Air Source Heat Pump Systems

Air Source Heat Pumps simply take the heat out of the surrounding air rather than a pipe network under ground or water. Inside the house the layout is the same as for the other two Heat Source systems.

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How might it work for you

While these systems are normally used for heating houses, they could also be used to heat a Swimming Pool or even an Aviary/Pigeon loft etc.

The first step is to assess how many kWh you use, or would like to use, on heating your property. This will give the supplier an idea of the size of installation required.

Air Source Heat Pumps (ASHPs) are ideal in situations where the garden is small, but there is space to attach a box of approx. 1 x 1.5m on a south facing wall. This is deep enough to contain a fan to draw the air into the heat extraction mechanism. Even on a winter's day there is some extractable heat in the air and such a wall can become quite hot in the sun. However, more electricity will be needed to power the compressor when the air is cold and therefore the savings would be reduced. In order to avoid using cold morning air, it is essential to have an accumulator tank built into your system.

These require the same inside space as the Ground Source heat pump.

The actual Heat Extractors for both: the Air, Soil and Water heat sources will require housing. This could be a lean-to shed; under-stairs cupboard; cellar; or a small room dedicated to housing the heat pump mechanism. This includes: accumulator tank(s); piping; compressor and associated electronic controls for the house's heating system. The volume of space needed will depend on the size of the house and the system installed, but a rough estimate for a 3 - 4bb house, would be a minimum area of 3 x 2m - it needs to be tall enough to stand/operate in.

There are two techniques to avoid an hour of a cold house in the early morning, or hours of cold offices on a Monday morning.

  • The Heat Pump can be left running all the time, preferable to avoid condensation forming in offices; and usually recommended by the manufacturers using the most efficient modern systems. However, this does mean that the installation is also using electricity all the time; and most people sleep better at a lower air temperature.
  • The Heat Pump can be turned off at night. In which case an insulated Accumulator/Bulk tank is used to store extra hot water from the day. This hot water can then be used to prime the system first thing in the morning until the cooled water in the piping can be returned to the pump and heated up from the fluid in the outside piping network/loop. This means that the house/radiators will warm up quickly as soon as the heat pump is turned on again.
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Facts - Energy Produced and Costs of Production

When working well, an Air Source Heat Pump can have a CoP (Coefficient of Performance) of 1: 4 i.e. 1kW of electricity used to drive the system will result in 4kW of heat energy abstracted. However, the lower the temperature of the incoming fluid, the more electricity will be needed by the compressor; in this case the CoP can fall to 2.5 or less.

Ideally the system installed should be reversible; so that in summer, excess heat can be returned to the air, soil, rocks or water, thus cooling the house and helping to restore the heat sink. This is an especially attractive feature in warmer climates.

Currently no payments equivalent to FiTs (Feed in Tariffs) are payable for non-electric heating systems. However, from June 2011 there will be RHI (Renewable Heat Incentive) payments for any installations completed after 15th July 2009. These may equate to the FiTs available for electricity generation; and will hopefully be calculated so that the payment bridges the gap between the cost of heating by conventional means versus the cost of installing renewables. A component will be towards the establishment of the system and the RHI will also offer a Rate of Return of approx 10% on the additional cost of Renewables, or 6% for solar thermal systems - the input from the sun being free - whereas Air Source has an element of electrical input to extract the heat and Biomass etc. requires growing and payment for the crop.

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Pros and Cons


  • The systems can be run with under-floor distribution or with radiators. In the first instance - suitable for new builds or conversions - a thermostat controls the temperature of the whole room or house. However, if radiators are to heat a whole room to an even temperature, then the radiators would have to be kept very hot. Inevitably there will be a temperature gradient across the room from the site of the radiator, and to deliver the heat to where it is required most will require careful thought. In this scenario the electrical input is often greater, depending on where the thermostat is situated; and a thermostat set to control the temperature of the circulating fluid may be more cost effective.
  • With all types of heat exchange systems, the distribution system does not have to be fitted throughout, but can be limited to specific rooms/areas in the house. It should be possible to switch the flow to specific rooms off and on as required e.g. for a guest bedroom.
  • It should also be possible to arrange the plumbing so that the house heating could be boosted by use of a wood fuel boiler or a solar thermal system; instead of using extra electricity to provide point sources of heat.

