Air Source Heat Pumps

We have considerable expertise in designing and specifying both large systems in the commercial and public sectors and also in providing cost-effective solutions for domestic users and integrating these systems with existing home energy systems.

Air Source Heat Pumps are very much in vogue at the moment. They can provide energy at very low cost. But they are not the universal panacea that they first appear. There are many factors to take into account and unless the right system is installed for the right application, your energy bills may actually increase.

What is an ASHP?

So what is an Air Source Heat Pump (ASHP)? It is most simply explained as a refrigerator in reverse. A refrigerator extracts the heat out of its interior (and by doing so lowers the temperature of the interior) and then dumps this heat through a radiator on the back (and so raises the temperature of the radiator). You may have noticed that the radiator on the back of a refrigerator is always warm. If this explanation is a little confusing, just be aware that 'heat' and 'temperature' are two different things. A body can have a lot of heat stored in it but be a relatively low temperature – the sea for example. Alternatively things can have a high temperature but not much heat, such as a match.

So a heat pump extracts the energy form a large body of low-grade heat and converts this into a smaller amount of high temperature heat. This heat can be extracted form the ground, air or in some cases water, particularly flowing water. So an ASHP creates the enrgy for heating from 'spare' energy that resides in the air outside a building. Energy for nothing then? Well not quite. Please read on.

Heat Pump Unit

Although there are different types of heat pump the majority of them work on the same principle as a refrigerator, using a vapour compression cycle. The main components in the heat pump are the compressor, the expansion vaIve and two heat exchangers (an evaporator and a condenser).

The Heat Pump Cycle

The heat pump recompresses the heat exchange vapour that evaporates as a result of being warmed by the air coming into the heat pump (and so absorbed the heat from the air). The technical name for this heat is called 'the latent heat of evaporation'. To understand this, Imagine you have been swimming and are standing on windy beach, you will soon begin to feel very cold as the water evaporates from your body. This is because the water, in order to evaporate, has to extract the heat from your skin.

  1. The refrigerant in the evaporator is colder than the incoming air (even when the outside temperature is very low). This causes the heat to move from the air to the refrigerant, which then evaporates.
  2. The vapour moves to the compressor which in compressing it raises the temperature and pressure.
  3. The hot vapour now enters the condenser and gives off heat as it condenses. This heat is transferred (via a heat exchanger) to a hot water circulation system.
  4. The refrigerant then moves to the expansion valve and drops in temperature and pressure, and then returns to the evaporator.

CoP and Financial Viability

The more compression, the higher the output temperature. So the final temperature of the output is, to a limit, up to us. Unfortunately, the higher we raise the temperature the less efficient the process becomes. Why? WeII, we have to put more effort into the compression, so the compressor has to work harder and consume more energy itself. This leads to the concept of 'Coefficient of Performance' or CoP. This is really a technical way of defining the amount of energy we have to put in compared to what we get back. If we restrict our output to (say) 35°C we can get a CoP of around 3.5. That is to say that for very kWh of energy we put into the system we get 3.5 kWhs back. In fact some of the power we put into the compressor goes directly into heat energy as well so the ratio becomes even better.

If a single kWh of electricity costs around 12p and the CoP is 3.5, for every 12p we spend we get more than 3.5kWhs of energy, so our energy is actually costing around 3.4p per kWh. However, if our electricity costs more than this - say 16p per kWh then our ASHP energy is costing us about 4.6p per kWh since the ratio is still 3.5 to 1.

So far so good. However, if we raise our output temperature higher than 35°C to something approaching 50°C - a temperature at which we might run a radiator central heating system, then our CoP drops to Iess than 3 (about 2.75). Now for every 12p spent, we are only getting 2.75 kWh back. So each kWh now costs 4.4p. Even at today's prices, gas costs around 4p per kWh. Modern condensing boilers are relatively cheap and operate at over 90% efficiency so given the investment required an ASHP scheme simply doesn't make economic sense.

Additionally, if our electricity costs 16p per kWh and our CoP is less than 3, then each kWh of ASHP energy costs us nearly 6p per kWh and it is now not just a question of economic sense; it actually becomes an expensive means of heating a building. So it is not as straightforward as it first appears but don't discard the idea yet, please read on.

Energy Extraction

Whilst the heat pump is running the energy is extracted by air being pulled through the heat pump, as explained below, and then expeIIed again at a lower temperature. Obviously this requires a continuous unrestricted flow of air through the heat pump otherwise the air in the surrounding area would be progressively cooled. Therefore air source heat pumps are normally installed outdoors, although they can be installed within a building if appropriate ducting is fitted.

