I'm hearing more and more about air source heat pumps and air to water heat pumps and i don't really know anything about them.

I think eventually the company i work for is going to be getting involved with them at some point so i'd like to understand a bit more about them, then
when i do eventually work on them, at least i will have a little bit of knowledge.

I have been looking on the net and have seen the Altherma and the Ecodan air source heat pumps.
As i understand these have are used for hot water and space heating and cooling?? and even under floor heating.

What about the air to water heat pumps, what are these used for and how do they work.