Takes advantage of electronic growth valve to exactly Command the movement of R290 refrigerant in the evaporator in addition to a four-way valve to divert the refrigerant flow to the defrost Procedure Lots of the maintenance requirements for air source heat pumps mirror that of standard air-con and furnace installations, https://purethermal.co.uk/media-news/q4-launch-lithium-large-capacity-r290-ashp-from-pure-thermal-and-enerblue/