I am a happy user of a DYI air source heat pump, that I've been using for 8+ years, mostly for heating in a residential building. First, it must be told that a

I am a happy user of a DYI air source heat pump, that I've been using for 8+ years, mostly for heating in a residential building. First, it must be told that an off the shelf AC unit is technically speaking also a heat pump. There has been this distinction (and confusion) between AC units and (mostly) ground source heat pumps because, you know the latter actually have a water 'pump' to circulate fluid in the ground piping. Ground source heat pumps are far more expensive that AC units and are nearly impossible to retrofit in existing buildings. Hence, companies came up with a 'middle ground' solution, called air source heat pumps. In these devices the outdoor unit is much like an outdoor unit of a split type AC, but the indoor unit is basically a refrigerant/water heat exchanger with a water pump and some sensors around it. There is also a monoblock type of air source heat pump, where all components are in the outer block and you only connect the piping. IMHO, the reason air source heat pumps are becoming popular now is because their price went down considerably during the last 5 years, whereas the price of fossil fuel heat sources has gone up. Back then when I had to choose between a gas boiler and a heat pump, the boiler was around EUR 2, off the shelf heat pump (24000 btu) was around EUR 7k and for the DIY option I paid a total of EUR 2.3k. The reason for this was that the market for air source heat pumps was underdeveloped and companies (e.g. Mitsubishi, Daikin, Fujitsu) were charging a lot more in this market segment (rich people with new houses). At some point in time, people started realizing the benefits of a heat pump over a normal AC (less units, no noise, nor air currents, better efficiency, can produce hot water, heating is decoupled from the source and more sources, such as solar can be seamlessly plugged in). Manufacturers albeit slowly responded to this demand. Now with solar power gaining more traction, they are becoming a very obvious choice.