Heat pump system
A heat pump system is a system that conveys heat from one area (heat source) to another (heat sink) using a compression heat pump or an absorption heat pump. Most conventional heat pump systems give off the heat directly the indoor air, or to tap water (ie situated in a boiler, see solar thermal collector), or to water circulated for heating radiators).
The heat pump can be used to heat or cool a space or the tap water. Also, it can be used to extract or pump heat either into or out of a space. For home heating, a heat pump can extract heat from outside air (even at temperatures below freezing), or from the ground through liquid circulated through subsoil pipes.This heat can then be transferred indoors. During hot weather, the cycle is reversed; heat is extracted from the indoor air and expelled to the outside as in an air conditioner or refrigerator.
 Operation of a compression heat pump
A compression heat pump uses compression to pump heat around. The principle is used in many appliances such as refrigerators, ... A compression heat pump is 108% efficient.
- Heating Cycle
During the heating mode, the outdoor coil is the evaporator and the indoor coil is the condenser.
- The refrigerant absorbs heat in the evaporator from the outdoor air, turning into gas
- The compressor raises the pressure of the gas, which also increases its temperature
- The hot gas then flows through the condenser coils that are inside the heated space
- The liquid then flows back through a pressure-reducing valve (capillary tube)to the outdoor evaporator coils, being cooled through expansion
- Cooling Cycle
 Operation of an absorbtion heat pump
See this wiki article. A schematic of the parts can be found at this Eandis document. An absorbtion heat pump is 170% efficient. However, whereas the compression heat pump can be powered by electricity, the absorbtion heat pump needs a fuel to operate. As such, it is still less ecologic (atleast if no renewable fuel is used).
 Types of heat pump systems
- Ground-source heat pumps
- Water-source heat pumps
- Air-source heat pumps
The mentioned types of heat pumps are, besides the heat source, further divised to the type of working fluid they use. As such we have:
- air/air-source heat pumps, and air/water-source heat pumps under the Air-source heat pumps
- ground/water-source heat pumps under the Ground-source heat pumps
- water/water-source heat pumps under the Water-source heat pumps
 Efficiency of the different types of heat pump systems
The efficiency used for heat pumps is not expressed in terms of the ratio of energy output to energy input, but expressed in terms of the ratio of energy output to work input. The measurement unit used for this is called the Coefficient of performance or COP. The higher the COP, the more efficient it is.
- air/air-source heat pumps: COP 2,9
- air/water-source heat pumps: COP 3,1
- ground/water-source heat pumps: COP 4
- water/water-source heat pumps: COP 4,5
 Which heat pump systems to use with private houses ?
Heat pump systems that attain their heat from the ground (water/water-source heat pumps use groundwater and so also fall in this "category") are the most efficient heat pump systems yet do require groundworks. Nevertheless, they are still the best choice to make for use in temperate countries (where it is cold during the entire day in winter, and where the air is hence cold all day too). It is best to use a system that can also run in reverse. This way, you can cool your house in summer with it, and immediatelly store this heat underground where it will be reused in winter. Systems that can run in reverse (active cooling) are often not recommended at all, but this is often due to the fact that they never take into account the stored heat (which helps to heat the house in winter).
In countries where it gets cold at night, but where it warms up during the day (ie subtropical countries/desert countries), an air/water-source system is best used.
Air-source heat pump systems are best not used at all. This, as it needs to be compatible to your ventilation system. So, if you don't use this you can freely decide your own ventilation system (for example a heat recovery ventilation system).
 Fitting a heat pump system to a house in practice
First of all, you need to determine whether your radiatiors have a suffiently large combined surface area (needs to be much larger than when running the radiators on burner-heated water). Often, the surface area will be too small, so you may need to add more heat exchanger surface. You could do this by using water tubes placed in the floor.
Next is the heat sink. Most systems simply transfer the heat directly to the indoor air or tap water in the house and so required the user to use it immediatelly. However, in situations where the heat can not be continuously stored (as in the case with the air-source heat pumps system used in subtropical countries, noted above and in temperate countries when using a heat exchanger placed at a shallow depth, ie 2m deep), it is best to include a purpose-built heat sink. This is generally an underground chamber (ie filled with broken glass, water, ...) In practice, most people will opt for a water pit. A water pit or water tank can, besides serving as a heat sink, also store rainwater which can be reused in the garden and in the house (ie for flushing toilets, washing hands, showering ...). Alternatively, one can also fill the water pit with water obtained from a clean lake, stream, groundwater, ... The water pit/tank also allows to connect other heating devices (such as a solar thermal collector) which also helps to heat the water.
 See also
- Growing under cover: the Strawbridge air/ground-source heat pump is a simple heat pump system for a greenhouse