Uses electronic enlargement valve to exactly control the movement of R290 refrigerant to the evaporator in addition to a 4-way valve to divert the refrigerant move for the defrost operation A lot of the upkeep requires for air source heat pumps replicate that of traditional air con and furnace installations, https://purethermal.co.uk/media-news/q4-launch-lithium-large-capacity-r290-ashp-from-pure-thermal-and-enerblue/