- You need 400CFM per ton. A 5 tons would need 2000CFM.
- Always check the minimal OD temperature your heatpump is design for. Some newer heatpump can handle -30F. Those are equipped with a crankcase heater to eliminate liquid refrigerant from compressor base (like paul.goodjohn explained) . You mus look in the installation manual of your heatpump. In fact liquid refrigerant always migrate to the coldest location. It heat ( 30W heater usually) the compressor to evaporate liquid refrigerant using this crankcase heater.
My own system is a 2-tons heatpump York YHM. it can run down to -29F. My balance point ( the point where heat loss of the house > heat produce by the heat pump ) is around 7F. below 7F my heat strip pulse (kind of modulation) to maintain a supply temperature of 90F in the supply duct. The heat pump continue to run with the heat strip. This way when it is 5F outside my heat strip run at maybe 5% + heatpump. when it is 0F it runs at maybe 10% + heatpump. My heat strip is only 5KW… It is the first winter i run my system. you wouldn’t believe how much i saved compared to last winter when i was heating with electric baseboard only. I’m in Quebec City, Canada, so winter is extreme here compare to southern climate.