I'm not sure if this is this right place for this post but apologies if it isn't:
I'm glad to see heat pumps are becoming popular, they seem to be fairly efficient from what I've heard, although not everyone seems to be that keen on them. There's a lot of confusion about how they work which is totally understandable. We're always told that they extract 'heat energy' from outside, but how is that possible when it's colder outside then in? I did 2 years at college training in aircon and commercial refrigeration many years ago, my career in it was very short-lived thanks to a teenage nitwit who shot out of a side road one day and wrote off my little Peugeot 205, which injured my hand and wrist (as I was gripping the steering wheel very tightly on impact). I couldn't write or anything for a couple of months and had to find a different career in the end (I got into computers and web design instead).
Anyway, I understand heat pumps because, as most people know, they work on the same principle as refrigeration does.
The interesting thing about heat pumps is that the way they generate sufficient heat to heat our homes is by rapidly compressing a gas. It's not actually a result of extracting any 'heat energy' from outside as we're often told.
I remember as a kid that every time I pumped up the types on my bicycle the pump would get so hot that it was almost to hot to hold sometimes. Obviously, when you rapidly compress air or a gas it gets very hot; that's basically how diesel engines work, the air is compressed and gets so hot it ignites the fuel. The heat pump works on pretty much the same principle, and the heat generated is used to heat water for the home.
The whole thing about pumping the gas outside your home and then bringing it back in is simply to make it possible to repeat the cycle of compressing gas to create heat. Why is that necessary? It's because that by the time the gas has been compressed and then given up its heat to heat our home it has cooled a bit and condensed back into a liquid, and you can't compress a liquid! You can only compress a gas. So the liquid has to be turned back into a gas, and that's where the outside unit comes in. The gas is pumped round and then forced through a tiny hole, which has the opposite effect of compressing it, it vaporizes it and turns it into liquid droplets and makes it very cold, colder than the air outside. And because heat energy always moves from a warm place to a cold place, the vapour will absorb any heat energy outside (because the vapour is colder) and it will then 'boil off' to become a gas again ready to be rapidly compressed again to generate heat. But It's the compression that gives us our heat NOT the actual absorbing of any 'heat energy' from outside.
When we think of heat we immediately think of warmth, but the term 'heat energy' means something a bit different. No matter how cold it gets, there is always some 'heat energy' in the air (until you get to near absolute zero which is about −273 Celsius).
So the job of the outside unit is simply to return the liquid vapour back to a gas so we can compress it again to get some more heat, in a cycle.
I wasn't too sad by the way to see the back of that Peugeot as it was so hot inside in the summer, it was like a mobile oven. It was very economical though.
Another thing connected with refrigeration/aircon that's interesting is that I was always taught that if an aircon system has a leak it is technically illegal to put gas back into it without fixing the leak first (as stated in the COSHH regulations}. To do so is effectively illegally releasing (or 'disposing') of refrigerate into the air, because it will just leak out again. All aircon and refrigeration is supposed to be in a SEALED system. It's not supposed to leak out.
How is it then that so many garages offer an aircon regass??? Your cars aircon should not need regassing. It's supposed to be a sealed/closed system. It's not like your car's oil or petrol that needs 'topping up'. It should last the life of the aircon unit. If if needs regassing then it means there's a leak, which should BY LAW be fixed BEFORE any gas is reintroduced.
It was something I read in a magazine in a dentist's waiting room years ago funny enough that got me thinking about this. It was a car magazine and there was an advert offering regassing aircon machines for mechanics, saying that a garages ability to offer aircon regasses was a very "lucrative" adjunct to their business. But the fact is, if you add gas to a cars aircon WITHOUT fixing the leak first you are technically 'illegally disposing a controlled substance'. Just a thought.
Of all the marvelous things that humans have invented the invention of a machine that can lower the air's temperature has always seemed to me to be an almost magical achievement. It's amazing really when you think about it.