The hype about heat pumps – Air source heat pumps (ASHP)

Air source heat pump

In Heat pumps, Heating

Hailed as the current answer to low carbon heating, heat pumps have been billed as the future for heating and cooling in the UK. But what do you know about the technology, and is it suitable for all homes and buildings?

Heat pumps have been used for decades, predominantly in the air conditioning industry. It is a proven technology, used worldwide, mainly to provide both cooling and heating in commercial and industrial properties. However the appliances have not been used widely in UK residential settings, where wet central heating systems are king.

In the UK domestic heating sector there are two main types of heat pump technology, air source heat pumps or ground source heat pumps. Both absorb heat from another element (the air or ground) via a refrigeration cycle to heat your home and hot water.

Whatever type of heat pump you go for, its good to keep in mind that heat pumps have a lower heat output than a gas or oil-fired boiler. Therefore, they won’t be as responsive as a traditional central heating system. Heat pumps perform best when they heat homes slowly and over longer periods of time, this is why they are a very good fit with wet underfloor heating systems.

Additionally, not all types of heat pump will supply hot water, and the ones that do, won’t deliver hot water on demand in the same way, or to the same temperature as a combi boiler. This means that you will also need to consider how you will heat and store hot water.

Depending on the type of heat pump you choose, ASHPs can be used for heating, cooling and hot water. In UK residential use, air-to-water ASHPs are the most popular as they are compatible with wet central heating systems. Many air-to-water ASHPs can also provide hot water too. If you are looking to replace your current heat source, the majority of systems will use this type of ASHP.

Alternatively, air-to-air ASHPs are often used in ducted systems. These heat pumps do not produce hot water, but they can provide very effective cooling.

How ASHPS work?

In the simplest terms, ASHPs move heat from one area to another. Ambient (outside) air is drawn into the heat pump by a fan, which then forces the air over a heat exchanger called an evaporator. Within the heat exchanger is a refrigerant, which is at a lower temperature than the air. Heat will always move from a hot place to a cool one, and so the refrigerant absorbs the heat from the air, and is warmed up. As the refrigerant has a low boiling point, it starts to boil and turns into a gas. This gas next passes through a compressor, which via pressure, increases the temperature further. This compressed, hot refrigerant gas then transfers heat via another heat exchanger called a condenser, into the heating and hot water circuits of the property. The refrigerant condenses back to a liquid and passes through an expansion vessel, which reduces pressure and temperature further. The cooled refrigerant returns back to the evaporator to start the cycle again.

ASHPs work like a fridge in reverse, extracting renewable heat from the environment, and transferring it to your heating and hot water system. While it does use electricity to do this, the heat output is higher than the electricity input, (around 1kW electricity per 3-4kw of heat generated) making it a sustainable system.

The Energy Savings Trust has a very good video and further information on this process at

Pros and cons of air source heat pumps

The pros:

  • ASHPs are highly efficient.
  • ASHPs can extract heat all year round. They can still work when air temperatures are as low as -25°C.
  • ASHPs work particularly well with underfloor heating and properties with larger radiators.
  • If you are replacing an electric heating system with an ASHP you will see a big reduction in running costs.
  • ASHPs will cut your carbon footprint.
  • ASHPs can be used with other renewable technologies such as PV solar panels and solar water heating.
  • Electricity generation is becoming increasingly greener thanks to renewables. The more the national grid decarbonizes, the more sustainable tech like heat pumps become.
  • Heat pumps can have a lifespan of 20+ years.

The cons:

  • ASHPs need to be installed outside, with enough space around it for good airflow so you will need room to either install it in your garden or attach it to the side of your building.
  • ASHPs don’t deliver hot water on demand, so you will need room for a hot water cylinder for hot water storage.
  • The more heat and hot water you need, the bigger the ASHP and associated system appliances (such as a hot water cylinder) will be.
  • ASHPs will use more electricity in winter, as they have to work harder to extract the heat from colder air. This means you will still have higher winter heating bills.
  • Heat pumps run at considerably lower temperatures to traditional boilers at around 50oC (55oC max), and so to get the best out of the system, your home will need to be well insulated. Homeowners will need to take into account any additional costs to make their homes energy efficient.
  • If you are replacing a gas central heating system with an ASHP, then your running costs are likely to be similar, but your installation costs are likely to be more.
  • Depending on your Local Authority and if you are in a conservation zone, you may need planning permission to install a heat pump.
  • For your ASHP to work effectively, you may need to make significant changes to your current heating system.
  • Depending on the age and type of property, it may be difficult to rely on a heat pump for your heating and hot water needs all year round. You may need to invest in a hybrid system, where the heat pump provides your basic heat and hot water, and a conventional boiler provides a boost to both when required.

Installation and costs

Changing from a gas or oil-fired boiler to an electric heat pump is unlikely to be a like-for-like swap for the majority of households. At present the costs involved in installing a heat pump, the associated adaptions to the heating and hot water system, plus home improvements for energy efficiency will be prohibitive for those retrofitting. In new builds, the installation is easier and more cost effective. This is why the legislation around low carbon heating is being introduced initially for new builds, rather than current housing stock.

Luckily, Government grants are set to bring the costs down, meaning they are becoming a more viable option, even for existing homes. However, the grant coincides with the closure of the Domestic Renewable Heat Incentive Scheme.

There is no ‘one size fits all’ cost when it comes to heat pumps, as there are many factors that affect prices. These include:

  • The type of heat pump you install.
  • The size of your property.
  • Your hot water requirements.
  • The current energy performance of your home – most houses will need improvements which could involve new loft or wall insulation, draught-proofing or even new windows and doors.
  • Your current system e.g. if you already have wet underfloor heating or oversized radiators, you’ll be in an ideal position to swap to heat pump technology. If you don’t, then you’ll need to pay for additional system alterations to ensure the system works effectively and efficiently for you.
  • The brand, performance and efficiency of the heat pump you choose will affect the cost.

Estimates on installation prices vary widely. As a rough guide an air-to-water ASHP installation can cost between £5,000-15,000. At present, if you are replacing an efficient gas combi boiler with an ASHP you are unlikely to see much change in your bills. The biggest reductions will be to your carbon footprint initially, but as the price of gas rises, it’s likely these systems will offer better value for money. Heat pumps have a longer lifespan than boilers, and are less prone to breakdown. Long term, the heat pumps do have benefits when it comes to maintenance and the whole life cost of the system.

However, if your house is not adequately insulated or your system is not designed and installed correctly, you could see a substantial rise in energy costs. It is therefore vital you have a qualified and knowledgeable professional design and install the system, taking the energy efficiencies of the whole home and the way you intend to use the system into account.

Search for renewable engineers in your area via our find a plumbing or heating engineer online tool.