Air Source Heat Pumps

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The Facts:

  • A heat pump is a simple system that takes heat energy from one place to another whilst increasing it from a lower to a higher temperature. You will find a heat pump within your domestic refrigerator; heat is removed from inside (the source) and discharged out the back into the surrounding atmosphere (the sink). In heating applications the process is reversed, heat is removed from atmosphere, water, or soil and delivered to the building where it is required.
  • It is possible to take heat from the air when it is at temperatures as low as -20°C, so these systems can work all year round.
  • They use approximately a third less electricity for heating than other forms of electrical heating, continually delivering heat at lower temperatures over much longer hours than a standard boiler.
  • Heat pumps will save you money.  They are cheaper to run than oil/LPG boilers and can be cheaper than using gas boilers. Because heat pumps can be fully automated they require less work than biomass boilers.
  • Heat pumps will save carbon emissions. Unlike burning gas, oil, biomass or LPG, a Heat Pump produces no emissions from your property, and if you already use a renewable source of energy like solar panels - then no carbon emissions at all!
  • Heat pumps are also safe to run. No combustion required and no emission of potentially harmful gases.
  • Heat pumps require very little ongoing maintenance unlike the combustion alternatives.

Glossary of ASHP Terms (HPA)

There are many abbreivations and terms used in the description of ashp's; most of them are fairly obvious, but these definitions will help to explain what they mean.

So, these are the most technical terms that concern the refrigeration systems found in all ashp's:

Air Source

Heat is taken from outside air and transferred into another medium. Outside temperaturs will effect the efficiency of the system: See also “Air-to-Air” and “Air-to-Water”.

Air to Air

The ashp will harvest warmth from the outside air and distribute the heated air within the property. The air will be in direct contact with the condenser and evaporator. Most commercial small split or packaged ashp systems are direct air-to-air. See also “Air Source”.


Heat is extracted from the external air and simply transferred to water which can be used for heating. This type of system uses both an acting outdoor unit coupled with an indoor section.

Closed Loop

Heat is collected by a pipe-work system that is placed within the ground (or lake, river sea) and water and glycol mixed is circulated within it. A heat pump will then extract heat from the fluid which will be replenished by the origional source of the ground, lake, river or sea. The fluid is always contained within the system and does not come in direct contact with anything, but the pipe it travels in.

Coefficient of Performance (CoP)

The ratio, CoP, is a simplistic equasion that describes the output of a system in heating mode as a proportion of power input and is the;  capacity divided by the power input. This is expressed as a single figure or sometimes a percentage.Example, an ashp that is rated in heating at 6.5kW, with a rated power consumption of 1.8kW will have a CoP of 3.61 or 361%. This is a simlistic ratio used in BS EN 14511:2007 now superseded by BS EN 14511:2011 (See CoSP, EER).

Coefficient of System Performance (CoSP)

A figure that describes the efficiency of an ashp that takes into account the input power from internal control circuits, compressor, as well as fan and pump power required to overcome any fluid resistances of its own heat exchangers, but not those external to the unit. It is the rated capacity divided by the rated total power input and is more representative of the systems efficiency than the more simplified CoP. In practice it has become the de facto measure of CoP and is expressed as a single figure or sometimes as a percentage. This ratio is now described in BS EN 14511:2011 (See also CoP, SCoP , SPF, EER).


The compressor is often referred to as the heart of the vapour compression ashp system. It serves two main purposes. The first is to circulate the fluid throughout the circuit as a pump, the other is to compress the pressure and raise the temperature of the refrigerant vapour so that it can easily be condensed back into a liquid to then restart the cycle once again.


In refrigeration systems the condenser is the heat exchanger where hot, compressed gas is condensed to a liquid and then cooled to restart its journey around the circuit again.

Defrost Cycle

When an air source heat pump system operates at low temperatures, the evaporator within the outdoor unit is below freezing point and the moisture within the air will freeze on the surface forming ice. This coating of ice is removed periodically with an automated defrost cycle. The frequency of this cycle is automatic taking into account the time and temperature of the refrigerant pressure within the external coil. 

