Uses Digital enlargement valve to exactly Manage the flow of R290 refrigerant to the evaporator as well as a four-way valve to divert the refrigerant flow to the defrost operation A lot of the maintenance demands for air source heat pumps mirror that of common air conditioning and furnace installations, https://purethermal.co.uk/media-news/q4-launch-lithium-large-capacity-r290-ashp-from-pure-thermal-and-enerblue/