It’s actually going into air conditioning to defrost the system. To hasten the melting process, it disables the fan outside.
But since A/C in the middle of winter isn’t cool (hehe), the system energizes your electric back up heat to mitigate the A/C. It wants to get that done as rapidly as possible.
Air conditioning systems, and heat pumps are simply moving heat (btus) from one place to another. If you place a cold Coke in your 70° kitchen a heat transfer takes place; Btus from the ambient air in the kitchen transfer into the Coke and in time the Coke will be the same temperature of the kitchen: 70°.
So…hot always “runs” to cold, right? Nature always want to equalize. It’s a little harder than our Coke analogy however.
How do we essentially “harvest” btus from the outside ambient conditions when it’s cold out side? (Of course, “cold” is just the absence of heat)
Answer: by creating a surface----a Coke can-----that’s colder them the surrounding ambient, from which heat (btus) will transfer— an arrangement where hot will run to cold.
So if it’s 20° outside------even though we perceive that to be cold—or, rather, lacking any heat----there is some heat in that air. If I put a surface----in this case a refrigeration coil----that I’ve engineered to be 5°, the btus in that 20° air will transfer to the 5° coil. Heat (what seems to be cold) has run to cold[er]. In other words, cold has run to colder.
Now I can take those btus and essentially concentrate them----kind of round them up—and heat your home.
The perfect medium to do that is refrigerant–largely because if it’s low boiling and condensing temperatures/pressure.
The refrigerant is pumped through the system where it “accepts” heat/btus through "boiling (and changes to vapor (think:steam) and them is pumped somewhere where the heat is “rejected” through condensing. (think:liquid)
And the process repeats again and again and again.
I agree with YamatoTwinkie. Of course, in the summer the heat pump is doing the reverse—it’s heating the world, while air conditioning your home.
In reality, of course, it’s simply moving heat from one place to another. (taking the heat of compression out of the equation for now)
So in the summer (and I assume you were talking about the air temperatures outside as opposed to inside) the air temperature will be higher when it leaves the system. (while in both summer and winter the opposite is happening inside)
The heat pump COP complicates the often discussed theory about whether setting back the thermostat at night during heating season saves energy. If you use strictly heat generating central heat, cutting back the thermostat will always save energy. Heat loss, which is what you have to make up, is always proportional to the delta T between the house and the environment. Actually, since as your house cools off it heats the outside, it’s the log mean square of the delta T, but the outside doesn’t heat much, so forget about it.
My heat pump (COP maybe 3.5, depending on the outside temperature), if the temperature set on the thermostat is 5 degrees higher than the temperature in the house, will start the emergency heat strips (COP 1, or a little bit less). So if I cut back the thermostat 10 degrees at night and crank it back up in the morning until my house heats up that first 5.1 degrees the efficiency of my heating takes a hit, because the folks that designed it figured that a five degree difference meant the heat pump wasn’t keeping up.
I have to bump the temperature in a couple of stages to get the heat pump to do the work without relying on the resistance heating.
Every stat I’m aware of will do this unfortunately; if you’re more than a couple (3 give or take 1) degrees off setpoin it will bring in the back up heat.
One potential fix is to install an outdoor thermostat that will disable the electric heat if the outside temperature is above the balance point.
It will force the heat pump to do the work (because above the balance point it can) although it will obviously take longer to get there.