I had heat pumps added to my home this past summer, primarily to provide air conditioning. This article is a good overview of what I learned in the process. I'll add a few lessons:

- Get a few quotes and iterate on the plan. Save the contractor you'd most prefer to work with for last, because you'll have a much better idea of what you want by then, and your knowledge will save you money.

- There's a fun puzzle to retrofitting older homes (no existing ducts): where you place the lines, where you place the units, and where you place the heads are all variables to play around with. This is part of the reason for getting different quotes. Only the last two contractors I spoke with were able to come up with ways to keep all the external lines on the side of my house and away from the street. Only the very last contractor offered a ducted system for bedrooms, so I have head units downstairs but the upstairs is vented.

- No one will run a real Man J calculation, they all use proxy calculations. This should be fine but you almost certainly want to slightly undersize your system. If your system is always turning on and then turning off (because it is oversized and heating / cooling the room very quickly) you'll end up with humidity and mildew problems, you want it to be running "low and slow" while it's on.

- They work amazing for air conditioning. For heat, your existing boiler + radiators is often nicer and more cozy. It's also often cheaper — heat pumps are electrically efficient, but natural gas costs vary significantly by location and for me (~Philadelphia) heating with my boiler is a lot cheaper than doing it with the heat pumps.

- I live in an older home, so before I installed heat pumps I had to replace my ancient windows for modern ones that didn't constantly leak energy. I'm happy with the results but it adds a lot to the startup cost.