I Don’t fit air to water heat pumps but do fit normal air con heat pumps and I’m sure at -5 they run at 95% and at -10 run at 85% of maximum heat load without fitting heater systems in condensing unit for defrost and anti frost. So when sizing the unit it should be sized based off worst working conditions going to be used in. If it’s the only form of heating I would size at -15 outside temperature so it would work in the worst of conditions. You will lose some efficiency in the milder days but better than not having heat when you really need it. |