[teruakohatu]

Modern heatpumps can go far lower than that. Many mitsibushi sold in my country can operate at -15 Celsius. I think the newest from mitsibushi can do -20 C.

Efficiency does decrease since they need to spend more time defrosting.

[8fingerlouie]

My 2022 Vaillant model can operate down to -25C, and has a backup electrical heating element built in for when it gets colder than that.

https://www.vaillant.com/home/products/air-to-water-heat-pum...