... Technically, you could also have a "staged" heat pump system, where you have one heat pump moving heat into or out of another heat pump using a different refrigerant that has a different performance envelope, but that's more of a cryogenics lab thing than a "heat the house in winter and cool it in the summer" thing.
That is a viesmann thing and has been done. Can heat water to 70C and so on.
First gens used 2 staged compressors, second gen had 1. Then there are also CO2 HP's that have 65°C heat by design.
That said I personally think it’s unnecessary for a mini-split to have a disconnect, but that’s code now.
Makes it great and easy to service. I personally hate disconnects with passion and wired my own with standard high current plugs.
I can't trust a disconnect I can't see inside and I've seen those fuckers weld and fry themselves in closed contact even tho the lever is pulled. The option to disconnect the AC and plug something else in outside is also a major +.
The issue with heat pump energy consumption comes from increasing compression ratio as the outdoor temperature decreases.
Not just that, the capacity is dropping as well.
A pump will do with 1kW compressor power at 5°C outside with EER of 3 about 3kW of heat.
At -5°C that compressor will only draw 800W or less with EER of 2 and that makes only 1.6kW of heat.
Since you also need a higher heat temperature that kills everything even further.
That is why ground sourced pumps are great as source temp is almost the same. Until you see the price of a well and the cost of maintaining it. It's cheaper to just put panels on whole house.
COP drops through the floor and the backup resistance strips have to kick in.
Once resistance heating kicks in the power goes thru the roof and efficiency thru floor. At that point it's better to use propane heating, even if it's the kitchen stove being lit. This heat should technically be only used during defrosts but it happens that thermostats are misconfigured that it runs all the time.
Assuming there's still anything left in stock anywhere that uses 410A. "Oh we're not making it illegal, we're just going to jack the price up on it and push something else at the exact moment the patent expires!"
You can get R32 units that will take R410a. Better to just get the R290 propane air-water unit tho. It is much better gas overall and you can refill it yourself.
Holy fuck, I forgot wall/window mounted. they're basically portables with the hot end outside though. So it's a similar, albeit marginally less bad, outcome.
They can be pretty efficient and they come in interesting packages. It's like mini - minisplit
Issue with all these is noise and heat leakage.
US homes sadly aren't built with ventilation in mind.
EDIT: Nevermind they got pulled out of market due to mold issues