Even though the heat pump cools the air as it is used it does not mean, as some people assume, that the heat pump cannot provide heating in cold weather. All air source heat pumps will operate satisfactorily in temperatures well blow zero, and there are some that can operate with the ambient temperature as low as 25°C.

However, working at such low temperatures is not 'cost free', because just as we've already described about the efficiency of the heat pump dropping as the output temperature is raised, the same is true about the input temperature falling – the compressor will have to work harder to raise the colder input temperature and the efficiency drops. So this should be borne in mind when considering the advantages and disadvantages of the different heat sources.

The main advantage of air source is that air is freely available and an inexhaustible source of heat. Also the installation of the heat pump is usually straightforward as very little outdoor space is required. The main disadvantage of using air, as a heat source is that air temperature changes throughout the year. This isn't a problem if, for instance, you want to heat an outdoor swimming pool which you only use in the warmer months – an ASHP will give greater efficiency when the ambient temperatures are higher. But for standard domestic heating it is unfortunately the case you need the most heat at the time when the outside air is coldest.


As we have seen above, the energy saving capability of an ASHP system is dependent on what use is made of the output. Lower temperature applications yield the greatest benefits. That is why ASHP installations are ideal for underfloor heating schemes (or for heating swimming pools).

Underfloor heating temperatures are much lower than traditional radiator systems and provide the same (or better) level of comfort. So to heat your building thorough underfloor heating via an ASHP is one of the best solutions. As energy prices continue to rise then the savings increase.

If you have a conventional radiator system then the benefits are likely to be marginal when compared to natural gas heating. For oil fired or total electric systems then the benefits are greater and can be easily calculated.

The importance of building insulation cannot be overstressed. Your heating requirements are basically based on how much heat your building loses through the roof, walls, windows and floor. If your building is well insulated then the amount of energy you need to input to maintain a comfortable temperature is drastically reduced.

If your building is not well insulated then you will need a heavily overrated fossil fuel boiler to keep it warm. This will mean that your radiators have to be maintained at a temperature that is beyond the capability of an ASHP system – certainly beyond economic viability. If your building is well insulated and there is a conventional radiator system then a ASHP scheme is still a possibility. However, the radiators may need to operate at a lower temperature and may need to be sized much larger than if operated at (say) 65 degrees C. If you are well insulated and so currently operate your radiators at a more modest temperature whilst maintaining good comfort levels then an ASHP system may fit the bill. It all depends on what your energy is currently costing.

From the above, it should be clear that to consider an ASHP system a considerable amount of investigative work is required. Firstly the heat losses of your building need to be calculated very accurately. This will then allow the correct size of system to be calculated.

If the scheme is to be used to power an underfloor heating system, then the operating temperature can be defined and the CoP of 3.5 established. If an electricity tariff can be selected, which may provide energy at a reduced cost (say 8p per kWh) the central heating cost can be reduced to very low levels.

Note that mixed schemes (radiators and underfloor) may be possible but the ASHP will deliver heat at the required radiator temperature and the underfloor part will require 'zoning' and temperature attenuation (as with a conventional mixed heating system).

Since the temperatures available from an ASHP are less than with a conventional boiler (and it makes sense to use lower temperatures) it can be seen that the provision of hot water presents some difficulty. An ASHP system will not alone provide water that is 'piping hot'. So what's the solution? The ASHP can be used to pre-heat your hot water, remember it takes as much energy to heat cold water to lukewarm as it does to turn lukewarm water into hot water. So an ASHP can be used to significantly reduce your hot water bills. If this is combined with a thermal solar panel scheme then most of your hot water requirements can be met throughout the year. A small immersion system can be installed to ensure that hot water is available when solar heating is not. Note that a solar hot water system on it's own can generate 60-70% of your annual domestic hot water requirements.

Our Policy

At Morri Consult we are happy to discuss all forms of alternative energy sources with you, to allow you to make the right decision. We won't try to sell you anything that is not economically right for you. We believe in giving it straight, in terms of the facts and figures, and what you can expect from an alternative energy source.

We do not supply equipment from a single supplier; we will advise and help you select the product that best meets your needs. Above all we are always around to answer your questions and deal with any issues that may arise. We are professionally qualified engineers and technicians with teams of trained staff.

Give us a call for an informal chat.


The government backed Renewable Heat Incentive (RHI) may provide a tariff and payment for the introduction of ASHP whereby people and organisations will be paid to generate their own heat from renewable sources. This will apply to ASHP as well as other technologies such as ground source heat pumps, solar thermal panels and biomass boilers.