Planning a Minisplit System for a Row House
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?
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