Air source heat pumps

No gas in your village? If you heat your house with oil, you can save as much as £1000- a year in a typical 4 bedroom house (2000 sq. ft)

With RHI on top of that, the saving plus RHI could be more than £2,500- a year, with total RHI income at more than £12,000- over its 7 year life.

So if you replace your oil fired boiler with an air source heat pump, which might cost around £10k for a large 4 bedroom house,  it could be paid for in around 4 years.

How do they work?

Air source heat pumps (ASHPs) absorb heat from the outside air. This heat can then be used to heat radiators, hot water and underfloor heating systems, or warm air convectors.  (Only systems which heat water are eligible for subsidy in the UK.)

Ground source heat pumps (GSHPs), use pipes that are buried underground to extract heat from the ground, which is then used to heat radiators, underfloor or warm air heating systems and hot water in your home.

An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside.

It can get heat from the air even when the temperature is as low as -15° C.

Heat pumps have some impact on the environment as they need electricity to run, but the heat they extract from the ground, air, or water is constantly being renewed naturally.

You can find a more detailed technical explanation below.


  • Lower fuel bills, especially if you are replacing conventional electric heating
  • potential income through the UK government’s Renewable Heat Incentive (RHI)
  • lower home carbon emissions, depending on which fuel you are replacing
  • no fuel deliveries needed
  • can heat your home as well as your water
  • minimal maintenance required
  • can be easier to install than a ground source heat pump.

Unlike gas and oil boilers, heat pumps deliver heat at lower temperatures over much longer periods.

During the winter they may need to be on constantly to heat your home efficiently. You will also notice that radiators won’t feel as hot to the touch as they might do when you are using a gas or oil boiler.

Is an air source heat pump suitable for me?

  • Do you have somewhere to put it? You’ll need a place outside your home where a unit can be fitted to a wall or placed on the ground. It will need plenty of space around it to get a good flow of air. A sunny wall is ideal.
  • Is your home well insulated? Since air source heat pumps work best when producing heat at a lower temperature than traditional boilers, it’s essential that your home is well insulated and draught-proofed for the heating system to be most efficient.
  • What fuel will you be replacing? The system will pay for itself much more quickly if it’s replacing an electricity or coal heating system. Heat pumps may not be the best option for homes using mains gas.
  • What type of heating system will you use? Air source heat pumps can perform better with underfloor heating systems or warm air heating than with radiator-based systems because of the lower water temperatures required.


  • Heat pump systems typically  come with a warranty of two to three years. Workmanship warranties for heat pumps can last for up to 10 years, for example through QANW (Quality Assured National Warranties).
  • Many manufacturers also offer options for warranty extensions for a fee.
  • You can expect them to operate for 20 years or more, however they do require regular scheduled maintenance.
  • A yearly check by you and a more detailed check by a professional installer every three to five years should be sufficient.
  • The installer should leave written details of any maintenance checks you should undertake to ensure everything is working properly. Consult with your supplier for exact maintenance requirements before you commit to installing a heat pump. You are likely to be advised to carry out a yearly check that the air inlet grill and evaporator are free of leaves or other debris.
  • Any plants that have started to grow near the heat pump unit will also need to be removed.
  • You may also be advised by your installer to check the central heating pressure gauge in your house from time to time. If so, you should be shown how to do this.
  • To prevent the heat pump from freezing in cold winter weather anti-freeze is used.
  • Levels of anti-freeze and its concentration is one of the things that a professional installer will check when he comes to service your heat pump. If your heat pump has external refrigeration pipes, (very unusual for a domestic system) these will need to be serviced annually by a refrigeration engineer


How does an Air Source Heat Pump  work – the detail.

Heat pumps do not create heat; they simply move available heat from one place to another. An outdoor unit absorbs warmth from the surrounding air and transfers it into your home. As outside temperatures drop, your heat pump must work harder to transfer the same amount of heat. As New Zealand’s best performing heat pumps in the cold, we guarantee superior heating down to -15ºC. All the while, heat pumps are the most efficient form of heating available.

Winter / Heating

In heating mode the outdoor units heat exchanger is used to absorb heat energy from the surrounding outdoor air using refrigerant as the heat transfer medium.

The hot refrigerant is pumped through copper pipes to the indoor units heat exchanger and the heat energy is transferred to the cooler indoor room air as the air passes over the indoor heat exchanger warming the room.

Summer / Cooling

Air to air units in cooling mode, use the heat exchanger to absorb heat energy from the indoor room air with refrigerant as the heat transfer medium cooling the room air.

The refrigerant is then sucked back to the outdoor unit through copper pipes and the heat energy is rejected into the surrounding air outside via the outdoor units heat exchanger.

There are two key parts to a heat pump system

There is an outside unit housing the compressor that is similar to the exterior back of your fridge. It draws heat energy from the outside air in even the coldest of weather. That heat energy is then transferred inside the home using a refrigerant process through a piping system powered by an indoor fan unit that is typically mounted to the wall. In summer, the reverse happens – heat energy is drawn from the interior room and expelled by the outside unit. While the system is often referred to as a heat pump rather than air conditioning – both are in fact the same.

The vapour compression cycle

Like a fridge, Mitsubishi Electric’s Heat Pump technology transfers heat energy from place to place using the vapour compression cycle.

A heat pump exploits the fact that a fluid’s boiling point is affected by pressure. Lowering the pressure lowers the temperature at which the fluid evaporates, changing from liquid to gas: raising the pressure raises the temperature at which it condenses, changing from gas to liquid.

  1. Refrigerant in the evaporator is colder than the heat source. This causes the heat to move from the heat source to the refrigerant as it evaporates.
  2. This vapour moves to the compressor where its temperature and pressure are increased.
  3. The hot vapour now enters the condenser where it rejects heat as it condenses.
  4. The refrigerant then moves to the expansion valve; drops in temperature and pressure; then returns to the evaporator.

Defrost cycle

All Mitsubishi Electric Heat Pumps are fitted with intelligent defrost technology to ensure that you get the best performance out of your heat pump when it gets cold.

In low temperatures, all heat pumps have to perform a “defrost cycle” to remove ice build-up on the outdoor coils. This can mean that the heat pump will temporarily stop operating for several minutes or may produce slightly cooler air.

Mitsubishi Electric has developed an advanced version of this, using Fuzzy Logic to learn, measure and record temperatures and running times. This data is then used to ensure defrost cycles are as fast, efficient and far apart as possible – so you can get heat quicker.