Air source and ground source heat pumps

What is a heat pump?


A heat pump is a system that extracts heat from a source, such as the ground, and upgrades the heat to a useful temperature. The heat is then used for space heating. 


Electricity is required to convert the heat that is harvested into usable heating. This means that it is important the harvested heat is more than the energy used to convert the heat. 


Heat pumps generally produce a lower temperature (around 40˚C) than traditional heating systems such as boilers, and therefore often use systems that require a lower output, such as underfloor heating (which require temperatures of between 30 and 35˚C) rather than traditional radiators (which require around 60˚C to 80˚C). Underfloor heating can run at much lower temperatures than traditional radiators but achieve a good overall room temperature. 


It is worth noting that all of the heat pumps systems are best suited to well insulated air tight buildings that have a very good fabric energy efficiency standard. 


Types of heat pump


The types of heat pumps available are ground source heat pumps, air source and water source heat pumps – some being more effective and more utilised than others. 


Ground Source Heat Pumps (GSHP)

The ground source heat pump system extracts heat from the ground using plastic piping that is buried in the ground. The pipes are filled with a fluid that flows into the heat pump, which then upgrades the heat, before the fluid is discharged back into the underground loop for the process to start again. This is known as a closed loop system.


A ground source heat pump system is generally not suited to heating hot water output due to the lower temperatures that are achieved with GSHP. Hot water taps require a storage temperature of around 60˚C, while the maximum a GSHP can achieve is more in the region of 50˚C.

The ground loops can be configured in a couple of ways, selection of which is dependent on land available, cost, ground conditions an other variables. 

Horizontal loops are installed horizontally in trenches, at a depth of around 1.5 to 2 metres deep. Actual depth will depend on soil conditions and the design of the system. These horizontal loops will require a larger surface area than the vertical loops. We can expect to see about 200m of piping for a single dwelling. On larger scale projects, a horizontal loop system is well suited to be installed below a car park or perhaps play area for obvious reasons – large surface area with plenty of space for the system, with minimal underground interruptions or services. 

Vertical loops systems require far less space than the horizontal system, by inserting the pipework into vertical bore holes at varying depth. Some loop systems can reach up to around 60m depth, while others may only be around 15m. Soil conditions and system design will have an impact on the vertical loop depth. 

vertical loopFinally, the slinky coil system reduces the volume required by overlapping the coils of piping in either a horizontal or vertical fashion. This system can reduce land area requirements and depth requirements for a vertical system. 

Suburban or rural locations tend to be the most suitable for GSHP. Urban areas often prohibit use of GSHP due to lack of land area, but also underground services competing for space. 

Advantages of Ground Source Heat Pumps

  • Long life expectancy
  • Low running costs
  • Low maintenance
  • Good renewable heating and cooling system
  • No visual impact


Disadvantages os Ground Source Heat Pumps

  • Requires large amount of space
  • High installation costs
  • Electricity required to drive the heat pump
  • Not suitable for hot water heating
  • Pipework can limit landscaping opportunities

Air Source Heat Pumps (ASHP)


Air source heat pumps absorb heat from the outside air which is then upgraded using a heat exchanger to be used as space heating. As with the ground source heat pump, the air source heat pump is only suited to to low surface temperature heating systems such as underfloor heating. They are not generally recommended for hot water heating. 

Air source heat pumpThere are two types of air source heat pumps – air to air and air to water. The air to air system produces warm air that is then circulated directly through the building. Air to water systems transfer the heat to a thermal store which is then connected to the underfloor heating system. 


The air source heat pump is most suitable for urban areas, but consider the units have both noise and aesthetic implications. The units look similar to an air conditioning unit.  


As temperatures fall during the colder months, more electricity is required to increase the temperature of the fluid, for this reason, the ASHP are not suited to colder climates, such as Region 1 (Scotland) in the UK. 

Advantages of Air Source Heat Pumps

  • Low running costs
  • Renewable heating system
  • No fuel delivery required
  • Low maintenance


Disadvantages of Air Source Heat Pumps

  • Less efficient than GSHP
  • External air is cooler during winter, exactly when space heating demand is higher
  • Visual impact
  • Noise impact
  • No suited to colder regions of UK
  • May be planning permission requirements


Water Source Heat Pumps

A water source heat pump is similar to the ground source heat pump, but the pipes are immersed in what is known as a solar pond instead of the ground. The solar pond is designed to maximise on the suns energy, creating higher temperatures for the system to benefit from. This option also requires large amounts of open unshaded space, and again is only suitable for space heating and not hot water. 

Be sure to check out all the other posts in the Architecture and the Environment Series.

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