Heat Pump Selection

  • General considerations

  • Type of heat pump

  • Sizing

  • Heat and cooling system

General Considerations

A heat pump is not suitable for every situation and it is important to be realistic about the heating load and thermal properties of your building.

In general (excluding specialist applications) heat pumps suit well-insulated buildings where the temperature of the water in the heating system can be between 30 and 50 degrees C and still be effective.  A useful check is to fully turn down the flow temperature on your boiler (not the building thermostat) on a cold day and see if the building stays warm. If it does not stay warm, you may need to install low temperature radiators or fan-coil radiators for a heat pump to work effectively.

Type of Heat Pump

The two main types of heat pump are air-source and ground source.  Water-source is also an option where the situation permits.  These terms refer to the medium from which heat energy is being drawn.  Surprisingly this heat energy can exist at sub-zero C temperatures as above absolute freezing (-273C) everything has energy. The heat pumps takes this heat energy via a refrigerant, and using a compressor/evaporator system, delivers the heat energy at a higher temperature.  

Each type of heat pump therefore works in a very similar way, but they are tuned to perform at their optimum based upon the source of the heat and the requirements of the application.

At Aquarius, we focus on heat pumps for the domestic property market and certain specific industry applications.  For domestic properties, air source heat pumps are more common due to their far lower installation cost despite slightly poorer efficiency.


Correct sizing of the heat pump should be done by a competent, MCS*-certified installer or qualified engineer.  The building fabric's thermal properties need to be assessed room by room in conjunction with creating a record of heat emitters (radiators).  A spreadsheet is completed which then provides the recommended size and expected performance characteristics.

*Microgeneration Certification Scheme

Heating and Cooling System

Heat pumps can be used with an existing heating system, but due to the lower flow temperatures required, will perform more satisfactorily with either an underfloor heating system or fan-coil radiator units that allow greater air circulation and therefore sufficient energy to be imparted to the room.

The performance figures for Aquarius are truly impressive, the build quality is reassuringly high, and the price is unbeatable.

Carl Benfield, Director