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Air source heat pumps in Northamptonshire: How do they work?

Air Source Heat Pumps in Northamptonshire, are classed as Permitted Development for the purpose of building and planning regulations – so no planning application is necessary unless the property is in a conservation area or a listed building.

ASHP’s can be installed in place of oil/gas or electric boilers and adapted to work with existing radiator/underfloor wet heating systems.

Home owners should see a reduction in energy and resulting fuel costs, in many cases they will be eligible for government grants designed to defray some of the capital cost.

What is an Air Source Heat Pump?
Air source heat pumps (ASHP) usually have either one outdoor unit (mono-block system) that produces hot water or an outdoor unit connected to an indoor unit (bi-block system) that produces the hot water. The mono block ASHP connects directly to central heating pipework and in the case of retrofit, usually replaces the existing oil/gas/electric boiler.

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Mitsubishi Ecodan ASHP

A bi-block ASHP has an outdoor unit that connects by refrigerant pipe to an indoor unit where the hot water is produced. Outside air, at any temperature above absolute zero, contains some heat energy. An air-source heat pump moves (‘pumps’) some of this heat energy to provide hot water or space heating.

The ASHP uses this outside air as its heat source and utilises a refrigerant system involving a compressor and a condenser to absorb heat at one place and release it at another.

The main components of an air-source heat pump are:
1. An outdoor heat exchanger coil, which extracts heat from ambient air
2. A heat exchanger coil, which transfers the heat into a water tank or indoor heating system, such as radiators or under-floor circuits.

An efficient heat pump can provide up to four times as much heat as an electric heater using the same energy. The four times multiple is called the COP (Coefficient of Performance) usually quoted as a seasonal average based on the outdoor temperature. So if a heat pump manufacturer or installer tells you their equipment has a COP of 4.0 then you should ask what temperature range they are referring to.

A realistic COP for the UK climate, supported by independent laboratory research, is about 3.5* as a seasonal average. *(Mitsubishi Electric BRE Research on the Ecodan 8.5kW Air Source Heat Pump)
If you have mains gas as a primary heat source, the heat pump savings are not so immediate based on the current relatively lower price of gas.

An air-sourced heat pump designed for most domestic applications can extract useful heat down to -25C. If you are lucky enough (or unfortunate) to live in a large period house that is also a listed building, you can still make use of a heat pump however, it makes sense to insulate as well as you can first of all. One alternative configuration for a hard-to-heat property is a bi-valent system, designed to provide hot water and heating for say, 80% of the year, using the backup oil/gas boiler when the temperatures fall below a predetermined level.

Air source heat pumps use technology developed over a number of years and have a life span forecast over 20 years. With few moving parts, ASHP maintenance is confined to ensuring the heat exchanger and fan is kept clean and free from leaves and debris. The service however, is important to ensure you are using the correct settings and getting the best value in terms of energy usage. This should be balanced against the fact most ordinary electric heaters and fuel burning devices have less moving points but do require maintenance that is more essential.

How Do They Work?
Heating is accomplished by moving a refrigerant gas through the heat pump’s outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change the state of refrigerant gas between a liquid and a gas. With the liquid refrigerant at a low temperature (-30C) this passes through the outdoor evaporator heat exchanger coils and the surrounding air appears very hot causing the liquid to boil. This change of state from a liquid to a gas amasses energy as latent heat.

The vapour is then drawn into a compressor, which further boosts the temperature of the vapour. Remember school days pumping up a bicycle tyre and noticing how hot the end of the pump became the harder you worked on that tyre. For compressor now read bicycle pump! The now superheated gas passes through heat exchanger coils where it transfers heat to water piped through the system. (Depending on the model of air source heat pump system this stage may vary however, the result is hot water pumped through radiators/under floor pipes and delivered to a hot water cylinder.)

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