Direct Expansion (DX)

Refrigerant in an ashp is used to heat (or cool) the final medium being heated. For instance to heat a room, the air from the room would pass directly over the condenser containing the refrigerant, or in the example of domestic hot water heating the refrigerant would have to pass through the cylinder coil (condenser) itself — the latter being very rare. It offers an increased thermal exchange efficiency by reducing the number of thermal interchanges, however efficiency reduces in cooling modes if the separation between condenser and evaporator is large (over 5m). VRF Air/Water to Air systems are good examples of DX systems.

Energy Efficiency Ratio (EER)

Describes the efficiency of an air source heat pump machine in cooling mode. The rated capacity is divided by the rated total power input. In practice this is expressed as a single figure or again, sometimes as a percentage.

For example, an ashp system rated in cooling at 6.5kW, and a rated power consumption of 1.8kW will have an EER of 3.61 or 361%.(See CoP, SEER).


The refrigeration cycle of an air source heat pump, the evaporator is the heat-exchanger where refrigerant fluid is evaporated and it absorbs heat from the surrounding air or water, reducing its temperature.

Expansion Control Devices

Within a refrigeration based air source heat pump system, the flow and evaporation rate of the refrigerant within the evaporator is controlled by devices at the entrance to the evaporator. These devices are available in a variety of forms; the 3 most common types in commercial systems are as follows:

Capillary tubes that allow fluid flowing through an appature will experience a drop in pressure. A capillary tube is a precisely measured length of a narrow tube with a pre-determined internal diameter that produces the desired drop in pressure due to its length.

Thermostatic Expansion Valve (TEV) is an automatic mechanical valve that self-compensates for the pressure losses in the evaporator and controls the leaving superheat temperature of the refrigerant.

Electronic Expansion Valve (EEV) is a valve that is driven by a small dc stepping motor. It operates in the exactly same way as a TEV, apart from the valve positioning, as that is determined by a microprocessor. EEVs are enable a more precise metering for mainly automatic systems.

Ground Source Heat Pump

A GSHP system is when heat is extracted from the ground by passing a fluid (water/glycol) through tubes buried in the ground either in horizontal arrays (looped or straight tube) or vertically through thermal boreholes described as ‘closed loop’. In the UK the temperature below ground at a few tens of metres depth is stable within a small tolerance year round. Heat can be stored or obtained from shallow ground, about 2 m depth, or by deeper bore-holes where space is a consideration. Sometimes the phrase Ground Source ‘Open Loop’ is used to describe heat taken from the ground by extracting the water within the ground.

Geothermal Energy

Geothermal energy (or“hot rocks”) is obtained by very deep drilling in suitable areas providing heat from the earths core. This is a separate and distinct technology and should not be confused with ground source heat pumps.

Air Source Heat Pump

A heat pump is a device for transferring energy in the form of heat from outside to inside. It cannot store, make or destroy heat energy, it just moves it. There are a number of systems that eploy heat transfer technology; the commonest in use is the Refrigeration Cycle. An air source heat pump is capable of transforming a large quantity of low grade, low temperature heating. Some air source heat pump systems will operate in winter ambient conditions to as lows as -20ºC. Air Source Heat pumps are available that can operate in all weathers.

A European standard for testing and rating heat pump performance, EN 14511 – Part 1, defines a heat pump:“[a] heat pump [is an] encased assembly or assemblies designed as a unit to provide delivery of heat. It includes an electrically operated refrigeration system speicfically for heating. It can also have the means for cooling, circulating, cleaning, and dehumidifying. The cooling is by means of reversing the refrigeration cycle”.

Heat Exchanger

The heat exchanger is a device for the transfer of heat energy from one medium to another. It can take a variety of different forms; the commonest in everyday use is a central heating radiator or underfloor heating where hot water is circulated through pipes or plates and distributes its heat up to the surrounding air.

Inverter / Variable Speed Drives (VSD)

Rarely are heat pumps required to run at full capacity for long periods of time,  due to variable conditions such as external temperature, and varying heat loss within buildings. By altering the speed of the compressor the energy output of the equipment can be varied for increased efficiency as both evaporator and condenser are sized for peak duties and are able to conduct heat even more efficiently at part load. Such drives are used in most electrically driven vapour compression heat pumps, whether two piece ‘split systems’, multi split or VRF and use such inverter technology. A variable speed drive is also possible with Gas Engine Heat Pumps by conventional throttling of system speed via a carburettor.

