I recently installed an air-source heat pump for water heating system. Usually, they cost around 10000 EUR/USD, but I got used and Chinese made one for just 500 EUR/USD. As you probably know you should never buy used heat-pump, fridge etc., just because owner bought a new one. Usually, they do not break often and if they do or they have some other problem, only then the owner would sell it (if he is evil..). Anyway, I did not consider this and was willing to risk. Also important to notice is that I am living in northern part of Europe (Latvia), so in winters we get temperatures below 15-20°C / -5°F . Air source heat pump might not be the most effective, but it is more effective than for example shallow ground (2m) heat pump that would freeze up the whole yard till summer. Maybe deep bore (+100m) heat pump would be more effective + much more expensive.

Now when we fixed minor problems, like replacing water circulation pump and some leaks in freon system, everything was working fine, but as soon as the ambient temperature was close to water freezing point -5°C till 5°C Heat pump's radiator began to froze-up. As I tracked down the problem I understood that Chinese microcontroller improperly executed defrost cycles. It almost did not execute them at all. Also as this manufacturer do not provide any information about the microcontroller, I had to create my own control panel/microcontroller for the whole system.

After implementing new control panel, I can control every aspect of heat-pump using PC and now it is also much more economic than before and do not build-up ice.

Also, I added an external sensor ("robot eye") that is tracking how much ice has been build upon external radiator/evaporator.