Heat pumps are generally divided into :

• geothermal
• air-to-air
• air-to-water

The geothermal heat pump uses a brine fluid to collect the heat from either ground or deep holes into the bedrock. This is the most reliable source.

Air-to-water heat pumps are normally connected to indoor storage tanks. Ordinary water is used to transfer the heat between the outdoor unit and the tank(s). Anti-freeze agents are recommended.
They have a high capacity, often 7-20 kW.

Air-to-air heat pumps are commonly used where the capacity don’t have to exceed 4 kW. The indoor and outdoor units are interconnected with pipes containing a refridgerant agent, e.g. R410a.

The COP value tells you the efficiency factor of the pump.
A value of e.g. 3, inplies that it will output three (3) times the effect in heat, in comparison with input effect.