Pro's of the Air Heat Exchanger

  • Relatively low cost compared to the other Heat Pump systems.
  • Quicker to install than the other forms of Heat Pump systems.
  • No ground space needed and no ground disturbance
  • Air warms up, on a winter’s day, much more quickly than does even the surface soil. But it will rarely reach the 14ºC of the bedrock in a vertical Ground Loop system.

Con's of the Air Heat Exchanger

  • That the outside temperature of the air is very variable and therefore the associated amount of electricity used (from the Grid etc.) is also variable.
  • This system is not ideally suited to reversal to act as an internal Air Cooling system during the summer months.
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How and Why it works

For a detailed explanation see

The 35% of the infrared radiation that reaches the earth causes the behaviour of our winds, tides, water cycle (rain and evaporation) through the action of heating and cooling; as well as powering all of the flora and Fauna on the planet. A 1sq.m area of plant leaf intercepting 0.8kw is able to make use of ca. 0.1% of this to produce the chemical energy needed to power photosynthesis. A lot of the infrared radiation is reflected and much of the ‘stray’ energy causes the soil to heat up from the surface downwards. The top 3m are also heated from the earth’s crust, but the temperature in the top 1m is influenced more by the solar radiation reaching the surface and the season. The deeper one goes, the warmer the temperature becomes and the less it is affected by events on the surface; after all the molten core of the earth (centre at 6,500km below the surface) has a temperature of ca. 5,500°C. As cavers will know: in summer, caves always seem cool and refreshing when one enters from the heat outside; but on a frosty winters day, the same cave seems warm and cosy by comparison. In fact cave entrances are often found in winter by looking for patches of vegetation that, warmed by the air from below ground, are not white with frost/snow.

A simple heat exchanger is just like your refrigerator, in that the fridge takes heat out of the food put in to it and pumps it out into the kitchen. The heat exchanger takes heat out of the air or fluid that is pumped into it from outside, bulks it up using a compressor, and passes the final amount of heat on to the fluid circulating in your heating pipes. Electricity is used to power the compressor, as in a fridge, and to pump the circulating liquids around the pipework; though systems can be set up - and the early ones always were - that use the difference in density between hot and cold water to move hot water around the system. However, with the electric pump, the energy used works out at approximately 1 unit in for 2.5 - 4 units of heat out; so over all, it saves energy. The electricity needed to needed to run the heat pump can be supplied from the Grid, or it it could be generated by Wind or Solar installations, and stored in modern, highly efficient accumulators (batteries).

The Heat Pump itself consists of a closed loop of piping containing a condensable gas, or a gas such as carbon dioxide that remains in its gaseous state at normal atmospheric pressure until cooled to -78°C. When it is allowed to expand, so that its molecules are well spaced out, the gas becomes cold - or liquid - at which point it can absorb heat from the incoming air. This heating makes its molecules move faster and if a liquid it will be converted back to the gaseous state in the 'evaporator' chamber. The warmed gas is now passed through a compressor causing the molecules to knock into each other in the smaller space available. This procedure increases the heat of the gas, which is then transfered to the fluid in the central heating pipes. The molecules of the cooling gas in the 'condenser' chamber move more slowly until they either change back to the liquid state, or are so spaced out that they no-longer interact, this stage ends by passing the gas/liquid from the 'condenser' through an expansion valve into the 'evaporator' where the cycle begins again.

NB in carbon dioxide (CO2): the C = an atom of carbon and the O2 = 2 atoms of oxygen. Together these make up 1 molecule of carbon dioxide.

Basic chemistry explains this phenomenon by noting that in a substance in its gaseous state the individual atoms/molecules are far apart and rarely come into contact. However, as they are compressed into a smaller volume, increasing numbers of the molecules collide; the individual atoms are also jostled and the result is that the electrons surrounding the atoms lose energy with each collision and move from an higher to a lower energy level releasing heat in the process. A more pictorial way of thinking of it, is to consider the electrons as toddlers and the nucleus as their teacher. Whilst their teacher stops to talk they are are told to stay at her side. Stationary, they cool down, but their metabolism continues to store energy rich compounds in their muscles, where it remains as a potential source of energy. However, as soon as the teacher moves on they are free to run around again and the stored energy is released becoming kinetic energy. This energy is expended in mechanical activity and the heat produced.

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Top Tips new and used

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