[jrkatz]

Heat pumps use energy to _move_ heat, not create it. Your refrigerator, for example, moves heat from inside the fridge to outside the fridge. Your AC operates on the same principle. Pump the refrigerant in the other direction (note: mechanically this isn't possible in garden variety ACs) and you reverse the effect. Given the same amount of energy and typical conditions - this won't work to absurdly cold temperatures - it's several times more efficient to use it to run a heat pump than to convert it to heat directly.

[pedrocr]

I'm always amazed at how in the US there are "garden variety ACs" that lack the very small amount of parts needed to be reversible. I haven't seen any models like that for sale in Europe. Since having AC is also much more frequent in the US than in Europe it's really a shame that isn't just regulated as mandatory or something so that electrifying house heating becomes that much easier.

[ninkendo]

This also leads to the counterintuitive notion that it’s more efficient to burn natural gas to spin a turbine, and create electricity used to run a heat pump, than it is to burn that same natural gas directly for heat. It’s one of those situations that just shouldn’t make sense, until you do the math and see that it does (although only for ambient temperatures above a certain threshold.)

[bombcar]

They start to lose efficiency roughly around freezing, so not very cold, and can cease working entirely at zero Fahrenheit or so. But geothermal can work much longer because you dig holes deep enough to use the ambient earth temperature instead of air.

This won't work in permanently frozen areas like Antarctica.