How do heat pumps keep working in very cold weather?
Answered
Answer
Modern cold-climate heat pumps use variable-speed inverter compressors that keep running at low output instead of cycling off. Many add vapour injection, which pushes extra refrigerant into the compressor and holds capacity near 100% down to −15 °C. Below that, efficiency falls, but field data still shows a seasonal COP around 2.5, so they deliver more heat than the electricity they use. Backup resistance strips only switch on during the coldest hours.
Passage· latest cited
Configure auxiliary resistance heat to stage in only when the outdoor unit cannot meet the load, typically a few hours per year.
4Installer guide: auxiliary heat · docs.heatline.dev
Sources
How inverter compressors handle part load
Vapour injection in cold-climate units
Three-winter field study, 42 homes
Installer guide: auxiliary heat