other appliances (stove, water heater, etc.).

      According to Coolcalc, the energy needed to raise the temp by 5F on my entire second floor (three 100-150 sq ft bedrooms, tiny hallway, tiny bathroom) is roughly 1,100 btu/h. Say we want to do that for 12 hours a night, 30 days a month, 7 months a year - that's 2,772,000 btus/3412 = 812 kWh. If we only want to do that in that one hard to reach bedroom (because the other two bedroom doors can be left open basically 24/7), that's roughly 812/3 = 271 kWh. So we're not talking about all the electricity in the world. And if that means I get to tell the gas company that I'm closing my account and they need to come take their hideous gas meter off the front of my house, I'm all for it.

  3. Expert Member
    Akos | | #3

    Are you sure you mini split cuts out at 5F? 5F sound more like the rated minimum temperature but most will continue to run well bellow that, a call to the manufacturer should get you the right answer.

    As for supplemental heat, some form of resistance heat is probably your best option. Since the heat will be running for around 8h/day, spending money on something more efficient is most likely not worth it.

    If you do want something more efficient, you can look at an air to water heat pump. Something like a SpacePak Solstice LAHP48 would do it. Not cheap, but not that expensive either. Since the air to water is around the same efficiency as your wallmounts, you can run both at the same time to improve comfort.

    P.S. Any infrared radiant heat, such as the first post, it is mostly marketing fluff (or link spam in this case). When it comes to resistance heat, all setups have a COP of 1 (1 Watt of electricity makes 1 Watt of heat), there is no way around that.

    1. Expert Member
      PETER Engle | | #5

      Not entirely true. Yes, all resistance heat has a COP of 1. But radiant heaters can provide spot heat that allows you to be comfortable with lower air temperatures in the rest of the room. They can work well in workplaces, for example where a big, open room does not need to be heated if a workstation can be kept warm with radiant heat. OTOH, the recommendation of using electric blankets is even more efficient, as the heat is provided right where it's needed. But if electric blankets are not possible for some reason, a ceiling mounted radiant panel located over the bed could save energy over electric resistance baseboard heat. Might look better and make furniture location easier, too.

    2. kieran973 | | #8

      Hi Akos,

      You're right about the Fujitsu's rated minimums. Negative 5F is just what the specs say the units work down to, but I haven't had a chance to test that - we haven't seen temps even close to that since I've lived here.

      Re: air to water heat pumps: I looked into them, and I actually would have preferred this option over the ductless mini-splits, but I just couldn't find any local contractors who install them, let alone service them. I did find one local hvac company who supposedly installs SpacePak units, and I had them come out to my place to give me an estimate, but when I told the guy that, at that point, the SpacePak would be as a backup/supplemental heat source since we'd already committed to the mini-splits by then, he said the total installed price for SpacePaks was so astronomically high that I should honestly just stick with the mini-splits and not even think about air-to-water ever again. He actually didn't even give me the price - he said I didn't want to know. It's too bad, because I already have the hydronic system, and I even have extremely oversized radiators in parts of the house that could be ideal for lower temp air-to-water heat pumps since they have so much surface area, but the local reality here seems to be that no one really does them. At the same time, maybe that could change in the next five years or so? Part of me thinks that I should keep my hydronic system and install an electric boiler now for exactly this reason: maybe I could switch it out for an air-to-water heat pump if/when the local installer network matures in the future.

  4. rondeaunotrondo | | #4

    I have a very similar scenario as yourself. We gutted our oil boiler and radiators. We put in a total of four Ductless 1 to 1 units. We’ve only used two out of the four units 100% of the time on the first floor because as you said The units downstairs heat upstairs well. We have a third unit on The first floor that we will likely move to the basement. I’m having the same issue for my upstairs toddler‘s room. I put an envi 500 W Cove heater in With thermostat that will turn on when the temperature drops below a certain temperature. This worked out well and is pretty easy. We also have solar. It doesn’t make any sense to me to install gas anything in your scenario because of your solar. We purposely got it for gas to not have to pay for the meter monthly.

    I would like to retrofit a CERV Unit to help redistribute the heating as well as dehumidifier for the summer. We do not have an E RV now and the CO2 levels can get quite high even though it’s an old house. I’ve been working on their ceiling and the last floor door was six ACH 50 but likely closer to five at this point.

    Other things people with mentioned on this forum to improve room to room differences is improving airsealing. I have terrible old leaky Aluminum windows that need to be replaced Winter likely contributing to the temperature drop I’m seeing similar to yours.

    1. kieran973 | | #9

      Hi Will, I looked into that Envi wall heater too, but I got scared off by the Amazon reviews that said it's a fire hazard. According to some customers, it heats the wall so much that the wall gets painfully hot. Some people allege 135F. I'm guessing yours doesn't do this?

