An in-depth look at this energy-efficient, renewable way of heating your home
WORDS DAVID HILTON
An air-source heat pump (ASHP) can supply a home with heating and hot water, heating alone, or heating and cooling. It may cost more than a fossil-fuel powered boiler, but there are other benefits.
How does it work?
There are two types of ASHP, with both able to generate warmth even if the outdoor air is below 0°C.
Air-to-water systems include a fan unit fitted outside the property. It contains a special coil that absorbs heat, known as an evaporator, plus a compressor and another heat exchanger, known as a condenser. Warmth in the outdoor air is absorbed by a refrigerant fluid inside the evaporator. The refrigerant passes through the compressor, becoming a gas as it does so, which increases the temperature. This warmth is transferred via the condenser to a water-based heating system with radiators and/or underfloor heating (UFH) as well as the hot water supply.
Air-to-air heat pumpsare air-conditioning units that provide warmth and cooling. They extract heat from the outdoor air and increase the temperature in the same way as air-to-water heat pumps, but they do not supply hot water. The refrigeration cycle can be reversed to deliver cooling.
What’s the energy demand?
An ASHP consumes more electricity than a fossil fuel-powered boiler, but its heat energy output is greater than the electrical energy it consumes. An ASHP might use around one kilowatt (kW) of electricity for every three or four kWs of heat it produces. This ratio is lower if the heat pump is working hard, such as in very cold weather or when the demand for heat is high.
Measures of performance
A heat pump’s efficiency is indicated by its co-efficient of performance (COP), which shows the energy delivered compared with how much electricity powers the pump. Different ASHP models have different COPs – one with a COP of four indicates that for every one kW of electricity used, four kW of heat is generated. This information is included with the manufacturer’s product specifications. The COP is a snapshot of the efficiency of the unit. The average COP of the ASHP is the seasonal co-efficient of performance (SCOP), where the season is one year.
It’s important to determine an ASHP��
