Choosing a Modulating Minisplit
My wife and I are building a small off grid cabin in Leavenworth, WA, climate zone 5, 2,560 elevation, 300 ft2 + loft, 1 small bedroom, one tiny bunk room, small kitchen/living room and 3/4 bath. The cabin will be used on weekends, holidays and possibly weeklong stints with our two small children and close friends/family. We ultimately landed on a double stud wall, dense packed cellulose heated with a PV powered mini-split backed up by a solar thermal system we will utilize for DHW. Manual J calculation puts our max heat load @ 9 deg F between 2,200 and 2,900 BTU depending on final insulation level, window placement and SHGC/U Value, etc. Nov – Feb average temps are in the high 20’s to low 30’s. We’ll spend 5+ months per year almost entirely between the high teens and mid 50’s. Heat load @ 47 deg F will be in the 800 BTU range. We’ve thought about buying a cheap mini-split on Amazon, (think 6k-9k BTU, $700 to $1,200 DIY type option) dropping some more batteries into the ole battery bank and crossing our fingers that the COP remains high enough at low temps that the unit doesn’t suck the batteries dry before 3 AM. The load is so low that the unit would also need to modulate pretty dramatically to keep from cycling constantly in 20-35 deg temps. This solution has the added benefit of lots of 110V options, which will allow me to do most of the wiring and probably saving us some money on an inverter. I wouldn’t call this an….elegant solution. Ideally we’d find a mini-split that is easy to install, runs on 110V AC, (or even better, 24v or 48v DC) modulates down to a very low BTU heating output and maintains an excellent COP at low temps. This may not exist. It’s also pretty hard to verify some of these details. Has anyone run into a similar set of circumstances, (off grid, minimal load, using PV for heat)? Any recommendations on products? Where to look? From whom to buy? For context, in case it matters a Mr Cool DIY install would be pretty easy for me…not so sure about a line set that needs to be charged, etc. Thanks!
UPDATE: 10/27/21 – Thanks to everyone that contributed their time/expertise to this thread! After much research, we’ve decided to move forward with an air source heat pump for primary space heating as planned. We decided against electric resistance, (need for high efficiency) wood, (for safety reasons with young children and difficulties maintaining clean low BTU combustion) and a heat pump water heater, (due to many fewer options, degree of disruption when equipment needs to be replaced and difficulties confirming expected COP at low temp operation). After going through this process, I suspect the following info will be helpful to other homeowners:
-If you want to buy and install your own mini-split and low temp efficiency isn’t of primary concern, (grid connected, mild climate) buying a unit from Amazon, ecomfort, acwholesalers or HVACDirect with charged lines probably makes sense and will save you the trouble of buying gauges/pumps/specialized tools to commission the unit. There are many good options.
-If you need a unit with substantial modulating range, (500-2000 BTU minimum on a 6k to 12k BTU rated unit) with excellent cold weather COP, the options are significantly more limited as of October, 2021.
The lowest modulating unit with good cold weather COP was an LG product that was discontinued in 2018. The replacement has terrible cold weather COP.
The best option available today for a DIY type install appears to be the 6k BTU Mitsubishi model (Outdoor unit: MUZ-FH06NA[-01 to include the condenser heater], Indoor unit: MSZ-FH06NA-U1) which modulates down to 1,600 BTU at 47degF (4.26 COP) and provides between 8,700 (5degF) and 14,000 (47degF) BTUs with COP close to or above 3. This unit is available through multiple online retailers in the $1,500 to $1,700 range plus additional parts bringing the total cost just North of $2k. This will require charging lines, etc. Other options that may work here with similar amounts of skill to DIY include TOSOT and Innovair units currently being sold by Signature Solar, (the TOSOT appears to be AHRI Cert # 10062018 which you can search for on the NEEP database and the Innovair specs appear to match the TOSOT unit). Fujitsu (AHRI Cert # 204752937) is the other pretty good unit easily available to consumers at AC Wholesalers and AC Outlet.
If you’re not interested in DIY’ing a mini-split install, the best options are from Mitsubishi, American Standard & Trane. If you don’t have an installer in your area that carries these products, the 47degF modulation will be in excess of 2,000 BTU. Great options include Fujitsu, TOSOT, Friedrich, Gibson, Gree, Ameristar, Direct Air, LBG, Lennox, AZUR, Direct Air and Bosch. Availability and market penetration of these products appears to differ dramatically by location.
You can check cold weather efficiency at https://ashp.neep.org/#!/ and cross reference information at AHRI https://www.ahridirectory.org/NewSearch?programId=41&searchTypeId=3&productTypeId=4611. These databases are kept up to date and should remain great resources for years to come. The advice above is obviously just a snapshot in time, but hopefully proves helpful to anyone with a similar design goal in the coming few years. Good luck with your project(s)!
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