      Here is what I mean:

      1. rondeaunotrondo | | #11

        Well, I hadn’t actually seen those reports so that’s helpful. There’s definitely off gassing for 72h so I set mine up in the garage at first. In terms of wall heat I’ll definitely keep track but similar To what you were set up would be, I have mine plugged into the thermostat and only comes on when The temperature falls below 67° at that thermostat. I have it set at that temperature because my Awair Device on the other side of the room reads approximately 63-64°. Data From the energy Monitor I have it plugged into shows it runs probably about an hour or two at a time. We haven’t had a polar vortex yet so I don’t know how much it will run during those days but it has been Low as 20° out. Again This is supplemental heat for when the door is closed for my toddler so much different than someone who has this thing running all out 24 seven. Certain parts of the plastic top can be pretty warm.

        1. kieran973 | | #14

          Good to know. Hopefully the negative reviews were from faulty devices. How do you like the Awair? Is it the Element?

  5. Expert Member
    PETER Engle | | #6

    Yes. Insulating and air sealing that room will reduce the rate of temperature drop. Doing it for the whole house will allow your minisplits to do an even better job.

    You could install an electric boiler with the downsides mentioned above. Or, you could install an air-to-water heat pump that will provide hot water for your radiators. But why? This would be a big investment for use just a few days of the year. Once you know your minisplits are adequate, I would tear out the old boiler and radiators and reclaim the living space. For the upstairs bedrooms, as much as we hate equipment in attics, you might be able to put in a ducted minsplit. That will give you upstairs A/C as well which is a nice bonus. Sometimes you can find space in a centrally-located closet for the equipment and use short duct runs to the bedrooms without anything in the attic.

    1. kieran973 | | #10

      Hi Peter,

      Yes, I am working on doing another round of insulation and air-sealing. I'm also going to replace the windows in that bedroom eventually. In terms of a ducted mini-split system for the second floor, my rough napkin math says it would only save me about 600 kWh a year compared to just using resistance heat for that last 5F, which means that at $10-15K total installed cost, I'd be looking at a 25-30 year payback period. It would be nice, however, to stop putting in my window units ACs up there every summer. I'm using Frigidaire 5,000 btu window units right now (Energy Star, 12.1 CEER) - I didn't know about the Midea U window units when I bought the Frigidaires....

  6. walta100 | | #12

    “they keep the downstairs at 70F and the upstairs at around 65F (with the three upstairs bedroom doors left open).”

    To my ear it sounds like you have more work to do on air sealing and insulating before you start throwing equipment at the problem.


    1. kieran973 | | #15

      That's definitely part of the plan, but it's part of a longer project that will take some time and money. Meanwhile, I'm wondering if I should just put down a 500 watt baseboard in that one hard to reach bedroom now. I'm also trying to get a sense of what I should do in the medium to long term with the hydronic system and the boiler. Drop in an electric boiler replacement? Rip out the whole thing and put resistance baseboards throughout the house as needed? If more air sealing and insulating can get the house to the point where it's both 70F downstairs AND upstairs, with no direct heat source upstairs, only the two mini-splits downstairs, then that would be great.

      In terms of air sealing and insulation, the next projects are 1.) spray foaming the rim joist and 2.) whole-house air sealing. But before I can do that, I first have to deal with a roof leak, and a small but nasty crawl space under the front entrance and walk-in closet. The crawl space needs to be cleaned out and probably mold remediated, then I need to put a vapor barrier down on the dirt floor, then foam board insulation on top of that, and then concrete on top of that to help deal with a larger radon issue in the basement. Then, after all of that, once the crawl space is dealt with, I'll have the insulation company spray foam the rim joist and do the whole-house air seal as part of the same project.

  7. kyle_r | | #13

    Wasn’t the original plan to install a ducted mini split upstairs in a few years for cooling? If so, you only need to bridge the gap until then?

    1. kieran973 | | #16

      Yes, that has been the plan in the past, and I may still do that in the future. My thinking changes about it sometimes, and I'm not really sure what the best long-term solution is for the second floor. One factor is that I didn't expect the first-floor mini-splits to heat the second-floor as well as they do (bringing the second floor up to 65F) - I was under the impression that I would really need a heating source located on the second floor to handle the majority of the second floor heat load. But if the second floor only needs a 5F bump with the first floor mini-splits running 24/7, that's only around 800 kwh annually of resistance heat. Sure, a ducted mini-split would handle this 5F bump with only 200 kwh, but that only saves 600 kwh a year, and even if I adjust my current electric rate of $.22/kwh for future inflation to say $.35/kwh, an $8,000 ducted mini-split install/$210 per year of electricity savings (600 kwh x $.35/kwh) = a 38 year payback period. The other issue is that our attic is a walk-up and we'd like to one day convert it to home office space. So I don't want to put a bunch of hvac equipment and ducts up there only to have it compromise that renovation (plus the attic is unconditioned at the moment). So the long-term future of the second floor might actually be: 1.) let the first floor mini-splits handle the bulk of the second-floor heating, 2.) spot-heat the bedrooms 5F or so as needed using baseboard electric heat, and 3.) cool the second-floor with window units (maybe one day with Gradient units, if that company isn't just all talk and they're actually going to produce an actual product). I would actually love to just put a bunch of 7,000 btu LG PTAC units ($1,000 a piece, a COP of 3.9) in the three bedrooms, but my wife won't let me....

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