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How Do Hot Water Heat Pumps Work

Heat pumps are not a new concept. First developed at the start of the 20th Century, they operate successfully in a diverse range of applications. Many of us have at least one in our homes in the form of a domestic refrigerator. Air conditioning units providing comfort cooling are also heat pumps.

Heat pumps give thermodynamic heating (or cooling) by means of a vapour compression cycle, in this case collecting heat from the air. First, it is important to understand a little more about the heat within air. Simply speaking, heat can be considered as a generic description of certain energy within a substance. Air contains two forms of heat that a heat pump can extract and convert into useful energy.

The most obvious form of heat in the air is ‘sensible’ energy, in other words, temperature. This energy accounts for something in the order of 70% of the heat pump’s energy source. The remaining 30% of heat that can be usefully extracted from air is the latent energy.

Latent energy is present within air as invisible moisture vapour, commonly expressed as humidity. By changing the state of this invisible moisture back to water (and even ice when the air is cold enough) the heat pump can convert the latent energy, which was required to evaporate the water in the first place, into useful sensible heat. From this you will realise that heat pumps do not rely upon the air temperature alone, rather the total usable energy content of the air.

Matter contains energy down to the temperature called ‘absolute zero’, the minimum theoretical temperature, or -273.15°C in more familiar units. Heat pumps are capable of practically extracting this energy in air temperatures as low as -15°C.

Next we will consider the basic components that make such a system work (the illustration below shows the main components):


The COMPRESSOR, where it is compressed, which has the effect of upgrading the gas to a much higher temperature. The electrical energy that is used to drive the compressor is also absorbed by the refrigerant gas. The hot gas now enters:


The EVAPORATOR collects the heat from the air. This is either atmospheric air, which has been heated by the sun, or internal air, which has been heated by heat lost from another process. High volumes of air are drawn into the unit by the fan and passed across the evaporator fins. The evaporator has liquid refrigerant passing through it, which is at a considerably lower temperature than the ambient air. Therefore, the air gives up its heat to the refrigerant which then vaporises to a gas. This pre-heated refrigerant gas then passes to:


The HEAT EXCHANGER (or CONDENSER), where it is surrounded by the water to be heated in adjacent tubes. The refrigerant vapour, at a higher temperature, gives up its heat to the water which is returned to a storage tank. As the refrigerant cools it returns back to its liquid state, i.e. it condenses, but remains under high pressure from the compressor. The pressure is released by passing the liquid refrigerant through:


The EXPANSION DEVICE ( Electronic Expansion Valve ), and from there, now at normal pressure, it is returned to the evaporator and the cycle starts again.

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