Air Source Heat Pumps


This online Renewable Technology Briefing will help to give you an understanding of the technology and application of air source heat pumps (ASHP) to provide heat for water in domestic and small commercial buildings.

This briefing will give the key outline and link you directly to fuller explanations on the internet (many of these are not official B&ES endorsed links), and downloadable documents (in case you want to know more about any aspect). Additionally there will be references to key books and pamphlets that can give you better understanding of the subject – many freely available.

The B&ES TR30 'Guide to Good Practice - Heat Pumps' also provides detailed guidance and the association also has a Heat Pump Interest Group that is open to all members. Contact Gareth Keller for more information.

The air source heat pump

An air source heat pump supplies domestic hot water and heating

Air Source Heat Pumps can be powered by electricity and gas and may represent an energy efficient alternative to fossil fuel boilers for heating new and existing buildings providing heat for hot water (and air). They can be relatively simple refrigerating machines - the cold section (the evaporator) being outside the building and the warm part (the condenser) providing the heat into the building.

When selected and operated properly they will provide more heat energy to the building than is supplied by their powering electricity or gas as they simply move and enhance the temperature of the heat that is freely available in the outdoor air - hence the term 'heat pump'. The use of air as a heat source is convenient as it is freely available and does require any specific access requirements (compared with ground source heat pumps). Their main weakness is that the temperature (and potential for heat pumping) of the outdoor air will be low (in winter) when there is the greatest need for heating.

Heat pumps work most effectively when supplying heat at lower temperatures. And so heat pumps are best suited for use with low temperature systems such as underfloor heating at 30-45ºC, fan coils at 35-55ºC, and radiators sized to operate at 45-55ºC. Advancements in heat pump technology (particularly CO2 based systems) are providing systems that can successfully heat domestic hot water at 60°C.

Heat Emitter Guide for Domestic Heat Pumps

In commercial applications the heat pump can also be used to provide cooling (in a similar way to 'split' air conditioning units). But unless there is an specific essential need for low temperatures the use of cooling in both commercial and domestic premises can be (and should be) avoided in all parts of the UK.

Air source heat pumps are particularly popular for refurbishment work as they require little associated building work although system alterations are likley to be needed to provide optimum performance. (A very useful free guide MIS 3005 -Heat Emitter Guide provides excellent advice on capability of heat pumps with existing heating systems)

Appropriate use of an electrically powered heat pump can be cheaper than using an oil fuelled, or a condensing gas, boiler, as well as reducing operational carbon emissions. Based on data collected in real applications in 2011 by Energy Savings Trust properly installed and operated systems can provide (at 2014 prices) typical savings in a 4 RAC80.jpgbedroomed detached house of £290-435 per year (or 1.4 to 2.4 t CO2 equivalent) when replacing non-condensing gas heating and £545-880 per year (or 2.3 to 3.6 t CO2 equivalent) per year when replacing oil heating. (This excludes any RHI income).

This guidance will focus on 'air to water' air source heat pumps however the technology is similar to the ' air to air' systems that are used in ventilation and air-conditioning such as 'split units', variable refrigerant flow (VRF) and 'through the wall' units. They work like 'split unit' air conditioning units but the refrigerant flow can be reversed to heat, as well as cool, the internal space. 

B&ES's RAC/80 'Design Specification for DX Packaged Air Conditioning Equipment in Buildings' has extensive further details on these applications.

Electric Powered Air Source Heat Pumps

A heat pump circuitA heat pump is typically based around a vapour compression refrigeration system that uses an evaporator, compressor, condenser and expansion valve linked by a closed pipe circuit in which a refrigerant circulates, This will transfer heat from the, typically, outside air to the indoor loads. At the inlet of the outdoor evaporator the pressure is maintained low enough to ensure that the volatile refrigerant is at a lower temperature than its surroundings. The temperature difference between the surrounding air and the refrigerant in the evaporator causes heat to flow to the refrigerant, that fully changes into a vapour, still at low pressure.

The refrigerant vapour is drawn into the electrically powered compressor where its pressure and temperature increase. The hot, high pressure gas passes to the condenser coil where it transfers heat directly, or indirectly, into the heating and hot water used in a building. By releasing heat the high pressure refrigerant gas condenses back into a liquid and then returns through a restricted pathway (an 'expansion devive') to enter the evaporator.

The efficiency, or COP (see below), is variable depending on the operating conditions and the refrigerant used but can range from under 1 (when external temperatures are very low leading to units that produce almost the same heat energy as the electricity it consumes) to a COP of 3 and above under moderate winter conditions when supplying low temperature heating systems. Theoretically as technology develops (considering a 'Carnot cycle') future COPs are possible of beyond seven.

