Help Selecting Ducted Heat Pump Size
This forum has been incredibly helpful while we are building our tight, passive house (lower case p) in Zone 6. We’ve settled on an HVAC approach, but my heat pump installer and I are stuck at the corner of Novice Building Science Nerd and Old-School-Fossil-HVAC-Pro-Turned-Heat Pump-Pro. ie, I want to go a little small, he wants to go a little big.
Manual J (which I did myself, definitely not perfectly, but well enough to be helpful) says we have a total heating load of 34,000, and an average cooling load of 9200, peaking at 10,900.
The plan is a Mitsubishi ducted heat pump for the main and second floors, and a separate 6000btu wall mount in the mostly finished basement (MSZ-FS06NA with MUZ-FS06NAH). We are between the 12k and 18k SVZ models for the ducted unit (SVZ-KP12NA vs SVZ-KP18NA).
The cooling capacity of the 12k unit is 5600-12,000 – perfect for our needs. However at 5F it only has a heating capacity of 15k btus and this makes my installer nervous since we do way more heating than cooling in Zone 6. My feeling though is that paired with the 6k basement unit and it’s 10.5kbtus at 5F, it’ll be fine. That’s enough btu’s to stave off disaster if we aren’t home, and if we are home we can fire up the wood stove.
His preference is to install the 18k SVZ unit, which is a better match heating wise, but its cooling range is 9360-18,000 – which means best case scenario we’re always using it at the lowest end of its capacity. I’m nervous that this will lead to cycling and poor dehumidification.
I would love some help resolving our friendly debate! My LoopCAD outputs are attached. Thank you!
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