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Heat Pumps

A heat pump is an electrical device that extracts heat from one place and transfers it to another. The heat pump is not a new technology and has been used around the world for decades. Refrigerators and air conditioners are both common examples of this technology.

Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of evaporation and condensation. A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings. The refrigerant is then compressed en route to the other coil, where it condenses at high pressure. At this point, it releases the heat it absorbed earlier in the cycle.

In the UK it is common for ground conditions to have a high thermal mass from which heat energy can be extracted and utilised for heating buildings. A ground-source heat pump (GSHP) uses the earth or ground water or both as sources of heat by using plastic pipes filled with an antifreeze solution laid in trenches 1.5 – 2m deep or boreholes to extract heat.

Outdoor piping systems can be either an open system or closed loop. An open system takes advantage of the heat retained in an underground body of water or borehole. The water is drawn up from the well or borehole directly to the heat exchanger, where its heat is extracted. The water is discharged either to an above-ground body of water, such as a stream or pond, or back to the borehole through a separate well. Closed-loop systems collect heat from the ground by means of a continuous loop of piping buried underground. An antifreeze solution, which has been chilled by the heat pump’s refrigeration system to several degrees colder than the outside soil, circulates through the piping and absorbs heat from the surrounding soil.

A heat pump will operate at a COP (Coefficient of performance) of 3-4, meaning for every unit of electricity the unit consumes during operation 3-4 units of heat energy is produced.

GSHPs typically raise water to a temperature of 40°C which is most suitable for integrating with underfloor heating which requires temperatures of 30-35°C as opposed to conventional boiler systems which require temperatures of 60-65°C. GSHPs can be combined with traditional wet central heating systems and domestic hot water systems however due to the higher temperature requirements of these systems an additional immersion is often installed to raise the temperature of the water.

Air Source Heat Pumps

Air source heat pumps consist of a compressor, evaporator coil and heat exchanger. A refrigerant liquid with a boiling point as low as -40°C circulates within the system which evaporates when absorbing heat from the outside air. The resulting refrigerant gas is then compressed adding further energy and raising the temperature to approximately 75°C. This heat is transferred into the water via a heat exchanger and used to provide space heating through wet heating and underfloor heating systems. Some air source heat pumps operate effectively at temperatures as low as -20°C.

Air source heat pumps can be installed on ground mounted or wall mounted frames. When installing air source heat pumps there are various factors to consider. Heat pumps should be positioned to provide shelter from high winds which can reduce efficiency by causing defrost problems. They should also be positioned outside the drip line of the house to prevent ice and water falling on it, which could reduce airflow or cause fan or motor damage. Finally fans and compressors make noise and therefore the heat pumps should be locates away from windows on dwellings. The level of noise varies depending on the manufacturer.