DIY Manual J Results in Cold Temperature
I recently finished building our house (1200sqft, SIPS, lots of windows east, zone 5B, 12°F 99% temp) with a focus on airsealing.
The HVAC contractor we started with wanted to install a 36k BTU multisplit. I felt this was too big.
Did my own calcs, depending on variables came in between 18-24K BTU. Had two Mitsubishi Split systems installed (6k in Master bedroom, 12k in living room), and electric resistance in entry way, and 2nd bedroom.
It finally got cold (down to 12-15 F last night), have been leaving the split units off most of the time because its only been in the 20-30s at night. Left both on last night, set to 66 in living room, 67 in bedroom. Both modulated (one indicator light, never 2, i.e. not full load) all night, and I was actually too warm around 4am. Leaving for work set the 12k to 65, and turned off the other one.
I know this is an N of 1, and that it’s likely different from sustained low temps, but makes me feel better about “undersizing” our split units. Combined with 4000W of resistance heat back up, and a woodstove, I think we managed to have an energy efficient, comfortable, and reliable HVAC setup.
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