Air Source Heat Pumps


What are air source heat pumps?

An air source heat pump (ASHP) is typically located on the outside of buildings and connected through the wall to an internal unit.

They can provide hot water, used for underfloor heating, over-sized radiators or potentially domestic hot water, or warm air, used to directly heat a room.

ASHPs require a lot less energy than a conventional boiler; while a conventional immersion heater will supply one unit of heat for every unit of electricity, a heat pump will supply 2-3 units of heat per unit of electricity.

Air-to-air ASHPs transfer heat absorbed from the air outside into the inside via central heating fan systems. In air-to-water ASHPs, the heat produced raises the water temperature in a storage tank which supplies a low-pressure hot water distribution system for space heating.

How do air source heat pumps work?

Small-scale biomass CHP boilers usually rely on pyrolysis (heating the fuel in the absence of air). This produces syngas which is a mixture of hydrogen and carbon monoxide with a little carbon dioxide. This gas is fed through a spark ignition engine, which in essence is the same as a gas-powered CHP unit. The goal is to produce ‘clean’ gas as the carbon released from burning the biomass is roughly equal to what was absorbed during its growth.

Maintenance of small-scale biomass CHP is far greater than with mains gas powered CHP. Medium and large-scale biomass CHP systems tend to use steam to drive a turbine or Organic Rankine Cycle (ORC) that uses a turbine driven by gas which boils at a lower temperature than water. Both are mature technologies with excellent durability and reliability, which call for far less maintenance than is required for gasification CHP.

While most gas-powered CHP units can ‘modulate’ (respond to reduced electrical or heat load, usually to around 50% of peak output), biomass CHP boilers will run at maximum output consistently – they cannot run on part load. As a result, the ideal biomass CHP project is for the unit to provide a base load of electricity and heat, with the unit running for as close to 8,000 hours as possible.

How are air source heat pumps applied to a building?

A fan draws air through the outside unit of the ASHP, this must not be covered in a way that blocks inlet to the fan. It can be hidden away to some extent, but there needs to be space directly outside the building to promote a good flow of air.

Depending on the site optimal installation would position the outside fan away from the direction of prevailing winter winds. ASHPs are sensitive to large drops in air temperature, shielding them in this way can help to safeguard the SPF.

For larger units, there may also need to be planning considerations. Noise can be an issue as the external fan units can generate a considerable level of noise; this can be exacerbated if the pump is not sited correctly,
e.g. where noise echoes or can vibrate against a wall or fitted close to a resident (bedroom) windows. Building Regulations such as the Scottish Building Regulations Section 6 for Non-Domestic Energy have targets for minimum air sound insulation and maximum impact sound transmission which must be verified with post installation testing.

An ASHPs line of sight and proximity to domestic and residential buildings will be a key factor in securing planning permission.

As with ground source and water source heat pumps, ASHPs require electricity to power the heat pump. Unless this electricity is provided by other onsite renewables (e.g. wind or solar PV), a running cost is incurred as well as greenhouse gas emissions. The extent of this depends on the SCoP of the heat pump which, in turn, relies on the ambient conditions of the site and the water/air temperature required. However, significant savings can be made compared to a conventional boiler system.

ASHPs can be retrofitted relatively easily, assuming there is space for the outdoor unit and, in the case of an air-to-water heat pump, a thermal store (water cylinder). However, ASHPs are best suited to low temperature applications, such as underfloor heating, thus installing them in poorly insulated buildings is not appropriate.
ASHPs deliver heat at lower temperatures over much longer periods so, if the heat losses from the building fabric are too high, the heat provided by the ASHP will simply be lost.

It may be necessary to upgrade the building’s insulation before installing an ASHP to get the best SPF possible.
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