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What is an Air Source Heat Pump?

An air source heat pump (ASHP) is a system that transfers heat from outside air into a building to provide space heating and hot water. It works much like a refrigerator in reverse, using a refrigerant gas compressed by an electrically-driven compressor to absorb heat at one place and release it another.

The main components of an ASHP are:

  • Outdoor unit: Contains a heat exchanger, expansion valve, compressor and fan. Extracts heat from the outside air and raises the temperature of the refrigerant gas inside the system.
  • Indoor unit: Contains a heat exchanger and fan. Transfers the heat from the refrigerant gas to the indoor air or water supply.
  • Refrigerant lines: Carry the refrigerant between the outdoor and indoor units.
  • Controls: Allow the user to adjust the temperature.

Unlike a furnace that generates heat by burning fuel, an ASHP simply moves existing heat from one place to another. This makes them much more energy efficient for heating than traditional systems, like a gas boiler. The only electrical energy they consume is to run the compressor, fans and pumps.

Types of Air Source Heat Pumps

There are a few different types of air source heat pumps to consider:

Air-to-Air vs Air-to-Water

Air-to-air heat pumps extract heat from the outside air and pump it indoors to warm your home. The same system can run in reverse during hot weather to move heat from inside to outside, acting as air conditioning.

Air-to-water heat pumps work a bit differently – they extract heat from the air outside and use it to warm water that is then pumped through radiators or underfloor heating systems to heat your home. The main advantage is that they can provide hot water as well as heating.

Split vs Monobloc

Split system heat pumps have an outdoor unit connected to a separate indoor unit. The outdoor unit extracts heat from the air outside while the indoor unit blows warm air into your home. Split systems are quieter and more efficient.

Monobloc systems have the heat pump and fan in one outdoor casing. They are simpler to install as there is no indoor unit, but can be noisier.

Ground Source vs Air Source

Ground source heat pumps extract heat from the ground rather than the air. They can be more efficient than air source heat pumps, but the ground loop installation makes them a lot more expensive.

Air source heat pumps are generally much cheaper to install as they simply extract heat from the outside air. Their efficiency can drop a bit during extremely cold weather, but they work very well in the UK’s climate.

How Efficient Are Air Source Heat Pumps?

Air source heat pumps are extremely efficient, as quantified by their coefficient of performance or COP. The COP indicates how much heat is generated by a heat pump for every unit of power consumed. For instance, a heat pump with a COP of 3, creates 3 units of thermal energy for each unit of electrical energy used.

The average COP of an air source heat pump in the UK is between 2.5 and 4, depending on the outside temperature. This makes them much more efficient than gas boilers, which typically have an efficiency rating of around 0.9.

The efficiency of air source heat pumps is impacted by the climate. As it gets colder outside, more work is required to extract heat from the air. This causes the COP to decrease.

However, modern air source heat pump systems are designed to operate efficiently even in colder temperatures. With an optimised design and proper installation, COPs of 2+ can be maintained even when temperatures drop below freezing.

By leveraging renewable ambient heat in the air, air source heat pumps provide a much more efficient way of heating UK homes compared to burning fossil fuels in a traditional gas boiler. Their high COP enables substantial energy and cost savings for homeowners.

Installing an Air Source Heat Pump

Installing an air source heat pump in your home is a relatively straightforward process, whether you’re retrofitting an existing house or adding to a new build.

Retrofitting vs New Build

For existing homes, air source heat pumps can replace oil or gas boilers without major disruption to your property. The external unit is fitted outside, usually on a wall or on the ground, while the internal unit can fit neatly where your old boiler used to be. Some refitting of radiators or underfloor heating may be required, but a good installer will take care of this.

For new builds, air source heat pumps can be designed into the plans from the start. This allows the property to be built with suitable radiators or underfloor heating and minimised pipe runs between internal and external units. The system can then provide heating and hot water from day one.

Space Required

The amount of space required depends on the type of system. Compact wall mounted external units need only about 1 square metre of wall space. Larger ground source heat pumps require 3-5 square metres of garden space for the external unit. Internally, units are similar in size to a washing machine.


Since air source heat pumps operate at lower temperatures than gas boilers, existing radiators may need to be replaced with larger models to maintain heat output. Adding one extra radiator to each room is often sufficient. Specialist low temperature radiators are also available. Underfloor heating systems will not need any changes.

Air Source Heat Pump vs Gas Boiler Cost

When deciding between an air source heat pump and a gas boiler, cost is often the deciding factor for many homeowners. There are both upfront installation costs as well as long term operating costs to consider.

Upfront Costs

Installing an air source heat pump system can cost £7,000 to £11,000 including installation, depending on the size of your home. This is quite a bit more expensive than a new gas boiler, which can cost £1,500 to £4,000.

