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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 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 well insulated buildings, and can be a good choice in new build. If you are hoping that a heat pump will lower your heating costs, this is unlikely if you are replacing a mains gas boiler. If you currently heat with oil, LPG or electricity, you may benefit financially.

To install a ground source heat pump you need plenty of outside space for the pipework which is generally buried in trenches. It can also be inserted in a borehole, which costs a bit more. In this case, you will need suitable access for the drilling machinery. Air source heat pumps take up much less space, but you do need a bit of distance between it and your neighbours.

Heat pumps heat water to a lower temperature than traditional boilers. As a result the ideal place for them is an extremely well insulated house with underfloor heating. You can 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.

With a traditional boiler the hot water cylinder tends to be heated to 60C or higher. With a heat pump, the hotter you heat your water the more electricity you use, which leads to higher running costs. 40 - 50C is generally hot enough for washing up and bathing, but the temperature in the cylinder needs to be boosted to over 60C once a week to avoid the danger of legionella. Some heat pumps come with an integrated immersion heater.

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. Noise from the fan must be below 42 decibels from a metre away to meet permitted development rules. 

For a ground source heat pump you need space outside to dig trenches, 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.

The geology of the ground around your property is important in determining whether a ground source heat pump is suitable. For example, sandy soil drains fast and does not hold heat well. Heat pumps will not perform well in an area with this soil. 

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SPF: How heat pump performance is measured

The Energy Saving Trust's field trial of heat pumps installed in homes found that the performance was variable. It's main findings were:

1. Heat pumps can be an efficient alternative to electricity, oil, LPG or solid fuel heating systems.

2. They are sensitive to design and commissioning. However, between the phase one trial in 2010 and this trial the EST says that the reasons for previous underperformance are understood, and have been addressed by the new, improved MCS standards.

3. While most people taking part in the trial were happy with the heating and hot water provided by their system, they wanted more information. Installers and manufacturers should work to develop customer understanding of their system.

4. Different aspects of a heat pump system have different impact on its performance. In particular, auxilliary and immersion heaters can have a significant impact on the efficiency of the system, and customers will benefit from knowing that.

5. It's often said that the most efficient way to run a heat pump system is to have it running continuously. However, the trial found that a number of well-performing systems were controlled non-continuously, and their owners were satisfied with their performance.

6. Users' understanding of their systems is varied. The data suggests that if people understand about system design and control, the overall performance is likely to be better.

Seasonal Performance Factor (SPF)

The performance of heat pumps is rated as a seasonal performance factor (SPF). It is the total useful heat generated from the heat pump in a year divided by the annual electricity consumption. For example an SPF of 3 indicates that the system will give an average three units of heat energy for each one unit of electricity used. 

 To be considered 'renewable' (under EU legislation) heat pumps must have a SPF of at least 2.5, and this is the minimum performance that is eligible for the domestic renewable heat incentive. 

The domestic renewable heat incentive will pay on renewable heat only, so the more efficient the heat pump, the greater the payment you will receive. This makes it worth investing a bit more in your system to make sure you have a good quality heat pump, with suitably sized radiators or underfloor heating, installed by a reputable installer (MCS accredited, and recommended by the manufacturer). Click the link to see a table illustrating the difference in RHI payments for systems with different star ratings.

However, there is likely to be a balance to be struck. The upfront cost of getting the most efficient system - ie installing new radiators or underfloor heating - may be prohibitive. In this case a cheaper, slightly less efficient system with less disruption and a lower purchase cost, but higher running costs may be a better alternative.

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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. The calculation should take account of your property size and location, how energy efficient it is, and whether the heat pump is for space heating, water heating or both.

3. Check how experienced the installer is and whether previous customers are happy. If possible, talk to people who have had their heat pump at least a year.

4. Make sure both installer and the product they are installing are MCS accredited, so you will be eligible for the Renewable Heat Incentive. Also check that they are members of the Renewable Energy Consumer Code (RECC).

5. 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. Also check out these questions suggested by Graham Hazell of the Heat Pump Association.

6. Ask for an accurate estimate of your likely annual electricity consumption once the heat pump has been installed.  

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

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

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

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How much do heat pumps cost?

A report commissioned by the government to inform the domestic renewable heat incentive found the following average costs per kW installed:

Air to air heat pump: £1,138 (5-10kW system); £584 (10-20kW system)

Air to water heat pump: £1,380 (0-5kW system); £1,187 (5-10kW); £556 (10-20kW)

Ground source heat pump: £2,403 (5-10kW system); £1,980 (10-20kW) 

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 or larger radiators suitable for lower flow temperatures.

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Renewable heat incentive

The domestic Renewable Heat Incentive

The domestic renewable heat incentive is a boiler replacement scheme, aiming to bridge the gap between the cost of a replacement oil or LPG boiler and the cost of a renewable heating system. Air, ground and water source heat pumps are all eligible systems.

RHI rates are:
Air to water heat pumps: 7.3p per kWh
Ground or water source heat pump: 18.8p per kWh
Payments are index-linked and paid quarterly in arrears for seven years. 

