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Notes -
The new Mitsubishis will still outperform baseboard down to at least -15F (realistically down to whatever their minimum temp is, usually -22-25ish), but since baseboard is so ridiculously expensive it's rarely a good metric unless electric is your only option. See the graph in that post for all the stuff that would also outperform baseboard lol.
It was between 25 and 40F for most of Nov, so basically ideal heat pump conditions as typical for this area. In late Dec it went down to the tweens for a few days, causing the grid to fail several times.(Yeah, the management is that pathetic. I feel pretty vindicated on the "have a backup rather than rely on cold weather performance" argument)
Other people's Mitsubishi FS units kept up fine at that temp (at least when the power was on) but I turned my cheapo unit off and used the wood stove all week instead. Mine would only get a COP of about 1.7 at those temps, vs 2.5ish for a mitsu. And the output on mine would drop to maybe 6kbtu, while the Mitsus were still putting out about 16k, still above their 12k rating.
Good to have feedback. I haven't worked directly in HVAC in several years but recall heat pump performance getting progressively worse as temperatures got farther below freezing.
It still doesn't feel hot when the heater kicks on, right?
Oh, they definitely get worse. Even the Mitsubishi has lost 1/3 of its capacity by 12ishF. But the good ones are way overbuilt for their ratings now, and the vapor injection where pre-expanded gas subcools liquid going into the evaporator does a nice job of boosting capacity at lower temps.
(If you can explain how those work, I'd be eternally grateful. Been trying to wrap my head around it for ages--I get how a regenerator heat exchanger works on a steam turbine, but this is way more confusing somehow)
What do you mean by doesn't feel hot? The outlet air from mine is pretty low by furnace standards: 125F on autofan, down to 95 if I turn the fan up to get more airflow over the condenser.
Subcools liquid going into the evaporator? Subcooling in HVAC and refrigeration usually refers to how much sensible heat(that is, over and above that required for condensation) was released in the system in the condenser coil. Unless of course they're(whoever "they" is) referring to the condenser for the heat pump as an evaporator because that's what it is when working as an air conditioner, in which case gas refrigerant used to cool the system from the heat of operation(mostly the compressor motor) is routed directly into the condenser to add heat when the reversing valve(part that switches between an air conditioner and a heat pump) is in use. That would be mildly unconventional terminology but not any worse than usual marketing lies from HVAC manufacturers.
Honestly a lot higher than I remember air coming out of a heatpump.
Nah, this is a weird new thing they do where some of the liquid coming out of the condenser is split off to a separate expansion valve, then through a heat exchanger where it subcools the main liquid line like a mechanical subcooler, then gets injected back into the middle of the compressor at medium temp and pressure without going through the evaporator.
https://youtube.com/watch?v=74o3bPemVUU
Good video on it. I understand how extra subcooling increases capacity and that the injected vapor cools(?) the compressor and reduces outlet temps by shifting to a lower isentrope curve(??), but the rest of it is beyond me. Apparently it works out thermodynamically similar to having a two-stage cycle with two separate compressors coupled with a heat exchanger, like they use for the big cold temp industrial stuff.
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