Air Source Heat Pumps

An air source heat pump is usually placed outside at the side or back of a property, and takes heat from the air and boosts it to a higher temperature using a heat pump. This heat is then used to heat radiators, underfloor heating systems or even warm air convectors and hot water in your home.
The pump needs electricity to run, but the idea is that it uses less electrical energy than the heat it produces.

Air source heat pump systems

Air source heat pumps (ASHP) look similar to air-conditioning units and are less disruptive to install than ground source heat pumps, as they do not require any digging in your garden. The main components of an ASHP are a heat exchanger, a compressor and something to transfer the heat into a hot water tank or heating system, such as radiators or an underfloor heating system.


How it works

An ASHP works a bit like a refrigerator in reverse. The process consists of an evaporator, a compressor and a condenser. It absorbs heat from the outside air and the heat pump compressor then increases the temperature of that heat further to create useful heat. There are two main types of ASHP:

Air-to-water systems take heat from the outside air and feed it into your wet central heating system. As the heat produced is cooler than that from a conventional boiler, you may need to install larger radiators or undefloor heating in your home to make the most of it.

Air-to-air systems take heat from the outside air and feed it into your home through fans. This type of system cannot produce hot water.

In the summer, the ASHP can be operated in reverse, like an air-conditioning unit, to provide cool air for your home.

Heat pump costs and payback

ASHP are cheaper than ground source heat pumps. The Energy Saving Trust (EST) estimates that the cost of installing a typical ASHP system into a detached home ranges between £6,000 and £10,000. The payback period (the time it takes for the cost of the system to be recouped in energy savings) is equally elusive and depends on how efficiently your system works, the type of system you're replacing and how you'll be using the heat generated from the pump. Here are some general rules of thumb to bear in mind:

Like ground source heat pumps, air-to-water ASHP work better with underfloor heating systems. If underfloor heating is not possible, large radiators should be used. This is because the heat generated by the heat pump is not as high as that produced by a conventional boiler, so a larger surface area is needed to achieve similar temperatures in your home.

Air-to-water heat pumps could be better suited to new-build properties than retrofit - this is because costs could be reduced if the heat pump is included as part of the building specification, rather than having to retrofit underfloor heating later on.

Heat pumps can save you more on your heating bills if you're replacing an electric, LPG or coal system.

A well-insulated house is essential to best optimise the heat generated by your ASHP - otherwise the heat the pump is generating escapes more easily.

The payback period will be shorter if your current heating system is electric, LPG or oil.

Once in place, the heat pump should require little maintenance.

Heat pumps will qualify for the Renewable Heat Incentive (RHI), a government scheme that pays homeowners for generating renewable heat that will be introduced from October 2012.

Heating bill savings?

How much money you'll save on your heating bills depends not just on the factors listed above, but also on whether the system is the right size, installed correctly and used correctly by householders. The EST says that a 'typical' air source heat pump could save you up to £330 (replacing electric heating) a year. But be aware that only some of the heat pumps the EST monitored in recent trials achieved reductions in heating bills. It found wide-ranging variations in performance, with the biggest heating bill reductions for households off the gas grid. However, the trials found that, without the RHI, an ASHP could actually cost you more if you are currently using gas or heating oil to heat your home.

Which? thinks more evidence is needed to better understand the cost savings possible per household. The EST has also highlighted the need for further study of heat pumps on an installation-by-installation basis. But, as of October 2012, payback time for heat pumps will be reduced thanks to the Renewable Heat Incentive which will pay householders who generate heat using renewable technologies like heat pumps.

Pros of air source heat pumps

Air source heat pumps can generate less CO2 than conventional heating systems.

They are cheaper than ground source heat pumps and easier to install, particularly for retrofit, although their efficiency can be lower than with ground source heat pumps.

ASHP can provide heating and hot water.

They require very little maintenance.

They can be used for air conditioning in the summer.

You need to use electricity to power the pump which circulates the liquid in the outside loop, but for every unit of electricity used by the pump, you get between two and three units of heat – making this an efficient way to heat a building.

Cheaper Economy 7 electricity tariffs can be used to lower the cost of electricity to power the heat pump and special heat pump tariffs may be available from some electricity suppliers – alternatively consider solar photovoltaic panels or a wind turbine (if you are in a suitable area) for a greener source of electricity.

Cons of air source heat pumps

The EST says that a 'typical' air source heat pump could save you up to £330 (replacing electric heating) a year, but be aware that only some of the heat pumps in the trial reduced heating bills and that if you are on the mains gas grid, at the moment, some ASHP might not lead to any fuel savings.

You'll need enough space in your garden for the external condenser unit (comparable in size to an air-conditioning unit). Condenser units can be noisy and also blow out colder air to the immediate environment.

ASHP lose their efficiency when the external temperature falls below 5 degrees Celsius. So in cold weather, an additional source of heating may be required to supplement the ASHP.

You still need to use electricity to drive the pump, so an air source heat pump can’t be considered completely zero-carbon unless this is provided by a renewable source, such as solar power or a wind turbine.

Air source heat pumps are also recommended for newly-built houses with insulation and air-tightness meeting the latest building regulations.

How green is an air source heat pump?

An air source heat pump system can help to lower your carbon footprint as it uses a renewable, natural source of heat – air. The amount of CO2 you'll save depends on the fuel you are replacing. For example, it will be higher if you are replacing electric heating than natural gas.

However, the EST's recent trials found only 13% of pumps tested (air source and ground source heat pumps) reached the level of a 'well-performing' system, with many systems appearing to be installed incorrectly. A heat pump also requires a supplementary source of power, usually electricity, to power the heat pump, so there will still be some resulting CO2 emissions.

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