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Comment by dralley | Hacker News Reader

My understanding is that high-end residential heat pumps (e.g. Mitsubishi HyperHeat) will do this at lower temperatures as well. If the temp drops into the zone where the heat pump starts losing effectiveness (which on such systems is still pretty low, supposedly mine provides the full heat rating down to around 0F), they can run the blower motor in a less efficient regime to supplement the heat pump with a bit of resistance heating, without needing to fall back to a true "auxillary" heat source like heat strips or a furnace. That can extend the effective range down to a few degrees below zero (Fahrenheit).

Since I live in the southeast, that's more than good enough to skip installing any auxillary heat source in the first place.

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I find it interesting that you can find stuff like this even in biology: When you shiver in cold weather, your body is basically doing the same.

This is an aside, but as a native of winter cold: shivering is an excellent strategy to avoid death, but it’s a terrible strategy for actually feeling warm. (Better is moving, relaxing, and massaging the ears/hands/feet to promote blood flow.)

Sort of makes sense to me. From what I understood from the wiki [1], you can compare the temperature regulation (very) roughly to a thermostat: There is a "target" temperature that the body "wants" to reach and a "current" temperature that the body senses it currently has.

If "current" is higher than "target", the body invokes several mechanisms to bring the temperature down, such as turning up sweat production and making you feel hot; if "current" is lower than "target", it does the same to raise the temperature e.g. by shivering and making you feel cold.

(Fever also works by changing the target temperature of that regulation system, which is why you feel cold at the beginning when the target was raised to the fever temperature and hot at the end when it was lowered back to the normal body temperature)

So it makes sense to me that shivering doesn't make you feel warm, because both, the feeling and the shivering are efforts by the body to raise the temperature.

[1] https://en.m.wikipedia.org/wiki/Thermoregulation_in_humans

Easy upvote.

I think there’s something else mentioned on that page that is a major contributor to feeling cold: redirecting blood flow away from the skin and in to the muscles (/organs)

> shivering is an excellent strategy to avoid death, but it’s a terrible strategy for actually feeling warm.

Sounds like an excellent survival strategy - saves you, but leaves discomfort so you have an incentive to get somewhere safer.

Don't know about high end - I literally have the cheapest unit from Midea and it does exactly this when it's cold.

Interesting, that does not sound efficient? How much more efficient than using electrical heating directly instead?

0F ? That can't be right?

My Mitsubishi gives heat to around -31F (MSZ-RW Sumo)

The efficiency of electrical resistance heating is exactly the same whether you use motor coil resistance or resistance wire.

However, they simplify their system design a little (fewer contactors, fewer wires, fewer components to buy/test)

It's not the same if you generate the heat outside and bring it inside, though. I don't know enough about where the fan is located to know if that's what you mean, but that would be different.

air/air heat pumps have a fan on both the inside and the outside. In modern designs, both those fans will be brushless DC, which enables deliberate heating through software.

Note however that both fans will typically be ~150 watts - which means that when heating they can't produce orders of magnitude more heat than that. So heat produced by the fan motor, even when run to generate as much heat as possible, probably won't be a very significant contributor to overall system heat output.

Logically you'd keep as many parts as possible inside, where the temperature is easier to control?

Maybe if you expect the heat pump to mainly move heat in, the heat generating parts should be on the inside, and reverse if you’re mainly moving heat out (A/C).

Don’t know if this is considered during installation?

The compressor is the only component that generates a substantial amount of heat. That's really it's purpose: trading a pressure increase in the refigerant for a rise in temp.

The compressor is usually in the outdoor unit, because of noise, size, and proximity to the controls. It also is the item that consumes most of the power, so for minisplits, the outdoor unit is the part wired for high currents. It usually is wrapped in a nice insulating blanket (for sound as well), and vapor injection techniques used in cold climate pumps means the heat wasted is minimal.

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