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Heat Pumps

Heat pumps: an introduction

Heat Pumps take heat from the ground, air or water and use it for space heating, and sometimes to heat hot water. They are like a fridge in reverse. The fridge takes heat from the food you put in it, and pumps that into the kitchen, keeping the food cold. The heat pumps take heat from the ground (air or water) and pumps it into your house, keeping it warm.

Is a heat pump suitable for my home?

Heat pumps aren’t suitable for every home. They work best in houses off the gas grid, or in new build. You also need plenty of outside space for the pipework (for ground source heat pumps); and a bit of space between you and your neighbours (for air source heat pumps). Do not install one in a poorly insulated building.

Heat pumps heat water to a lower temperature than traditional boilers. As a result they are most suitable for extremely well insulated houses with underfloor heating. It is possible to use a heat pump with radiators, but to get the same level of heat you will need larger radiators. Many older buildings are not energy efficient enough to use underfloor heating or low temperature radiators.

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 more electricity it will need to run (which means higher running costs). You can use a heat pump to pre-heat water, and then boost it to the necessary temperature. Some 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. As a rule of thumb, you'll need twice the area of the property for the ground arrays.

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 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, or to replace electric heating. 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.

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How do heat pumps work?

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 1.5m 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. Water source heat pumps take their heat from a lake, river or stream.

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Heat pump performance

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. A good performing system should give a CoP of 3 or more.

The Energy Saving Trust has also developed a second measure which it calls the System Efficiency Ratio (SER). This measures the amount of heat the pump produces for each unit of electricity needed to run the entire heating system - ie including hot water and supplementary heating such as an immersion.

System efficiencies for ground source heat pumps in the Energy Saving Trust's field trial of 83 heat pumps (published September 2010) ranged from 1.3 to 3.3, with most in the mid range of 2.3 to 2.5. Air source heat pump system efficiencies ranged from 1.2 to 3.2, with a mid range near 2.2.

It also found that performance is affected by many things: how well insulated the house is, existing heating systems, attitudes and behaviours of the pump's users and the quality of the installation are the main ones.

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Choosing a good heat pump installer

1. Make sure that responsibility and liability for the complete installation lies with one company, ideally with a contract to guarantee consistency in after-sales service. The EST research found that often there was no single contractor responsible, and installations might involve a ground works contractor, a plumber, a heat pump installer, and an electrician, none of whom had liability for the overall system.

2. Ask the installer to explain how they have calculated the appropriate type and size of heat pump for your house, and explain how they calculated the heat demand of your house.

3. Make sure the installer explains how the heat pump will work with your existing heating system and hot water.

4. Ask them to show you how to use the system and controls, and how to get the best out of the system.

5. Ask how often you should run the heat pump.

6. Check how experienced the installer is. One of the conclusions of the Energy Saving Trust field trial is that heat pumps are sensitive to design and commissioning, and that many of the installations in the trial had not been effectively designed and commissioned. They recommended improved training for installers. In the meantime, it's important to check that previous customers are happy, and if possible talk to ones who have had the system at least a year.

7. Make sure both installer and the product they are installing are MCS accredited, so you will be eligible for the Renewable Heat Incentive, if / when it is introduced.

8. One way of checking out a potential installer is through the questions they ask you. Click on the link for 10 things a good installer should ask you.

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How much do heat pumps cost?

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, although researchers found some correlation between total cost and thermal output.

The Energy Saving Trust puts the price range for ground source heat pumps between £9,000 and £17,000; The Centre for Alternative Technology says around £1,000 per installed kW as a rule of thumb.

Air source heat pumps are significantly cheaper: £5,000 - £9,000 (says CAT); £6,000 - £10,000 (EST)

These prices do not include the cost of installing an underfloor heating system which would be around £2,000 depending on the size of your house.

There is some help with the upfront costs of installation available from the Renewable Heat Premium Payments until 31 March 2012 (unless the money runs out before then). These are one off capital grants of:

  • £1,250 for ground source heat pumps; 
  • £850 for air source heat pumps

Apply at the  Energy Saving Trust website.

Phase 1 of the Renewable Heat Incentive (RHI) is open to non domestic installations only. Rates are:

4.5p per kWh for small heat pumps, less than 100kWth
3.2p per kWh for large heat pumps of 100kWth+

Ground source and water source heat pumps heating water are eligible for the RHI. Air source heat pumps may be added to the scheme in 2012 after further research into cost and performance. Click here for frequently asked questions on phase 1 of the RHI.

Phase 2 of the Renewable Heat Incentive will cover domestic renewable heat installations. A consultation document, with proposed tariffs is expected in early 2012. The scheme is due to come into force in October 2012, to coincide with the start of the Green Deal. To qualify you must use a MCS accredited installer and product. Eligible systems installed since 15 July 2009 will receive the incentive. However, until the consultation is published we will not know what the eligibility criteria are.

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Planning permission for heat pumps

Ground and water source heat pumps are permitted developments.

Air source heat pumps are also permitted development as long as they meet a long list of additional criteria. Noise is one of the main issues. To meet the MCS020 planning standards, noise from ASHPs must be below 42dB at a position one metre external to the centre point of any door or window to a habitable room of a neighbouring property as measured perpendicular to the plane of the door or window. Most ASHP cannot currently meet this standard (December 2011), and so would require planning permission unless you don't have neighbours.

It is a condition of permitted development that the ASHP can only be used for heating purposes.

Other exclusion criteria include:

  • there's not another ASHP already installed on the building
  • there's not a wind turbine installed
  • the volume of the pump's outdoor compressor is not bigger than 0.6 cubic metres
  • any part of the pump is installed within one metre of the boundary
  • listed buildings and scheduled monuments
And there are additional criteria if you are in a conservation area or World Heritage Site.

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.

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