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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

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.

However, there is likely to be a balance to be struck. The upfront cost of getting the most efficient system - i.e. installing new radiators or underfloor heating - may be prohibitive. In this case a cheaper, a 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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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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