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Planning a Minisplit System for a Row House

cmaloof | Posted in Mechanicals on
We’re in a 1918 ductless Philadelphia rowhouse with gas heat and no AC. We want to put in a multi-split for AC. For carbon footprint reasons, we’d like to use it for heating too, plus then we could reclaim floor space from the radiators.

It’s 3-stories, about 1350 sq feet, ~36×14. I’ve done room-by-room load calculations using hvac.betterbuiltnw.com and coolcalc.com. I don’t know our exact insulation types so I’ve had to use my best guesses, which is kind of unfortunate since the calculations are sensitive to them. I’m figuring not much insulation in the walls due to knob-and-tube wiring (U=.24). We have a EPDM roof (no attic) and I’m assuming/hoping there’s an insulated board under it (U=.11), but no idea how to tell short of hiring someone to go peel it up and look.

Overall 17-degree heating load seems to be around 24K given those assumptions. (Cooling load in Philly is similar or less so I’m ignoring it.) But, there’s basically no airflow between rooms except via doorways, which are mostly open. Adding ducts is probably not realistic. I’m concerned that the third floor in particular might need two units for adequate heating. Floor plan including results from hvac.betterbuiltnw.com is attached (please ignore the circuitry notes).

The first salesperson wants to do a 42K Mitsubishi with 5 units (MXZ-5C42NA2) which seems like too much? Admittedly its 17-degree rated capacity is 24K, but it seems like it would be huge overkill 99% of the time. I could probably talk them down to the 4-unit 36K one (https://ashp.neep.org/#!/product/29041) but I’m not sure how much better that is. Second salesperson is from a company that won’t recommend minisplits for rowhouses at all without supplemental heating; they quoted us on a 24K Mitsubishi unit (could be https://ashp.neep.org/#!/product/29025 or https://ashp.neep.org/#!/product/29040), basically for AC and spring/fall heating. We’re talking to a third tomorrow.

As far as I can tell, most or all the reputable companies in the area only sell Mitsubishi units because they need less maintenance. But you can’t get four ports nor 24K 17-degree output without going to 36K.

What do we do? Compromise on the 36K? Focus on AC and give up on ditching the gas? See if somebody will sell us a 24K Mitsubishi HZ unit (https://ashp.neep.org/#!/product/26170) even though Philly isn’t that cold, and/or figure out how to make 3 indoor units work?

Many thanks for any advice. I’ve learned a lot from this site, but all our options feel iffy. Even though we’re willing to live with some temperature variations, I wonder if it’s a bad use case for minisplit heating.

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Replies

  1. Paul Wiedefeld | | #1

    I'm in a rowhouse in Baltimore about the same size, but luckily had existing ducting for most of the space. I had success using this heat loss method: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
    I suspect you'll need less than 24k btu and it's easier than guessing about what's inside your walls.

    You're correct about not wanting to oversize the multi-split. I think you have many options here, but above all run the fuel use heat loss:
    1. Keeping the boiler is not the end of the world (you won't gain floor space though) if you pair it with a 4 head multi-split. You'll probably cut gas use by 90% if you keep it for just the coldest days. If the piping supports it, you could add TRVs to some radiators to add some zoning.
    2. You could keep the radiators, ditch the gas boiler for an electric one. Same concept as 1., just use it sparingly.
    3. Another option would be to use an assortment of minisplits. Perhaps a 1-to-1 or 1-to-2 for some rooms. Using mini-ducted systems on floors 2 and 3 might work. There's no reason to be tied to 1 outdoor unit.
    4. Strategically use electric baseboard and window AC units for some rooms. Since your heat loss for any one room is low, the efficiency hit shouldn't be too bad. Will probably be cheaper than adding that 4th or 5th indoor unit.

  2. cmaloof | | #2

    Fascinating! I'm not exactly sure what I just did, but Dana's method gave me a result of 18-19.5K (sizing 25-27K), which gives me much more confidence that my numbers aren't vastly low.

    I love your analysis, that's really helpful. The option of electric baseboard is particularly interesting, I'd never thought of that -- it makes sense that if it's only to supplement a few rooms, it could increase winter comfort a lot without being super wasteful or expensive. Our radiators aren't zoned and might not play as well with other sources.

    Will have to sleep on it, but I think that gives me a lot more confidence in the smaller system.

    1. Paul Wiedefeld | | #3

      Great! Heat pumps fit rowhouses so well - the furnace I replaced was 88,000 Btu input, when all I needed was 18,000. Since the heat loss is so low, a baseboard is a great hack vs. running ductwork or adding an indoor minisplit head for some rooms.

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