Ground source heat pump

• A long loop of pipe, filled with water and anti-freeze, is buried in the earth. Depending on available space it can be in a trench at least 2m deep or down a borehole
• The liquid in the pipe (or ground loop) absorbs heat from the ground which is a fairly stable 8 - 12 degrees C all year round
• As it passes through an electrically powered heat pump, the absorbed heat is extracted, and the liquid goes back into the underground loop
• Using mains electricity, the heat pump boosts the heat from the ground to the level needed by the heating system, heating water in a buffer tank
• The heating system (ideally underfloor heating) is fed from the buffer tank.

Air source heat pumps work in a similar way, but instead of taking air from the ground, they extract heat from the air outside your home, using an evaporator coil. This looks like the big fans on air conditioner units and is fixed on an outside wall of the building.

The performance of heat pumps is rated as a Coefficient of Performance (CoP). This measures how many units of heat are generated per unit of electricity used to drive the heat pump. For example CoP3 indicates that the system will give three units of heat energy for each unit of electricity used.

The standard figures given for heat pump performance by manufacturers, the Energy Savings Trust and the GSHP Association are CoP 3-4. However, GreenSpec and others suggest that these claims are not always realised in practice. Debate on the Sustainable Building Forum gives evidence of high CoPs for pumps installed in houses in two social housing projects. These were installed in highly insulated houses, running underfloor heating.

There aren’t currently accreditation standards for agreeing manufacturer’s claims, so it makes sense to ask potential installers to refer you to previous customers, and ask them how their system performs.

A detailed guide to the design and installation of closed-loop domestic ground source heat pumps, produced by BRE’s Sustainable Energy Centre is available to download free of charge. For people who are technically minded, a heat pump efficiency estimator is availble to download from John Cantor Heat Pumps.

There are people who argue that heat pumps are not really renewable energy, because they require considerable input of electricity. In terms of reducing carbon emissions, their effectiveness will depend on how the electricity used is generated, and what they are replacing.

Labelling heat pumps as ‘renewable energy’ is ‘highly misleading’ according to AECB - the sustainable building association. In evidence submitted to the Department for Business, Enterprise and Regulatory Reform’s energy group in February 2008 it said: ‘Coefficients of performance from plant that has been monitored are not as high as the manufacturers state, with as much CO2 being emitted as a result of the electricity being used to run them as is saved by using the heat pump as a heat source. A highly efficient gas, LPG or oil boiler would usually emit less CO2.’

Low carbon energy consultant Casey Cole argues that heat pumps have higher emissions than the worst gas boiler that you can legally install, and should not be included in the renewable heat incentive (which is due to be introduced in April 2011).

Some would argue that for a heat pump to be a truly renewable form of energy, you need to power the heat pump with electricity that is 100 per cent renewable, such as that from Good Energy, or with electricity you generate yourself using photovoltaic panels, wind or hydro. Others, including the AECB, question the validity of treating electricity from the grid as ‘green’ or ‘brown’ given that green electricity is not in surplus. In 2007 the proportion of electricity generated from renewables did not even meet the Government’s minimum requirement of 7 per cent.

The other environmental issue that may concern some people is the use of hydrofluorocarbon refrigerants. These are powerful greenhouse gases which can cause significant damage if they leak during or after the pump’s working life. CAT has a list of suppliers that use natural hydrocarbon refrigerants which have a lower impact if leaked, and other low impact refrigerants are expected to reach the market soon.

Heat pumps aren’t suitable for every home. They heat water to a lower temperature than traditional boilers. As a result they are most suitable for extremely well insulated houses with underfloor heating. Many older buildings cannot be made energy efficient enough to use underfloor heating or low temperature radiators.

It is possible to use a heat pump with radiators, but to get the same level of heat you will need larger radiators. Alternatively you can use traditional radiators, and boost the heat of the water with a conventional boiler or immersion heater.

Heat pumps don’t tend to heat water hot enough for washing and bathing. The hotter you heat it the more electricity you use, and the lower the CoP is likely to be. You can use a heat pump to pre-heat water, and then boost it to the necessary temperature (as above). Some air source heat pumps come with an integrated immersion heater.

For a ground source heat pump you need space outside to dig a trench, or sink a borehole, for the ground loop. A typical installation ranges from 6 to 12 kW in size. You’ll need trenches that are 1.5 to 2 metres deep and long enough to lay 50 to 80m of pipe per kW or 10m of slinky (coiled) pipe.

Boreholes use less land, but are more expensive to drill. They tend to need between 20 and 50m of pipe per kW. Boreholes are usually 100 – 150mm in diameter and up to 120m deep. More than one pipe can be put in each borehole, but some systems will need more than one borehole.

Because of the need for good insulation and extensive ground works ground source heat pumps are most suitable for new build or renovation projects. For other homes, it will depend on what fuel the pump is replacing. It is most likely to be suitable for people who do not have access to mains gas.

Because they do not take up much space, air source heat pumps are more likely to be used in flats and in urban areas, particularly in places where there is no mains gas supply. The fan can be noisy – described by one proud owner as ‘about the same as a modern washing machine on spin cycle’. The noise can be a problem for neighbours, and you wouldn’t want to site it too near to windows or doors.

The cost of ground source heat pump installations varied by a factor of four according to research done by Oxford University's Environmental Change Institute on the first two years of the low carbon buildings programme grants. They found some correlation between total cost and thermal output, and also the disruption involved in laying the pipe work will contribute to differences in price.
Fixed costs: average nearly £3,000, but varied considerably
Equipment: £500 per kWh/year
Example: a 2kWh/year system should cost an average of £4,000.

Air source heat pumps are significantly cheaper – around £3,500 for a 6kW system and £6,000 for a 12kW system. These prices will not include the cost of installing an underfloor heating system.

Grants are available from the Low Carbon Building Programme in England and Wales for 30 per cent of the cost of the installation of a ground source heat pump, up to a maximum of £1,200, whichever is lower. For air source heat pumps the maximum is £900.

To get a grant you must meet LCBP criteria for energy efficiency in your house and use an LCBP-accredited installer and product. Air source heat pumps have only recently been added to the LCBP scheme and only two products and a handful of installers have been accredited (August 2008).

There are additional grants programmes for Scotland and Northern Ireland. To find out if there are any local grants available in your area you can ring your local energy efficiency advice centre on  0800 512 012.

Installing a ground source heat pump does not usually need planning permission. Air source heat pumps will become permitted development as soon as standards and safeguards have been established to deal with noise. More information can be found on the Greener Homes planning portal.

If you live in a listed building or in a conservation area, you should check with your council. Up-to-date advice is available on the government’s planning portal.

For those of you living in Wales, from the beginning of September 2009, the Welsh Assembly Government announced new planning rules to encourage householders to install renewable energy equipment. A leaflet has been published to explain the changes - Domestic microgeneration permitted development: A guide for householders.

From the blog:

Calculators shed light on renewable choices

Sources, and more information:

AECB - The sustainable building association

Centre for Alternative Technology

Chris Goodall's Carbon Commentary - Domestic heat pumps: enthusiasm needs to be tempered

Earth Notes: saving the planet @ home

Energy Saving Trust

GSHP Association

Green Building Forum

GreenSpec

Heat Pump Net