None as far as I can tell. Some people I know here in Norway turn off the heat pump when the outside temperature is below -25°C, but those pumps were installed

None as far as I can tell. Some people I know here in Norway turn off the heat pump when the outside temperature is below -25°C, but those pumps were installed some years ago. New ones do have a significant gain at that temperature. For instance this Samsung Nordic Home: https://www.elkjop.no/product/hjem-og-husholdning/oppvarming/varmepumpe/215947/samsung-nordic-home-25-varmepumpe-nordichome25 https://www.elkjop.no/product/hjem-og-husholdning/oppvarming...