There is a trade-off between Air Source Heat Pump sizing and noise pollution. Air Source Heat Pumps work by drawing large volumes of air across a plate heat exchanger. To keep noise at a minimum, the fan blades on the outdoor unit need to spin as slowly as possible, but as you slow the fan speed the size of the blades themselves need to increase. Manufacturers try to balance having the outdoor units as small as possible, whilst also keeping noise levels to an absolute minimum.