A video showing the underlying refrigeration cycle in a heat pump unit is shown here - it shows the unit reversing that can be used for cooling (or as part of a defrost cycle).

Gas Fired Heat Pumps

Gas fired absorption heat pump circuitFor commercial applications air sourced gas absorption heat pumps that use a gas burner to drive the refrigeration cycle are an alternative. The gas absorption heat pump comprises a sealed thermodynamic circuit normally containing an ammonia water solution with ammonia being the refrigerant and water being used to carry the ammonia through the system, known as the 'absorbent'. (The water used as an absorbent is quite separate from that circulating in the heating system).

A circuit from a unit is shown here with heat being drawn in to the system from the surrounding air by the evaporator as the ammonia evaporates - the evaporator is shown wrapping around the other components to gain any heat given off by the components.

Depending on application the COP is between 1.1 and 1.3 (this compares with an efficient condensing boiler having an efficiency of around 0.9). They can operate effectively at much lower external temperatures than a standard vapour compression heat pump and so can provide the sole source of heating for a building without the need for auxiliarty heating.

Although the COP is far less than for vapour compression machines the gas energy being supplied is cheaper (about 33% the cost) and less carbon intense (about 60% per kWh) compared to grid electricity.

There is research being undertaken to develop domestic sized adsorption heat pumps that again would be gas fired and expected to provide a 'COP' of around 1.3. Adsorption systems use materials that work by 'soaking up' molecules (for example silica gel or active carbon) of the refrigerant and releasing it when heated to power the 'heat pump' effect. They are likely to need less operator attention compared to absorption systems.

Gas engine heat pumps (GEHP) are mostly sold in the commercial market in Japan and Korea and have become available in the UK in the last couple of years. They use a gas engine rather than an electric motor to drive the compressor. The heat released by the engine is recovered and used for heating the water. The heat can be collected from the engine cooling water and from the exhaust gas. This article provides a good overview of the technology.

This EU project document provides some more detail of gas fuelled heat pumps.

A more detailed description of the main heat pump types can be seen here.

upbullet.gifWorking fluids - the refrigerants

Traditional air to water electric heat pumps typically use hydrofluorocarbon (HFC) based refrigerants (such as the R134A, R407C and R410A). These refrigerants have zero ozone depletion potential (ODP) but must not be deliberately released into the atmosphere as they have high global warming potential.

The thermodynamics of HFCs restrict the upper limit of the working temperature to about 55°C. This restriction limits the heating temperatures that can be produced and also means that some auxiliary heating is required to meet the legionella protection requirements in hot water storage.

Carbon dioxide, CO2 (R744) heat pumps (using supercritical carbon dioxide as the refrigerant) use a modified vapour compression refrigeration circuit. R744 runs at higher pressures than HFCs and so is likely to be more demanding in manufacturing and operating standards - the refrigerant is completely enclosed in the unit.

They can produce higher temperatures than conventional heat pumps - to around 65°C and so beyond the minimum temperatures to avoid legionella multiplication. Manufacturers are producing units specifically for the European markets that claim SPFs of 3 and above. They can operate at outdoor temperatures below zero whilst still maintaining reasonable COPs.

Absorption heat pumps are likely to use ammonia (R717) that has zero ODP and zero GWP but is flammable, toxic and an irritant. Commercially available units are designed for outdoor installation that minimises the risk from leaking refrigerant.

Any refrigerant should be properly handled

upbullet.gifAir source heat pump performance

The Coefficient of Performance (COP) is a standard method of evaluating the basic efficiency of a heat pump (or refrigeration) machine. It is the ratio between the heat pump heat output and the energy input supplied by the electricity and/or gas driving the heat pump - a higher COP is better. The closer the temperatures of the heat source (the air around the evaporator) and the system that is being heated (hot water or indoor air at the condenser) the higher the COP. See this article for more detail on COP. The COP provides a 'snapshot' of the efficiency of the heat pump and will reduce by between 2 and 4% for every 1K increase in the heat output temperature or for every 1K reduction in outdoor temperature.

Manufacturers use sets of 'standard' temperatures when quoting COP - frequently 7°C outdoor (this is roughly the average outdoor temperature during the 'heating season' for central England) and 35°C heating temperature - this complies with part the electric heat pump test standard BS EN 14511. COPs are likely drop significantly at lower outdoor air temperatures.

