Air to water heat pumps explained

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Air to water heat pumps are a type of air source heat pump that can be used to heat water for home heating applications. The system works by absorbing heat from the outside air into water and transporting it into the building where it can be used to provide through a heat distribution system, normally underfloor heating.

Underlying Principles

The science behind air source heat pumps is the same as for any refrigeration cycle and hence these systems work in a very similar way to an ordinary fridge - but in reverse. A domestic refrigerator absorbs heat from the air inside the fridge and transports it via a fluid refrigerant to the outside air (the kitchen). This is why the rear of your fridge will often be warm. An air to water heat pump in an air source heating system absorbs heat from the outside air into a fluid (water), which is pumped into the building where it is used to heat the air inside the house through underfloor heating or radiators.

Temperature and Distribution

The temperature of the water heated using an air to water heat pump will be considerably lower than that heated using a more traditional heating system (such as a gas boiler). It is therefore more suited to underfloor heating systems rather than traditional radiators, because with underfloor heating the heating area is much larger meaning the lower temperature water is still effective in heating the whole house, whereas a normal radiator would only warm the small area around it. As underfloor heating systems are expensive and can also be difficult to retrofit, it is not always possible to install underfloor heating. In these cases the best option for heating distribution from a air source heat pump is to install oversized radiators, which will heat more effectively using the lower temperature water.

Efficiency

The efficiency of an air to water heat pump is given by a single number called the coefficient of performance (CoP). This number represents the amount of heat produced by the heating system compared to the amount of energy required to power the pump. Thus, an air to water heat pump system with a CoP of 2.5, produces heat equivalent to 2.5 times the electrical energy used to power it. A CoP around 2.5 is a fairly typical value for an air to water heat pump, which obviously gives a decent energy saving. However, it is not quite as effective as ground source heat pumps, which typically have CoP's around 3.5.

Key Considerations

If you are think of installing an air to water heat pump system for your own home then there are several important things that you should consider. Firstly, you will need a good location for your air to water heat pump. This obviously needs to be outside your house, where it can be fitted to either a wall or the ground. It also needs to be a place where there will be good airflow so the pump can work effectively. Secondly, you need to recognise that your energy savings will depend on the type of fuel that you are replacing with the air to water heat pump. If you already have a modern and efficient gas boiler, it will take a very long time for the air to water heat pump to pay for itself in energy bill savings, because the day to day efficiency savings will not be large. However, if you are replacing more expensive forms of energy with an air to water heat pump (such as electricity, LPG or coal) then you will get your capital expenditure back in savings far more quickly, making the installation of an air to water heat pump much more worthwhile. The final factor you should consider is home insulation. Whilst insulating your home is wise in any case, it is particularly important when using an air source heat pump due of the lower temperature water the system produces: this makes good insulation essential for effective heating.

Costs

Although, as mentioned earlier, a ground source heat pump has a higher CoP than an air source heat pump, this does not mean that a ground source heat pump is better for all applications. The cost of an air to water heat pump system capable of heating a well-insulated detached house is between £5,000 and £9,000, depending on the exact circumstances. This could be quite a bit cheaper than a ground source heat pump. An air source heat pump also require much less space because it doesn't require long trenches or deep boreholes to be dug and is therefore much more appropriate for circumstances where such excavations are not possible.

Overall, an air to water heat pump is a good method of supplying heat and hot water to a house as it will save you money by reducing your energy bills as well as being environmentally friendly and reducing the carbon dioxide emissions of your home. However, it should be remembered that the pump will require electrical power to run and the savings made (both monetary and CO2) are highly dependent on the type of fuel being replaced by the heat pump. Also, as the water supplied by an air to water heat pump is significantly lower than that supplied by ordinary heating systems it is essential that your house is well insulated and it is advisable to use underfloor heating to distribute the heat. Therefore, an air to water heat pump is often a much more sensible option for a newly built development where it is easy to install these things, as opposed to retrofitting to an old property, which could be considerably more difficult.

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