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Community and Q&A

Need new boiler, considering mini-splits & HPWH instead

Cliffdive | Posted in General Questions on

Hello All – we have a ~1700 SF place (not counting a full unfinished basement) in NH that has an old 3 zone oil boiler which is starting to leak.  At present it is a second home, so there are stretches that we aren’t there for several weeks.  As we don’t have A/C, I’ve been researching the possibility of switching over to mini-splits and adding a heat pump water heater.  

The house has a bit of an odd layout, and a couple of contractors that I’ve spoken with suggest 3 units on the first floor and 2 floor consoles on the second floor which only has 2 bedrooms and a half-bath.  The contractor I have the most confidence in suggested 2 outdoor -15 degree units to support the 5 indoor wall/floor, as he indicated he couldn’t get an outdoor unit that runs more than 4.

The big question I’m wrestling with is whether I can get rid of the boiler.  I’m getting mixed feedback from people on whether I’ll be happy with the comfort profile of mini-splits alone if I’m used to FHW heat.   The house isn’t particularly well insulated at this time, but I can solve that problem, but I’d still have to use 5 wall/floor units.  If I do go the mini-split route, is having 2 outdoor units the best/only option?  

Another wrinkle to this is that the house came with a wood stove, but the chimney needs a liner to operate.  One thought I had was swapping that out for a propane gas stove that I could put on a thermostat for aux heat if needed.

If I were to solve the heating problem only at this time by getting a new boiler, is it fair to say I could downsize the mini-splits at a later date for cooling and to take some load off the boiler?  As far as boilers are concerned the Energy Kinetics line looks solid.  I have quotes for an Ascent Combi tankless and Frontier w/ external tank – generally it would be just me and my wife with guests and adult kids joining us on occasion. I’m Ok with the pros/cons of a tankless if that’s the lowest operating cost going forward.  As I mentioned we aren’t there for good stretches so I’d prefer not to keep water heated for no reason.  I know there are plenty of other good boilers but I only have quotes for those and for a couple of PurePro boilers.

I tried getting some info on the A2W from SpacePak but didn’t get much in the way of help from a couple of local guys from their website or the company itself.  It’s intriguing to me but probably overly complex and expensive for what I need.

So, advice on running the house on mini-splits alone, and on the boiler if I need to retain that?  

Thanks very much.

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  1. Expert Member
    Akos | | #1

    Around me heating with oil is more expensive than electricity, depending on your local energy costs, I would not be surprised that your simplest option is to replace the oil boiler with an electric one.

    Once you are off oil, the next best thing is to add a largish wall/floor mount mini split to your main living space and to the basement and use these to provide bulk of the heat and use your hydronic to provide supplemental heat around the perimeter of the house to even out temperature. This would close to the same operating cost of a full heat pump setup but cost way less and maintain the comfort of the existing setup.

    The next best thing is to deep six all the rads and convert to a ducted heat pump. This is very invasive and only makes sense if you are looking to do a major interior reno.

    The las thing you want is to install about 6 to 8 tons of equipment into a 1700sqft place, which is what your installer is suggesting with all those indoor units. Since the units will be grossly oversized, the chances of this running efficiently and providing good comfort is pretty much zero, the efficiency might only be slightly better than resistance heat.

    Your existing boiler is most likely already way oversized. Just because the boiler is 120000BTU, doesn't mean you need to replace it the same size electric boiler. You can run through the numbers here and size from there:

    My guess your place needs between a 7kW to 10kW boiler.

    1. maine_tyler | | #3

      Akos, do you suggest this electric boiler option vs just heat pumps primarily because his house may not be well sealed and insulated?
      Because I sort of thought the general consensus was that heat pumps could be left to do all the work (yes, they need to be properly sized) if the building envelope is at least decent. Are there likely to be comfort issues? If so, why? Or is it that the pumps may not be able to keep up on the coldest days?

      1. Expert Member
        Akos | | #7

        It is very hard to heat older space with point source heat and have good comfort. By keeping a mix of baseboard and heat pump, you avoid this problem. It also means fewer indoor units are needed which saves a fair bit of cost and generally avoids the oversizing issue with a 4 to 7 head multi split.

        1. maine_tyler | | #8

          So is it fair to say it's a distribution problem and not a 'meeting the load' problem? And that to have enough distribution with ductless heads you would be way oversized? So a ducted HP system might fit the bill (but not the most practical with existing hydronic heating)?

          I'm curious about air to water heat pumps for when there is hydronic distribution in place. My understanding is they are expensive though.

          Do you have a good way to determine whether a structure might have comfort issues with point source low temp heat (ductless heads)? Is it about total heating load or are wall temps a signoficant concern? Or does air leakage dominate the equation?

          For example, we are considering going all heat pump in climate zone 6a, an old but compact cape (1.5 story) The air leakage will be quite low, basement fully insulated, and we're putting an insulated over roof on. So the total heating load should be quite low in a small space, but the walls are still only 2x4 construction (though they are insulated) and windows are not great.
          Question really being: is it enough to have low infiltration and low total load, or will a 2x4 wall simply be too cold (radiative discomfort) for ductless HPs?

          1. Expert Member
            Akos | | #9

            Yes it is a distribution issue.

            For example, I have a small cottage up north, nothing special 2x6 construction with clear double pane windows. It is a 16x32 footprint with large windows on both sides of two corners. The place is heated with a wall mount in the middle of 32' side, so pretty far from these windows. The place is generally comfortable but by the windows it is cold. There are baseboards there, turning them on a bit makes it comfortable again.

            I can see a place with a lot of air leaks or older windows would definitely be uncomfortably cold.

