Ground, air and water source heat pumps

What are heat pumps?

Ground source, air source and water source heat pumps are three different ways of extracting ambient temperature and using that heat in your property. Think of them as working like fridges…only in reverse. Probably best to read the next bit on ‘how they work’ to understand what we mean by that.

How they work

The ground, air and water source heat pumps take heat from a source (the ground, the air or the water) and pump it into the hot water system in your house. If your house uses air heating, rare in the UK but common in some other countries, then it can be used to provide the heating for that system too.

Most models use one of three alternative sources of heat:

• Ground Source Heat Pumps collect heat in a fluid passed round the ‘ground loop’ – usually plastic pipe-work buried in the garden. Alternatively vertical ‘loops’ in boreholes can be used which is great for houses that lack large gardens or if you don’t fancy digging up your lovely lawn.

• Air Source Heat Pumps use the ambient air as a heat source, with a fan to blow the air over the heat exchanger.

• Water Source Heat Pumps are less widely used because they need a stream or pond to take their heat from. They can either use a closed loop with a heat exchanger in the water or ‘open loop’ where the water itself is drawn through the heat pump.

The heat pump uses electricity to extract the heat, but delivers typically 2½ to 4 kW of heat for every kW of electricity used. It can work even when the heat source is below freezing. The heat pump delivers its heat into the house through the normal hot water or hot air system.

Where they work best

Before we get onto where it works best, you will need to ensure that your property is well insulated and draught proof if a heat pump is going to work most effectively.

Heat pumps tend to heat the water to a lower temperature than gas boilers so are particularly suited to under-floor heating systems and hot water. In most cases however, normal radiators can also be suitable as they can remain warm over a long period and as such, raise the temperature of a room to a steady ambient temperature.

The requirements for the heat source are:

• Ground Source needs enough ground area accessible to digging machinery for a trench to be dug. The length of the ground loop (illustrated on the right) depends on the size of your home and the amount of heat you need - longer loops can draw more heat. The loop can be laid flat, or coiled in trenches about two metres deep. Alternatively you can drill vertical boreholes up to 100 metres deep and install vertical loops. These take up far less surface area.

• Air Source Heat Pumps need a place outside your house where a unit can be fitted to a wall or placed on the ground. It will need plenty of space around it to get a good flow of air.

• Water Source obviously needs a nearby stream or pond.

Ground and watersource pumps also need space indoors, for example in a garage, for the heat pump and associated equipment; this is typically rather larger than a domestic boiler.

How much energy they produce

Heat pumps are typically designed to meet the total heat requirement of your house. They can be used in conjunction with solar thermal systems in which case the heat pump's electricity consumption is reduced and in some cases a smaller heat pump may be specified.

What maintenance they need

Heat pumps require very little maintenance although we do recommend a service visit once a year to check on the performance of the system.

Tariff levels

The government has proposed the following tariffs in its consultation on the Renewable Heat Incentive (see here). It proposes that tariffs for heat pump systems will continue for 18 to 23 years depending on size and type (see table below). Water-source heat pumps will also be eligible for tariffs - probably at the same levels as ground-source.

Government has not said if it will introduce degression for any of the heat technologies, but these tariff levels should continue at least before April 2012.