[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.