Kilowatt (kW)

The SI unit of power. It is used to specify the thermal performance of a Heat Pump as well as the power it consumes. It is a Kilojoule of energy per second (kJ/s).

Kilowatt Hour (kWh)

The of sale of electrical energy for a standard unit; it is the equivalent power consumed by a purely resistive load of 1000 Watts (1kW) for 1 hour. Because a kW is kJ/s, a kWh is equivalent to 1 kJ/s for 1 hour. Therefore 1 kWh = 3600 kJ. Your electricity supplier will specify the price of this.

Open Loop

A heat collection system whereby water is extracted from ether the ground or an open water source (lake, river or sea) and is passed directly through a water source heat pump. This water may be re-injected or passed to waste. Environment Agency approval is required for all systems extracting more than 20m3 which restricts use to domestic small to medium size without such approval.


The heat transfer fluid contained in an air source heat pump refrigeration circuit. This is a chemical contained in a hermetically sealed circuit that has a low temperature boiling point. Refrigerants can be one of a number of man-made Fluorocarbons(e.g. HFC) or a Hydrocarbon compound (refined Propane or Isobutane). All refrigerants currently used have Zero Ozone Depletion potential, but many have quite high direct Global Warming Impacts if released to atmosphere. All refrigerant handling should be undertaken by qualified and certified personnel with an F Gas certificate. 

Refrigeration Cycle

All air source heat pumps use a refrigeration cycle in some way. Heat is extracted from a source, upgraded in temperature and then delivered to its destination. The most popular method is the vapour compression cycle.

Reverse Cycle Heat Pump

A reverse cycle system is a refrigeration system that can, by means of a valve that reverses the flow of the refrigerant fluid, change the operation of the system from heating to cooling. This process can also be used to defrost.

Seasonal Coefficient of Performance SCoP

This describes performance of the heat pump over a typical season where the source temperature varies. Used mostly with Air Sourced Heat pumps where the source temperature varies considerably over the year and taking into account the efficiency and output varieables. The SCoP is dependant on the local climate. In order to compare units the EU has been divided into 3 principle regions and is defined in BSEN 14825 and will be used for the Energy Using Products directive (ErPEner Lot 1 and 10). Often referred to as SPF.

(See also CoP, CoSP, SPF and SEER)

Seasonal Energy Efficiency Ratio SEER

An efficiency metric of reverse cycle heat pumps which describes performance of the unit over a typical cooling season, again where the source temperature varies. Used mostly with Air Sourced Heat pumps where the source temperature varies over the year and hence efficiency and/or output varies. Thus SEER is very much dependant on the local climate. In order to compare units the EU has been divided into 3 principle regions under BS EN 14825 for both heating and cooling functions.

(See also CoP, SCoP and EER)

Seasonal Performance Factor (SPF)

Seasonal Performance Factor is similar to SCoP in that it is a ratio of the efficiency of a heat pump by describing the heat output to total energy input taking into account the variations in performance over a heating season. Under BS EN 15316, input energy includes auxiliary energy which could be all or part of pump/fan power. Care must be taken as to whether this is intended to include any additional boost heat from other sources, e.g. immersion heater, and the full pump/fan power to overcome all resistances of circuits (i.e. not just the heat exchangers of the heat pump). For this reason the predictive SPF, if based on say CoSP or SCoP, may vary from an empirical SPF based on the data measured on site because this may include the total power consumed by circulating pumps/fans and direct electric additional low ambient boost heaters, all of which are bespoke to predict in advance.

Water Source

A heat pump system whereby heat is extracted from water which is either directly extracted from the ground (e.g. a buried aquifer) called “open loop” or from an open water source such as a lake, river or sea. These systems are invariably indirect and need careful filtration to remove particles from the water source before it enters the heat exchanger. This term is sometimes used interchangeably with Ground Source with a sub designation of ‘open loop’.Permission is usually needed in the UK for extraction of ground water.

Water to Water

A heat pump where the collecting medium or 'source', is either ground water or a glycol solution and the destination medium is also water or glycol. These systems are generally indirect.

Water to Air

A heat pump where the collecting medium or 'source' is either ground water or a glycol solution and the destination medium is air. Hence the source side is indirect but the delivery side is likely to be direct.

Call us on 01392 823623 and we can talk you through the process.