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External air-source heat-pump unit

(Click to enlarge photo)

This is the other common type of heat-pump but, in this case, the heat comes from the air. The large fan cuts in to keep the unit at working temperature. The hot refrigerant is pumped along a pipe through the wall to a unit inside that dumps the heat into the room, then the refrigerant returns through a second pipe to be heated again and so on. Often, as in this case, a heat-pump is designed so that it can provide heat in the winter and air-conditioning in the summer. Its exactly the same system with a valve that simply reverses the flow of refrigerant as required. There is one big drawback with air-source heat-pumps, if it is below about 10 degrees C outside and there is any rain or the air is humid, the fins on the pipes of the outside unit clog with ice and the unit must be switched off. The unit can still extract heat at that temperature but there is no way of getting the heat into the pipes from the air because it can't flow between the fins on the pipes. Some units (not this) auto-defrost to avoid this problem.