Sizing is one of the most important factors determining a heat pump's efficiency and reliability. In Western Washington, heat pumps are sized according to the heating requirements of your home. If you lived in a climate where heat pumps are primarily used for air conditioning, then you homes cooling needs would determine the size of the heat pump.
In order to determine size, we must do a heat loss for your home as well as analyse your home's duct sizing to determine air circulation requirements. For a proper installation these calculations must be done. This helps us "balance" the system to ensure that the heat pump delivers the correct amount of air to all rooms in the house.
heat pumps are sized in two ways; tone and BTUs. Tons do not refer to the weight of the heat but rather are a measure of cooling capacity. Each ton of heat pump capacity produces about 12,000 BTUs of heat. A BTU is a measure of heat output; most heat pumps are in the 24,000 to 60,000 BTU range or 2 to 5 ton range.
As the temperature outside drops, the ability of most heat pump systems (except inverter) to deliver heat also drops somewhat and the heat needs of the house go up. It is possible to install a system large enough to heat the house no matter how low the temperature is outside. But a system which is sized to heat your home during the coldest day of the year will be over sized the rest of the year. This is not only uneconomical, but also places unnecessary stress on the compressor because the heat pump cycles on and off constantly.
In the Pacific NW's maritime climate, a heat pump should be sized so that it can heat the house when outside temperatures are as low as 30 degrees, below that you will need to invest in a 2-stage or Modulating Heat Pump.