The seasonal coefficient of performance (SCOP) is the overall coefficient of performance across the heating season. It makes assumptions about the amount of extra heating needed to top up the space and water heating available from the heat pump in accordance with BS EN 14825. This is used National Calculation Methodology for calculating carbon dioxide emission rates from buildings.

A heat pump’s efficiency measured across an entire season can be expressed by the SPF (Seasonal Performance Factor) - this either requires a measurement of the seasonal electricity/gas used and the heat produced at the outlet of the heat pump throughout the whole period (measured by a heat meter) or a prediction based on historic weather conditions as well as the parasitic (additional) use of electricity for pumps and controls. The are variants of SPFs in use - it is important to know which one is being used.

Work by the Energy Savings Trust uses the measure of System Efficiency that compares seasonal electricity/gas with the useful heat produced (eg leaving the hot water tank) to provide a more complete picture of the system performance as it will take account of the energy used in pumps, controls, coil deicing and 'top-up' heating. (A useful guide to how to undertake measurements to evaluate performance is the free Guide CE298)

TheHeatIsOn-EST2013.jpgThe 2013 report by the Energy Savings Trust showed the measured UK installations have a average SPF of 2.45 whereas a review indicates that in Germany and Switzerland, where the technology is better established, SPFs for ASHP installations are typically between 2.5 and 2.9. So it would not be unreasonable to expect a COP of UK installations would head towards 3.0 in the near future.

As well as improved standards of installation technology will also improve and reportedly heat pumps are already available in the Japanese and far eastern market with higher COPs.

Evaporator coil deicing - In the UK climate ice accumulates on the evaporator of an air source heat pump when outdoor air temperatures fall below about 5°C due to moisture in the air condensing and freezing on to the sub-zero evaporator surface. The ice decreases the efficiency of the coil by reducing its ability to transfer heat to the refrigerant.

To remove the ice the heat pump will switch into the 'defrost mode' normally by reversing its action and directing hot gas to the outdoor coil (whilst switching off the evaporator fan) to melt the frost. Some systems use hot-gas bypass (that sends hot uncondensed gas back from the compressor direct to the outdoor evaporator) or direct acting electric elements to undertake defrosting. De-icing contributes to a reduction in the SPF that would not be obvious in the manufacturer's published COP. This partially wasted energy can be minimised by effective control and, in a well controlled system, is likely to reduce operating efficiency by 5-10%.

upbullet.gifAir source heat pump applications

Microgeneration Installation Standard: MIS 3005 provides detailed approved guidance on design requirements for air source heat pumps - this is particularly important if government funding is being sought for the project.

Air to water heat pumps are typically arranged as:

An outside unit of a split air source heat pumpOutside air to water heat pump packages are used as complete packages for external mounting (with hot water flow and return leaving the unit) or as split units with only the evaporator unit mounted outside linked by appropriately vapor sealed insulated refrigerant pipes to the condenser and hot water heat exchanger inside the building.

Exhaust air to water heat pumps are usually mounted internally and recover heat from the heat in the air leaving the exhaust air ductwork.

The heat is then added to a thermal store, (or a domestic hot water pre-heater), or immediately used to heat the building or domestic hot water (that itself acts as a 'buffer' or thermal store). It is preferable, wherever possible to use the heat directly and reduce the additional pumping and standing losses inherent when using a thermal store (or buffer tank).

To get best performance systems should be designed to return the lowest temperature possible back to the heat pump condenser. The heating would preferably use low temperature water under 45°C to supply heat emitters that are appropriately sized (eg under floor heating). Otherwise traditional radiators may be used (with top-up heating as necessary) but should have modulated flow temperatures (using weather compensation and room sensors) to keep flow water temperature at a minimum.

Monovalent systems - When heat pumps are installed to meet the full load capacity (a 'monovalent' system) the heat pump is sized to cover full load, even on the coldest day of the year and so runs at part load for the rest of the year. In new build applications this will save the need for an auxiliary heating system. The maximum temperature of the heat produced by a simple heat pump would not be high enough for supplying traditionally sized radiators or fan convectors or safely heating domestic hot water. Particularly in monovalent systems the outdoor unit must be kept in operation throughout the year and so should be readily accessible to clear snow or leaves that may block the air flow through the evaporator.

When a building is initially being 'dried out' supplementary heating may be required.

c An example of a 'bivalent' system with the heat pump being used for heating and to preheat domestic hot waterBivalent systems - When installed to meet only part of the building load (known as a 'bivalent' system) the heat pump should cover approximately 40-60% of the capacity at the outdoor design temperature. The outdoor design temperature will only occur a few hours each year and on average the temperature will be much higher, so the heat pump can provide 90-95% of the yearly heat energy requirement.