To apply you will must have a green deal assessment (unless you are a self builder, in which case an EPC is required), and install loft and cavity wall insulation if it gets a green tick on the green deal advice report.

The RHI will be paid on each kWh of renewable heat generated by your biomass boiler or stove.

To calculate the payments you will receive, follow the formula in this example:

House with a heat demand (space heating plus hot water) of 18,000kWh per year. The heat demand figure will be taken from your EPC.

Installing an air source heat pump with an efficiency rate averaged over the whole year (seasonal performance factor or SPF) of 3 - ie it generates an average of 3kWh of heat for every 1kWh of electricity used. 

Tariff rate of 7.3p per kWh.

The RHI is only paid on the renewable element of the heat, not the electricity used. To find that figure use the formula 1 - 1/SPF:

1 - 1/3 = 2/3 of the electricity will be counted as renewable.

18,000 x 2/3 = 12,000kWh x 0.073p = £876 per year x 7 years = £6,132 

In addition, you will make some savings on fuel costs if you are swapping from oil or LPG, but probably not if you are on mains gas.  

18,000kWh of oil @ 6.1p per kWh = £1,098 a year heating bill
18,000kWh of LPG @ 6.8p = £1,224
18,000kWh of electricity @ 16p = £2880
18,000kWh of mains gas @ 4.6p = £828
6,000kWh of electricity for an ASHP @ 16p = £960
(source for fuel prices: Renewable Energy Installer, April 2014). 

For full details of the scheme see our renewable heat incentive page and our RHI Q&A blog.

Application is via the Ofgem website, where there is also a lot of helpful information and advice. You can also contact Ofgem via Twitter @AskDomesticRHI.

Heat pumps are approved measures under the Green Deal, so green deal finance may be available to help with the upfront cost of installing a domestic heat pump. This is repaid through the savings in your electricity bills.

Non-domestic Renewable Heat Incentive (RHI)

Rates are:

4.9p per kWh for small heat pumps, less than 100kWth
3.6p per kWh for large heat pumps of 100kWth+

Ground source and water source heat pumps heating water are eligible for the non-domestic RHI. Air source heat pumps may be added to the scheme after further research into cost and performance. Results of the consultation are expected in autumn 2013. Click here for frequently asked questions on phase 1 of the RHI.

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How do heat pumps work?

Air to water source heat pumps

  • extract heat from the air using an evaporator coil. This looks like the big fans on air conditioner units and is fixed on an outside wall of the building.
  • Using mains electricity, the heat pump boosts the heat from the air to the level needed by the heating system, heating water in a buffer tank.
  • The heating system is fed from the buffer tank.
Air to air heat pumps extract the heat from outside, boost it to the required level, and blow out hot air into a room, or through vents around the building. They can also be used for cooling (air conditioning). Air to air heat pumps are not eligible for the renewable heat incentive. 


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  is fed from the buffer tank.

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

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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. 

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.

Installation of either a ground source or air source heat pump will have to comply with the Building Regulations. Make sure that your installer belongs to either the Microgeneration Certification Scheme or the relevant Competent Person Scheme.

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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More information on heat pumps

From the blog:

Practical advice for those considering a heat pump system

Key things to consider before installing a heat pump

How to write a brief for a renewable heating system (Jan 2014) 

Which is the best heat pump?

Should I use an MCS accredited installer? 

Heat pumps: 12 tips for people thinking of installing one

Is a heat pump suitable for my home? 3 key checks

The results of a field trial of heat pumps (2013)

Is there such a thing as a ground to air heat pump? (Jan 2014) 


Choosing an installer 

10 Questions to ask when choosing your air source heat pump installer

Heat pumps: 10 things a good installer should ask

7 questions to ask a heat pump installer before you buy (July 1013)

Practical advice - ASHP

An introduction to air source heat pumps

When to consider using air sourced heat pumps

Can you use radiators with an air source heat pump?

Comparing air source heat pumps part 1 

Comparing air source heat pumps part 2

Air-to-air or air-to-water: which air source heat pump is best? (Feb 2014)

Get the best out of your air source heat pump in sub-zero temperatures

How to maintain & service your ground source heat pumps

How (and where) to position your air source heat pump

Using air source heat pump with an existing heating system: 5 things to consider 

Practical advice - GSHP

Ground source heat pumps don't need immersion he qaters

Heat pumps and underfloor heating: perfect partners?

How to install a heat pump into an existing property 

Three things to consider before running heat pumps with solar or wind power

Does your ground source heat pump cost too much to run? 

Your questions answered

How will the new EU ErP directive impact heat pump RHI?

Can heat pumps provide domestic hot water?

How to use an air source heat pump to heat your hot water 

Case studies

Living with an air source heat pump

Heat pumps a no brainer says property developer

Solar PV plus air source heat pumps: a case study of Donyatt Village Hall

Eco renovation transforms Devon village hall

Heat pumps keep hotel guests warm, happy and intrigued 

Ground source heat pump warms guests at country house hotel (Dec 2013)

Solar powered heat pump in Northamptonshire village hall (Oct 2013) 

More information

Heat pumps: 7 tips for installers

Heat pumps field trial: good or bad? 

Five energy sources for heat pumps 

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