[jfengel]

If you wanna get technical, it's about heating your apartment using heat from the outside. Remarkably, it can do that even when it's colder outside than it is in your apartment.

The technical distinction is important, because you get out more heat than you would by just running the electricity into a resistor. And not just by a little: you might get 3x as much heat out.

It's not magic, though it sure seems as if it would be. You really are extracting heat from the outside; the outside gets even colder.

Electricity is involved, but to drive a mechanical motor. It's thermodynamically different from pumping hot water around, where 1 joule of fuel turns into 1 joule of heat. In a heat pump 1 joule of electricity can turn into 3 to 5 joules of heat.

[hash0]

Thank you for that - NOW it's starting to make sense to me. Question remains, though: Why have we not done this before? It seems like the tech has been around for a couple of decades at least, yet I have never heard about heat pumps in this context before last year.

[jfengel]

It's not easy to add a heat pump to an existing building. You can't add it to a radiator system. You need to add a whole air handling system.

The heating system in place was good enough. It used existing infrastructure.

Heat pumps are great for new construction, and they're even more important now that you're adding air conditioning. You didn't need air conditioning nearly as much a couple of decades ago.

Heat pumps are very common in US homes. One advantage is that they run off electricity rather than gas, so you don't need to add a gas hookup to the house. Existing European infrastructure is mostly already rigged for gas, so that's not as big an advantage.