Where heating requirements are provided by a combination of heat pump and boiler, the heat pump should be controlled as the 'lead boiler' and the use of non-renewable supplementary heating should not exceed five percent of annual energy requirement (as detailed in BS EN15450:2007 Heating Systems in Buildings – Design of Heat Pump Heating Systems). Supplementary heating should not be designed as the means of providing enhanced heat output to speed start-up.

Radiators and under floor heating can be combined within the same heating system but the water temperature must be set at a level suitable for the radiators and it is likely that a mixing circuit would be provided to reduce the water temperature in the floor heating loops.

For refurbishment projects the boiler can be connected in parallel to the heat pump and the heat pump can then be of a lower capacity, so reducing the investment cost. The heat pump covers the main load at moderate external temperatures when the COP is at its highest. When the ambient temperature drops below a predetermined point, the boiler (if sized appropriately) can cover the entire capacity. The predetermined point between the use of the heat pump and that of the boiler is dependent on the difference in cost between electricity and fossil fuel.

When retrofitting radiator systems originally supplied by a boiler with 80°C flow and 60°C return a typical heat pump would provide temperatures that will be only half the original heat output - many existing radiator systems are oversized and this can help overcome this potential deficiency. The MIS 3005 -Heat Emitter Guide provides guidance on the capability of existing systems to be used with heat pumps.

Installation - The MCS scheme has supporting documents and for air source heat pumps - specifically MIS 3005 - this provides comprehensive requirements (in a very readable way) for the installation of air source heat pumps. For air source heat pumps, these will include consideration of factors that may detrimentally affect the performance of the heat pump system such as the unwanted recirculation of chilled air across the evaporator.

upbullet.gifAir source heat pump elements

thermal store or a buffer tank can be used to improve the performance of the heat pump (and reduce unwanted compressor cycling) when the heat emitters are low temperature radiators or convectors (underfloor heating has a greater inbuilt 'store'). Thermal stores can be used to bring together heat from various sources (eg solar, ASHP, biomass) and then heat is taken when needed by the building through a coil heat exchanger.An inside storage unit of a split air source heat pump

The maximum domestic hot water temperature available from typical vapour compression heat pumps using conventional refrigerants is around 50°C. To get the greatest benefit from the heat pump large heat transfer coils (larger than that used with a traditional boiler system) are used in the store (or hot water cylinder). Higher temperatures can be achieved using de-superheating circuits (at the expense of efficiency) or by using a CO2 heat pump.

In most air source heat pumps to provide legionella safe domestic hot water a booster heater will be required to perform the Legionella prevention cycle (as required by HSE L8) - this could be by electricity, or an auxiliary heat source. The auxiliary heat source may be included within the heat pump package or within the heat distribution system. In small systems the heat source is frequently an electric immersion element but whatever method is used careful consideration should be given to ensure that the 'sterilisation' cycle is undertaken at the most effective time to reduce energy use.

The outdoor unit should be inverter (speed control) driven so that the output of the heat pump can match the demands of the building. Controlling the internal water temperature according to the heat load of the building ensures that the temperature difference between the evaporator and condenser is as low as possible - this maintains a high COP. (This might be done using weather compensation). Inverter speed control will also reduce the rate of cycling and the nuisance noise.

Historically it has been thought that heat pump systems performed significantly better with continuous operation. Research that looked at both steady state performance and a range of on/off cycling shows that provided run-times exceed a minimum value (of between 4 and 8 minutes), on/off cycling has little discernible effect on heat pump COPs.

Refrigerant pipework connecting split units should be insulated complete with a vapour barrier to prevent ice build-up, adequately supported and protected against corrosion and accidental damage.

Condensate that will collect on the evaporator should be properly collected and drained (and may need deicing provision).

The blog entry Common problems with Air Source Heat Pumps provides some discussion on areas that cause regular concern.

upbullet.gifPlanning and Regulation

In 2010 the Department of Communities and Local Government (DCLG) extended permitted development rights (PDR) to various categories of renewable energy technology, including air source heat pumps. PDR removes the requirement to submit a planning application to the local planning authority for developments meeting specified conditions, saving costs and time. (Practically this applies in England only as the requirements in Scotland are particular onerous and Ireland and Wales have not established PDR for ASHPs). But in the case of air source heat pumps, potential impacts such as noise still need to be carefully addressed and they must meet criteria set out in the MCS planning standards.

