Air Source Heat Pumps: ASHP

How does an air source heat pump work?

An air-to-water ASHP acts like a domestic fridge working in reverse. A fridge takes heat out of the air in the fridge and expels it out into your house, cooling the fridge and warming your house slightly. A heat pump takes heat out of the external air, cooling it slightly, and warms the water which heats your house and provides hot water in your cylinder to feed the hot water taps. The main component is shown on the left, situated outside the property, taking in warmth from the outside air with an electric pump and refrigerant.


This warmth is then released inside the home at a higher temperature and sent to radiators and/or underfloor heating.


Hot water is also stored in a cylinder and can be used for showers, baths etc.



ASHPs look similar to air conditioning units and are less disruptive than ground source heat pumps as they do not require any ground excavation to install them.


However, consideration needs to be given to the aesthetic impact on your home and while they can be screened, you can’t put things like plants or trees too close because air needs to circulate around the unit so it can’t be too restricted.


ASHPs should be positioned in a location where they are not exposed to high winds because anything above 60mph could result in the unit shutting down temporarily.


While they’re ideal for well-insulated properties and work best with underfloor heating or homes with large radiators, houses subject to draughtiness, with open chimney flues or very cold walls can experience problems.


Bear in mind they are electrically powered so would be more expensive to run than gas unless they generate more heat from the refrigeration process and heat exchanger (used to warm your home and provide hot water) to cover costs. They an ideal replacement for night storage electrical or oil fired central heating, but can be a workable alternative to gas heating also.


Therefore it’s vital you don’t make a common mistake and turn them on and off – the house should be kept at a fairly consistent temperature for optimum results.


We have specified a number of ASHPs. The latest was on a recently completed eco-home at which is anticipated to have energy bills of less than £100.00 per year including the standing charge; this will cover heating, hot water, cooking, ventilation and lighting. Clearly this is a unique house utilising the Passivhaus concept for the construction – in this circumstance the ASHP runs at optimum performance.

Costs and savings

There is an initial outlay between £6,500 and £14,000 depending upon the size and complexity. The work to the 4 bedroom house above cost £8,000 for the heat pump installation.


The period in which this amount is recouped depends on various factors: how efficiently the system works; how energy efficient your home is (i.e. how well insulated); eligibility for the government’s Renewable Heat Incentive (RHI).


If you qualify for RHI, you will receive an extra payment of between £800 to £1,300-a-year for seven years. This is a reward payment for people who use renewable energy to heat buildings to help the UK achieve its target of producing 12% of its heat from green sources by 2020.


The Energy Saving Trust estimates an average performing ASHP in a four-bedroom home could save between £545 to £880-a-year if replacing oil and between £550 to £1,060-a-year if replacing electric heating.


To view our eco-design work on line please visit:

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