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Heat Pump Size Guide

Getting sizing right for heat pumps is crucial for maintaining their efficiency.


If the heat pump is too big for the space requiring heating (or cooling) it will cycle on and off repeatedly (as the target temperature will be reached very quickly), which is inefficient and reduces the life expectancy of the unit.


If the heat pump is too small, the unit will run continuously as it attempts to reach the set point, which reduces the outside coil temperature and causes it to defrost regularly.

A general recommendation is to avoid using extremes of annual temperatures when selecting the ambient design condition and also to avoid exaggerating the set points for indoor heating and cooling.

Heat pump sizing calculations must take into consideration:

  • region, location and orientation of the building
  • specific local conditions
  • orientation of windows
  • seasonal high and low ambient temperatures
  • levels of insulation
  • number and type of occupants and their levels of activity
  • solar gain

Ducted heat pump systems require careful design. They can have heating energy loss of more than 30% if the ducts are overlong and/or have many bends, or travel through uninsulated spaces such as attics. Also, Ducted systems will have higher than necessary pressure, which can cause duct leakage.

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