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Mini split heat for an 1880 remodel

BCC22 | Posted in General Questions on

Hi, I’m new here but I’ve been reading these Q&As extensively in trying to plan a heating system for my home. We are currently working on a large remodel of the house and I’ve been delaying the HVAC because I wasn’t able to get any reasonable quotes for installation.

The house is ~1550sqft with a full basement (unfinished), main floor of about 950sqft, and upstairs of about 600sqft. Upstairs is currently 3 small bedrooms, master is 170sqft, secondary BR is about 150sqft and the third BR is about 140sqft. We are planning to open up the third room into an upstairs lounge area connected to the stairway/upstairs landing bringing it to around 170sqft. with no door. Next to that room is an unfinished attic space that is on the same floor as the upstairs bedrooms. That area will eventually become a bathroom of roughly 100sqft with sloped ceilings. Above the current bedrooms is another unfinished attic which is currently loosely insulated, plans to improve the insulation up there and maybe do something fun with it at some point.

The existing heat is an oil furnace in the basement ducted to the first floor but no ducts running to the second story. The house is in southern VT and this past winter we were able to keep the heat at about 62º and ran through about 800 gallons of oil between Christmas and now (end of April). Not very comfortable but I really hate oil and we were working on the house most days.

There is no insulation in the walls and I don’t plan to add any. House is balloon framed with plaster and lath except one room where there were no walls. I added R-15 mineral wool to those walls and finished with 5/8 drywall. At some point I plan to insulate the sills in the basement to improve the walls a bit.

The house gets great southern exposure and warms up quite a bit when the sun comes out. The temperature is also holds fairly steady after sunset and starts to get colder about 2 hours later.

I hope that’s enough info, let me know if more is necessary. I’d really like to get a heating system (cooling is less important) that runs on electricity. I’d like heat pumps/mini splits so that: 1. no more oil  2. potential for solar  3. chimney is in rough shape and needs to be removed or rebuilt.

I had a couple installers come out and quote the job, the lowest price I got for the full house was ~$25k for ductless splits in every room. The other most reasonable contractor was willing to let me do the electric and only install splits for nearly all the rooms for ~$15k. This would leave only one bedroom without a heat source, the upstairs lounge area being heated ambiently by the adjacent bedroom, and one central room downstairs also being heated ambiently. Not a terrible setup but the units in the small bedrooms seemed hugely oversized. 9k each for less than 200sqft.

This lead me to researching and designing a system myself. Eventually I settled on a small ducted unit upstairs installed in a corner at the upstairs landing that shares walls with the master BR and second BR. The idea here is that it would heat with vents straight through those walls to the two rooms and a large return up above in the “hallway”. A third vent would point towards the upstairs lounge room. Those two BR’s would have transfer grilles above the doors for when the doors are closed and probably one supply each run straight through the wall. Is this crazy? Do people do this sort of setup? I can’t seem to find examples, but I’m thinking a 12k Fujitsu would do the trick here.

Second, I’m open to ideas for the downstairs. The rooms are large and open enough for the most part that they might be fine with a ductless wall unit, but we also have the option to use the existing floor registers and do something ducted in the basement.

I’d love to hear thoughts and ideas and I will happily send some floor plans if people think it would be helpful.

Apologies for the long post and looking forward to talking with you all!

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Replies

  1. Austin G | | #1

    I'm a mini-split fanboy, but honestly I'm not sure they're going to be the right solution for your application. You DO NOT want a mini split head in every room. I learned this the hard way - it will short cycle and be generally oversized leading to high power bills and uncomfortable rooms.

    IMO, the only way to use mini-splits in your application would be EXRENSIVE air sealing. Cap the wall opening with fire block in the attic and in the crawl space. Seal every penetration you can think of. When a home is properly air sealed, mini split conditioned air will stay inside of the envelope and you won't have to try and cram a head in every room.

    You said you don't intend to insulate, so I'm assuming you wouldn't want to undertake the air sealing project either. If you want electric heat, but floor mounted baseboard heaters. They're cheap and effective - I've heated homes as old as yours with them before.

  2. Walter Ahlgrim | | #2

    The way I see it this house survived not because it was so well built but the contrary. Your old walls are poorly flashed by today’s standards so lots of water gets into your wall and that’s is fine so long as the water evaporates and is carried out of the wall before things get rotten and moldy. Thank goodness you don’t have any insulation and lots of air moving thru your walls. It is a balanced system If you stop the air or the heat from moving thru the walls things could get ugly fast.

    Unless you are prepared to reflash your walls insulating and air sealing is a very risky proposition.

    Until you are ready to redo the flashing you can’t insulate and air seal and install modern heating. As it stands you current heat load is just so unimaginably large throwing oil at the problem seems like the only answer.

    Walta

  3. BCC22 | | #3

    Thanks for the responses so far, just to clarify I’m really not planning on insulating the walls beyond what was already done in the one room. Further insulation that I plan to do will be in the attic and basement at the sills.

    As a thought experiment let’s imagine that the oil fired furnace is either completely broken down or nonexistent. Is there something else that is recommended for a house like this? I’m *really* not interested in heating with oil and additionally there are looming problems with the current setup that I may have mentioned briefly (chimney issues, nonexistent ducting upstairs, oil tank installation problem).

    Is there any reason not to use a ducted heat pump?

    Thank you all for your opinions and replies thus far and let me know if you need any other info.

  4. Walter Ahlgrim | | #4

    First you need to know how many BTUs will be required to heat this building.

    If you have accurate oil fill data combined with local weather data a very accurate heat load number can be generated.

    If that is not possible you will need a manual J calculation.

    Given your lack of insulation and guessing that you have single pane drafty windows I would not be surprised if you needed many tons of equipment to get heat a -15°

    Note each ton of equipment needs to move 400 cubic feet of air per minute it could feel more like a wind tunnel than a home.

    Walta

    1. Expert Member
      Dana Dorsett | | #5

      >"If you have accurate oil fill data combined with local weather data a very accurate heat load number can be generated."

      Details on how to go about that are spelled out here:

      https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

      If it's only going through 800 gallons per heating season you won't need more than 3.5-4 tons of cold-climate heat pump to cover the load at design temp. In my area that would run about $13-16K in competitive bidding if all ductless. That is explicitly NOT going to be a "ductless head in every room" solution. In most cases it will make more sense to use at least a couple of ducted cassettes serving multiple rooms as a single zone.

      Even if you're not planning to insulate the walls (which can be risky with some antique wall stack-ups) it is still WELL worth a serious round of blower door & IR imaging directed air sealing. It's especially worth insulating and air sealing the unfinished basement walls, which is low risk/high reward, even if not using it as living space.

      With a leaky basement and a leaky attic the 24/7 stack effect infiltration is substantial, leading to cold first-floor floors and dry wintertime indoor air. Sealing both the attic and the basement can create a dramatic uptick in comfort, even if not insulating the framed walls. It also lowers the freeze-up risk of any plumbing in the basement, and the distribution losses of any uninsulated ducts in the basement.

  5. Irene3 | | #6

    I have a 1901 house with an unfinished basement and did a ton of air sealing as well as putting mineral wool in the rim joists. I think it helped a lot. I am honestly not sure whether there is insulation in the walls of the older part of the house (some of which were taken down to the studs during a major remodel/addition about fifteen years ago). One bedroom wasn't touched in the remodel and is still quite chilly. At some point we will likely take the walls out there (there's a dropped ceiling that should go, and we suspect not great wiring behind it, based on what we found in other rooms) and I would like to put mineral wool in then (possibly even on the inside wall, for soundproofing). But we haven't got any plaster walls remaining.

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