Produce hot water for storage and house heat and for use, with a $6kW system. Open source it to get it to $3k. See Hot Water Heat Pump. Main consideration is temperate year-round, so requires down to -22F. Sizing is 24k BTU for 1400 sf 
Senville Aura 24000 BTU Installation Instructions
- Coefficient of Performance. Note that the Europeans have a more sophisticated Seasonal COP (SCOP) 
- US has SEER - Seasonal Energy Efficiency Ratio
- Heating Seasonal Performance Factor - HSPF -  - Energy out/ energy in in BTU/W. Ex - 24000 BTU with HSPF 10 means energy use is 2.4kW. This is essentially a nonsensical, unit-mixing version of COP created by someone who had nothing better to do than come up with nonsensical metrics? Or this is different than COP or SCOP, and has some additional value? Since SEER / HSPF is just a multiple of COP, it appears that there is no value added by SEER / HSPF. 
- Cost - note that the above states $7-11k system cost - large value that the owner-builder can create as the hardware involved is only 1/5 of the cost.
- Ground Source - tubing install cost is $13-24k according to above video. Extraction is 10-30W/meter of tubing. Water source heat pump is a bit better, and has extraction of 20-60W/meter of tubing.
- -13F 2 ton 
- 2 Ton, 21 SEER, $1389, but only to -4F. 
- 2 Ton, 21 SEER, $1388, down to -13F 
- 2 Ton, 21 SEER, $1788 2 zone - but down to -4F 
- 2 Ton 3 zone 21 Seer $2k but down to -4F - 
- Senville - 2 ton 1 zone 21 SEER, 10.5 HSPF down to -22F R410 $1480. But this specific one is only 5F, description is wrong if model number is right.
- Senville Senville SENL-18CD Mini Split Air Conditioner Heat Pump 18000 BTU 19 SEER - Only to 5F
- Mr. Cool DIY only goes from 5F .
- Mr. Cool DIY Multizone goes down to -13F 
- Mr. Cool Olympus - only 5F 
- Mr. Cool Advantage - only 5F
- Blue Ridge - 5F - 1.5 Ton - 
- Blue ridge 3 zone -4F 2 ton - 
- BLue Ridge -4f - 
- Ductless Aire 24000 but 5F min 
- 5F - Coliera - 
Narrowing Down Sourcing
The lowest temperature option is Senville, others going down to -13F with unspecified performance at the low temp.
- Senville Aura Series 24HF Model -  - says it's down to -22F with 'up to 75% efficiency' and precharged. Senville SENA-24IC-Z 24000 BTU
- But, is it 75% efficient at the low temp (of possible Carnot Efficiency?)
- Has heat belt for low temp climates
- Is self-cleaning, self-diagnosing
- Precharged on the exterior unit, but not in hoses? Must pull vacuum on hoses?
- Has a wall mount available for $44 for keeping it off the ground (out of snow in winter, out of rain splatter otherwise).
- Installation - 
- HSPF 11.5
- Ships in 3-5 weeks.
- Wayfair has a cheaper Sena 24HF - 
- Why is this one $2200? 
- Amazon looks good - 
- 1500 sf 2 zone - $2k - 
- $1500 in stock - 
- 2ton, 9 HSPF, 17 SEER, $1040 . Has outlets for up to 4 heat exchanger units. One unit comes with the basic package. This one needs to be vacuumed and pumped. DIY versio is precharged for $800 extra. You can set it for freeze proof - which keeps your space at 42F.
- Videos for Mr. Cool:
- Elevate so snow doesn't bury it 
- Base pan heaters are recommended for sub-freezing climates - . Different manufacturers sell these for their brands.
- Wind baffles help control the heat exchange in the outdoor unit in cooling mode. 
- Heat Pump Installation
- R410-a purchase requries EPA 608 certification. 
- Online EPA 608 certification for techs - $300. 
- Hot Water Heat Pump
- Tonnage calculator - 
- A Video by the Youtube Channel "Technology Connections" Titled "Heat Pumps: the Future of Home Heating" [[Category:Calculators]