However, an air source heat pump system should last 20-25 years, compared to 10-15 years for a gas boiler. So while the upfront cost is higher, an air source heat pump is a longer-term investment.

Air Source Heat Pump Running Costs

Over the lifetime of an air source heat pump, the lower running costs compared to a gas boiler can make up for the higher initial investment.

Air source heat pumps are much more energy efficient, extracting heat from the outside air. This can lead to energy bill savings of 20-40% compared to gas.

This lower energy consumption means an air source heat pump can save you money in the long run. The savings may offset the higher upfront cost within 5-10 years of use.

So while gas boilers are cheaper to purchase up front, air source heat pumps offer greater value and cost savings in the long term. The energy efficiency makes them a smart investment that pays off over years of use.

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Environmental Benefits of Heat Pumps

Installing an air source heat pump can greatly reduce the carbon emissions from your home heating system. This helps support the UK’s goal of reaching net zero greenhouse gas emissions by 2050.

Whereas gas boilers directly burn natural gas or oil, air source heat pumps use electricity to extract heat from the outside air. The amount of carbon emissions depends on how that electricity is generated. Even if powered by the National Grid’s mix of energy sources, heat pumps produce far lower emissions than boilers.

Some key environmental benefits of air source heat pumps:

  • Produce 2-3 times less carbon dioxide than the average gas boiler.
  • Emissions continue to decrease as the UK electrical grid uses more renewables.
  • Support the phase-out of natural gas for domestic heating.
  • Enable homes to be heated with little or no direct fossil fuel consumption.
  • Help the UK meet its legally binding target of net zero emissions by 2050.
  • Qualify for incentives aimed at reducing carbon footprints.

For homeowners concerned about their environmental impact, switching from a gas boiler to an air source heat pump is one of the biggest steps they can take. The technology helps decarbonize homes and plays a major role in fighting climate change.

Air Source Heat Pump Maintenance

To keep your air source heat pump running efficiently, some regular maintenance is required. Here are a few key maintenance tasks:

Filter Cleaning

Your heat pump unit has air filters that should be checked and cleaned every 2-3 months. Dirty filters put extra strain on the heat pump, reducing efficiency. Take the filters out and rinse or gently vacuum them to remove dust and debris. Replacing the filters annually is also recommended.

Antifreeze Levels

The heat exchanger coils in an air source heat pump contain a water/antifreeze mix that can freeze in cold weather. It’s important to check the antifreeze concentration yearly and top it up if needed. An antifreeze tester can determine if the levels are still sufficient. Using the wrong antifreeze type or inadequate levels can lead to damage from freezing.

Other maintenance tasks include checking refrigerant levels, clearing debris that blocks outdoor air flow, and cleaning the coils if they get clogged with dirt and dust. Following the manufacturer’s maintenance schedule will help keep your system running smoothly.

Making the Switch

When considering switching from a gas boiler to an air source heat pump, there are a few things to take into account before making the move.

Things to Consider Before Switching to a Heat Pump

  • Upfront cost. While an air source heat pump will save money on energy bills in the long run, the upfront cost of purchasing and installing the system can be higher than replacing a gas boiler. Make sure to consider any government incentives to help offset this cost.
  • Home insulation. An air source heat pump runs more efficiently in a well insulated home. Consider whether insulation upgrades like cavity wall insulation, loft insulation and double glazing could improve efficiency.
  • Heating requirements. An air source heat pump performs better with a lower temperature heating system like underfloor heating. Radiators may need to be oversized to give the same heat output.
  • Outside space. You’ll need space outside your home for the external heat pump unit and any ground loops if needed. Make sure you have adequate space.

Timeline for Installation

Are you now convinced of the financial and environmental benefits of investing in a heat pump?

If the answer to this is ‘yes’, here is a rough plan of the steps you would need to take in order to move towards a cleaner, greener heating and cooling solution for your home or business.

  • Speak to installers. Get quotes from air source heat pump installers for the cost and timeline of your installation. Ask if they handle any renewable heat incentives.
  • Apply for incentives. Submit applications for any renewable heating schemes or subsidies well in advance of your installation date.
  • Schedule work. It’s best to install a new heating system during the warmer months to avoid being without heat in winter. Schedule a date with your chosen supplier.
  • Installation time. Actual installation may take 1-2 days for air source heat pumps, potentially longer for any ground loops. Make sure your property will be vacant.
  • Commissioning. The installer will need to test and commission the new system, which can take a few days. Hot water and heating may be intermittent during this time.
  • Get trained. Make sure you receive training from the installer on how to properly operate and maintain your new air source heat pump.

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