Heat Pumps: Air source or ground source?
Posted by Aderanti Kudehinbu on 15 November 2016 at 10:30 am
Heat pumps are devices that use a small amount of energy to move heat from one place to another. During cold weather they extract heat from outside sources and pump it indoors. This process can be reversed during summer in order to act as an air conditioning system.
The heat pump circulates a liquid or gas refrigerant. This is cooler than the ambient temperature so absorbs heat. It is then fed through a compressor. Compressing the fluid increases its temperature so that it becomes hotter than its surroundings and can be used to warm up air and water. Pumping heat from one place to another requires relatively little electricity. Consequently heat pumps can provide a good alternative to off-gas oil and electric heating systems provided a property is well insulated. They supply more heat than the amount of electricity they need to run, giving them an efficiency as high as 300% .
There are two main types of heat pumps:
1. Air source heat pumps (ASHP):
Air source heat pumps extract energy from the air outside the house. They can
be used to heat a building in a number of ways.
Air to water system: This heats up water then distributes it via a wet central heating system. As the water coming into the radiators will be at a lower temperature than with gas central heating system, often radiators with a larger surface area (eg. double radiators) are installed.
Air to air system: This produces warm air, which is circulated through the house using fans.
2. Ground source heat pumps (GSHP):
Ground source heat pumps extract heat from the ground by pumping water through a series of buried pipes, known as ground loops. The ground loops circulate a mixture of water and anti-freeze. This is warmed up as it passes through the ground which stays relatively warm one metre below the surface in winter. This heat source can then be used to produce hot water, and warm a house via radiators, underfloor or warm air heating systems.
The heat pump performs the same role that a boiler does in a normal central heating system. However the energy comes from the ambient temperature of the ground, rather than using fossil fuels. The heat pump has to be carefully designed so that the size of the pump, the design of the ground loops and the selection of radiators are appropriate to the house.
The temperature of the ground one metre down tends to be at around 8-11oC in the UK. This stays fairly stable over the year, with minor changes which reflect the variation in seasonal air temperature. 
Comparing air source vs ground source
Ground source heat pumps are much more expensive to install. All air source heat pumps are fairly similar, but a ground source heat pump has to be set up differently depending on the location. Some houses will have space for large ground loops, while others will need a vertical system using bore holes. Digging or drilling holes to accommodate the loops will be a major cost so a GSHP install might cost between £9,000 - £16,000 whereas an ASHP might be £6000 - £10,000 according to the SuperHomes article here of 2013. The Energy Saving Trust quote more: £13,000 - £20,000 for GSHP and £7,000 - £11,000 for ASHP. Anecdotally, cheaper deals than both these quoted are possible where just a basic ASHP unit for a small property is needed.
Currently GSHP have the highest Coefficient of Performance (CoP), and Seasonal Performance Factor (SPF). This is because the temperature underground fluctuates less than the air temperature.
How much you save will depend on the type of fuel your heat pump replaces. Heat pumps have the potential to reduce running costs compared to oil boilers, direct electric heating and LPG or solid fuel fired systems. This can result in substantial savings over their lifetime, despite the high initial cost.
Trials have been conducted by the Energy Saving Trust and Mitsubishi Electric in order to estimate the savings from different heat pumps. It was found that:
- An air source heat pump with an SPF of 2.45 could give savings of around £150 and 1,400 KgCO2 per year when replacing an oil boiler, or £530 and 5700KgCO2 when replacing electric heating.
- A ground source heat pump with an SPF of 2.82 is likely to save £300 and 1900 KgCO2 per year if replacing oil, and £685 and 6300KgCO2 if replacing electric heating. These figures based on average performing pumps, installed in a well-insulated detached building.
Which is easier to install?
The ASHP is much easier to install. It consists of outdoor and indoor units linked through a wall. The GSHP has an indoor unit connected to the ground loop outside. Burying the loop requires a lot of digging or drilling.
A typical ASHP system consists of an outdoor unit with a large fan. The fan will make a low level noise while the system is in use. GSHP systems tend to be quieter as pumps tend to make less noise than fans.
The installation of GSHP systems on domestic premises is considered to be a permitted development, so doesn’t usually require planning permission. However this might not be the case with listed buildings or properties in a conservation area.
How it looks
ASHP systems are normally mounted on the wall outside a building. These can detract from the appearance of the property. In comparison the GSHP array is buried underground so has no visual impact on the outside. In this case the heat pump itself is likely to be a large box which will need to be accommodated indoors in a dry, safe place next with a hot water cylinder alongside. You may need to think about whether you have space to accommodate it say in your garage, utility room or extension.
Heat pumps are only suitable for very well insulated homes and sizing them correctly is a must. Otherwise your winter heat demand risks exceeding what the heat pump can comfortably deliver. With ground source heat pumps using the ‘boost’ mode in winter is to be avoided as it means extra electricity will be bringing the water up to temperature at significant cost.
In summary, which heat pump you go for will largely depend on space availability and your budget. Maintenance costs should also be taken into account. A GSHP is likely to require less maintenance over its life and will achieve higher efficiency in comparison to an ASHP. It thus has a longer life span. However the air pump is a cheaper and more convenient option.
GSHP and ASHP technologies are both eligible for for the Renewable Heat Incentive.
Are Heat pumps cost effective?
More information about Heat Pumps on YouGen.
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