Employs electronic growth valve to precisely control the circulation of R290 refrigerant into the evaporator in addition to a 4-way valve to divert the refrigerant circulation with the defrost operation They set to operate and at the conclusion of the day had eliminated Pretty much most of the old products.From https://socialbuzzfeed.com/story5977216/the-2-minute-rule-for-air-source-heat-pump