As the heat is transferred into the home, the gas falls in temperature, creating it to return to a liquid condition. With a global warming opportunity of only 3, It's a lot better than R32 but it does have a person dilemma. R290 is propane and, therefore, is highly flammable. https://heat-pumps02234.dm-blog.com/31007780/top-guidelines-of-high-temperature-heat-pumps