Environmental protection All systems must be designed to minimise the risk of accidentally introducing harmful or persistent substances to the environment. This
includes the accidental release of refrigerants or secondary fluid such as water with chemical inhibitors.

Noise (from the both the fan and compressor) can cause annoyance. For air source heat pumps, external fan noise can be a problem. If not properly controlled, fan noise could result in a nuisance covered by regulations. The location of the evaporator unit should be chosen to avoid nuisance to neighbours and take account of local reflection effects. Internal fans and ducts should be fitted with sound attenuation features.

A visualisation of the noise from an outside unit of an air source heat pump

A relatively simple calculation methodology for assessing noise nuisance to neighbouring properties is part of the Planning Standards (MCS 020) of the  Microgeneration Certification Scheme. This is intended to reduce planning issues but does not preclude the possibility of objections or complaints from neighbours.

Other guidance on acceptable local noise levels is contained in BS EN 15450:2007.

Refrigerant handling Any person accessing the refrigerant circuit of a heat pump for filling, evacuating, testing or maintenance must be competent to do so. The F-Gas Regulations require that there should be regular checks for leakage and that refrigerant should be recovered at during maintenance or at the end of the system's life. For equipment containing 3 kg or more of fluorinated gases (such as HFCs) there should be good records kept and anyone dealing with system should have appropriate qualifications.

Both DEFRA and ACRIB have specific information on refrigerant handling.

Building Regulations - Heat pump heating systems for should meet the recommendations of the Non-domestic/Domestic Heating Compliance Guide as a means of demonstrating compliance with the Building Regulations.

For those other than absorption heat pumps and gas-engine heat pumps the COP for for space heating (greater than 12kW) should be 2.5 and a COP of 2.0 for hot water when operating at the rating conditions as defined by BS EN 14511. Absorption heat pumps are required to have a COP of 0.5 and gas-engine heat pumps 1.0 when operating at the standard conditions.

And in terms of seasonal performance factor, for new build an SPF of 2.7 is required and 2.5 for retrofit

Legionella - Approved Code of Practice and Guidance L8 includes hot water storage arrangements and requires that provisions are made to heat the whole water content of the calorifier, including that at the base, to a temperature of 60°C for one hour each day.

Only heat pump packages that are CE marked in compliance with the relevant European Directives may be offered for sale in the EU.

upbullet.gifGovernment Funding

Heat pumps can therefore contribute to a reduction of carbon emission and are recognised as a 'renewable energy' technology by the UK Government. As from 2014 non-domestic air source heat pumps will attract RHI (renewable heat initiative) funding for 20 years. 

Domestic installations became eligible for 7 years of RHI payments - the scheme commenced in April 2014 but reaches back to eligible installations from 2009.

upbullet.gifGreat resources that will provide more detail on Air Source Heat Pumps

Books to buy or borrow

B&ES's TR30 - Heat Pumps - A comprehensive guide that is worth buying if any work is to be undertaken concerning heat pumps

BSRIA BG 7/2009 Heat Pumps – A Guidance Document that provides excellent coverage of the application of heat pumps

ASHRAE Applications Handbook 2011 (chapter 34) - gives a detailed technical background

Web Sites and freely downloadable resources

Heat Pump Association - A manufacturer's based organisation that provides a wide resource of materials and links.

IEA Heat Pump Centre - this has some excellent downloads as well as accessible and well illustrated explanations of heat pump technology

Natural Resources Canada - This includes a great free downloadable spreadsheet tool, RETScreen to determine the benefits of using renewable energy in just about any global location

The Carbon Trust document Solar thermal technology - A guide to equipment eligible for Enhanced Capital Allowances provides some explanations of technologies as well as giving useful information on the relevant ECA scheme

The YouGen heat pump web page has a broad ranging introduction with many links to other resources


AECB wrote an interesting article in the CIBSE Journal

Standards and Regulations

BS EN 378 Refrigerating Systems and Heat Pumps

BS EN 14511-2:2011 Air conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling. Test conditions

BS EN 15450:2007 Heating systems in buildings – Design of heat pump heating systems.

Microgeneration Installation Standard: MIS 3005 (Available at no cost) This provides excellent focussed design and installation guidance for air source heat pump

The DCLG Domestic Building Services Compliance Guide 2013 Edition and Non-domestic Building Services Compliance Guide 2013 Edition both include extensive requirements for both performance and control of air source heat pump systems