Uses Digital enlargement valve to precisely Regulate the movement of R290 refrigerant into your evaporator as well as a 4-way valve to divert the refrigerant stream for your defrost Procedure Air source heat pumps use precisely the same style of technological innovation that keeps a fridge or freezer interesting – https://purethermal.co.uk/media-news/q4-launch-lithium-large-capacity-r290-ashp-from-pure-thermal-and-enerblue/