            Insulated 2x4 walls in cold climate tend to be coldish as the surface is a bit bit bellow room temperature but this is nowhere near as bad as a window.

            I think windows and air leaks are the big issue for point source heat.

    2. Cliffdive | | #4

      Ok, I admit that I'd not considered switching to an electric boiler - expecting that the operating costs would be the highest option of all but it is intriguing. I'd love to get off fossil fuels. From, the average price of oil is $4.57/gallon or $/MBTU $41.21 and electric resistance is $0.3087/ kwh or $/MBTU $90.48. Heat pump is $/MBTU $36.19, and the link shows efficiency, etc. Is there such a thing as a hybrid resistance/heat pump electric boiler? If not, I'm assuming that I'm paying top dollar for hot water year round - but maybe that's not main operating expense killer?

      For the mini-split quotes, to add more detail, the primary quote I mentioned in my post was for a 30k unit for 3 heads on first floor and a 20k unit for 2 heads on the second floor. I was given a less expensive second option for a single 36k unit with 4 heads total (2 on first and 2 on second) - however this option leaves the kitchen/dining area without a heat source nearby.

      We're nearing retirement and will use the house more often for sure, but because we currently use it more in the spring/summer/fall, I can't use our oil usage as a guide on sizing as we keep it fairly cold when we aren't there. What you wrote and in the referenced article makes a lot of sense about not oversizing the boiler for sure.

      At some point we'll redo the kitchen and maybe bump it out a bit, so at least some of the walls will be open at that time. However, I'm kind of stuck now as I do have to do something about the heat pretty soon due to the leak.

      Thanks again for your thoughts.

      1. Expert Member
        Akos | | #6

        Your oil boiler is at best 75% efficient, so the actual cost is a bit higher, somewhere $55 to $60 /MMBtu. Your electricity is pretty expensive though.

        Even with high electricity the partial heat pump still ends up cheaper. Say you can cover 75% of your heat loss with a heat pump. Your overall heat cost would be:

        25%*90.48 + 75%*$36.19=49.15 which is still ahead of oil cost.

        Installing more heat pump capacity might make sense but you have to be very carful to match zones for the load. Generally, a wall mount in a bedroom is about 3x oversized, which should be avoided. A wall mount in a living space and wall mount in an upstairs hallway can work. These still have to be sized correctly for best efficiency and generally best installed on their own dedicated outdoor unit.

        I understand you are not using the place all the time, but it is still being heated. I would run through the calculations in the link I posted previously to bound your heat loss. A bit more work but you can also setup up a simple model of your house here which would give you an upper bound:

        Your actual heat loss will be between the two but you can now see how big the equipment needs to be and avoid gross oversizing.

        As for the domestic hot water, that is best served by a heat pump water heater. The last thing you want is keep a hydronic boiler running in the summer time.

        It is very hard to separate a basement from the rest of the house. The best is to assume it is part of the house and only insulate the basement walls not bellow the main floor. Heating the basement in this case also helps with comfort as the main floor above is much warmer and cost little extra.

        Instead of a main floor wall mount, one option would be to install a larger ducted air handler in the basement with a couple of simple duct runs to feed upstairs and the basement. As you modify the house, you can tie more and more of the spaces into this air handler so over time less and less would need to be covered by the hydronic baseboards.

        1. Cliffdive | | #10

          Akos - thanks, you've given me a lot of info I'll need to dig into. I'm on board that getting rid of the baseboard isn't a good idea for me. So now I need to focus on replacing the boiler as the priority - instead of an electric boiler with a separate heat pump water heater I may be better off replacing the oil boiler with a new higher efficiency one. The EK Ascent Combi is an 88% low mass tankless unit that has a clever mode you can choose that lets the boiler drop to room temp if neither heat nor HW is being called for.

          Having this boiler (or any boiler) installed will take the pressure off to let me look at the mini-split ductless as well as the ducted option you suggested.


          Thank you.

          1. Expert Member
            Akos | | #11

            Investing more in fancier dead end technology doesn't make much sense to me but a house must be heated.

            I would price out the options and see which way to go.

            With your current electricity price, a reasonable sized PV array would be a very good long investment.

            Sometimes leaky boilers are caused by water logged expansion tanks, not sure if the tech has checked it but might be worth while.

            Thinking more about it, installing a properly ducted heat pump in the basement to heat the main floor only and a couple of plug in heaters for the 2nd floor might be a good way to go all electric without having to deal with anything hydronic.

  2. walta100 | | #2

    How much longer do you think you will own the house?

    Will you tell us about the building? Has it been blower door tested? How much insulation is in the attic and walls?

    When I look at the cost of propane it seems to cost more per BTU than an electric boiler.

    The first step maybe to get an energy audit with a blower door test and IR camera.


    1. Cliffdive | | #5

      Walta - we'll likely keep this place for a good long time, and will use it more when we retire in the (hopefully) not too distant future. We've had a visual inspection but no blower test. The unfinished basement has foam board insulation over the concrete walls and insulation in the basement ceiling between the joists with gaps here and there. Not really sure what's in the walls, likely basic 1970 standard, and the attic isn't where it needs to be either. No way it was properly air sealed.

      My plan had been to upgrade insulation that I can easily get to at this time, without opening anything up.

  3. Cliffdive | | #12

    Akos- thanks for the input here, I’ll have some more homework to do. I appreciate the thoughts.

  4. jgold723 | | #13

    We're in a similar situation here in Maine. We've just put a deposit down for two mini splits for our 1978 split level (one up, one down, on separate units). I'm hoping these will work well enough so that when our now 20-year-old boiler gives up the ghost, we can tear it out and heat water with a heat pump hot water tank. It's a bit of an experiment -- if it works, we'll get a nice, quiet, efficient heating system.

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