The size of the heat pump that would be ideal for cooling is not always the same as the ideal heating size. If you are in a climate where the maximum heat gain on a hot summer day is similar to the maximum heat loss on a cold winter day, then sizing a heat pump is easy. You simply design it for the same heating and cooling capacity. If the heating load is lower than the cooling load, you design it for the cooling load.
Even if the heating load is larger, most heat pumps are still designed for the cooling load because if the unit is too large for cooling, it will make the house cool and damp, which is very uncomfortable. We talked about this earlier.
One exception is two-stage compressors which allow the heat pump to operate at different capacities. This can allow a heat pump to operate at lower capacity during the cooling season and higher capacity during the heating season. The typical cooling load in the southern United States is one ton for every 450 to 700 square feet of living space, whereas in moderate and cooler climates, one ton is adequate for 700 to 1,000 square feet.
Common residential heat pump sizes range from one to five tons of cooling. Generally speaking, less cooling is required in dryer climates, and two or three story houses need less cooling per square foot of living space than bungalows. As a rough rule for heating, 40 to 60 BTUs per square foot is recommended for poorly insulated, leaky homes in northern climates. Twenty-five to 40 BTUs per square foot is recommended for better insulated, tight homes.