This article will explain how Ground Source and Air Source Heat Pumps (GSHP and ASHP) work technically and how they can provide heating for your house, advantages and disadvantages, as well as the Renewable Heat Incentive that will be available to them.

Ground Source Heat Pumps (GSHP)

Depending on latitude, the ground remains at a relatively constant temperature all year, between 10⁰C and 16⁰C at a depth of 3m. By forcing water to flow in a pipe within this depth of 3m, heat from the ground can be transferred to the water. The pipework is normally looped creating a bigger surface area which enables the system to accept more heat quicker. Antifreeze is also added to the water for obvious reasons. Some homes may not have the advantage of space in their garden. If this is the case, a vertical single looped pipe can be installed within a drilled bore reaching 100m in depth.

This relatively constant temperature means that GSHP have an advantage opposed to other renewable technologies (such as solar thermal). It can provide heat for your home 24 hours a day for the entire year.

The heat produced from this system is much lower than what you would get from a normal central heating system. Radiators in your house would be warm to the touch as opposed to hot. However, as it is at a constant low temperature and not a few hours heating a day here and there, the efficiency as a system is high. To improve this efficiency further, larger surface area radiators or, even better, under floor heating could be installed.

The system requires electricity to run its motor and pump. The approximate ratio of electricity to renewable heat generated is for every unit of electricity needed to run the system, 4 units of heat is generated.

Warm water from the ground loops passes through a heat exchanger. The heat is then transferred into a refrigerant fluid (refrigerant is also used in refrigerators and is used because of its thermodynamic properties suitable for heat exchange). The fluid is then compressed giving it a higher thermal energy content. This fluid then heats up the heating and water circuits in your house.

Spent water within the ground system is now cooler and goes back into the ground.

As a system, GSHP is continuously becoming more efficient. GHSP is a feasible Renewable Technology that can replace you existing heating system. It can save you several hundreds of pounds a year particularly if you are replacing an electricity fuelled system. Carbon savings are in the region of 5 tonnes a year for electricity replacement.

The table below is taken from the Energy Saving Trust and shows savings figures for carbon emissions and money.

Suitability and Costing

Primarily the most important suitability factor in installing a GHSP system is what fuel you are replacing it with. As you can see from the table above, not all conventional fuels are feasible. However, it does not take into account the Renewable Heat Incentive (RHI) due to be launched in April 2011. Electricity, Oil and Solid fuel types are beneficial to replace in terms of savings, however due to the cost of these fuels, the payback period will differ. Gas is a fuel which is relatively cheap, and as you can see from the table, at the moment it is not really feasible to replace it with a GSHP system.

According to the Energy Saving Trust, the typical cost for a GSHP system can be anywhere between £9,000 and £17,000, leaving you with a payback for an electricity replaced fuel of 18 to 34 years! Although figures for the RHI have not been published, the payback period is sure to be lessened greatly.

If the house is new build or going under refurbishment, this will reduce the cost further. GSHP works more effectively if its generated heat is distributed differently to conventional heating systems. Underfloor heating, low temperature fan convectors or larger radiators will distribute the heat more effectively than conventional radiators. These will be costly to install, however, as said before, if the house is going under refurbishment or a new build then these will be a credible alternative to installing conventional radiators.

As talked about in previous articles, ‘low and no cost improvements’ GSHP as with any Renewable Technology should only be installed if these improvements have been made, particularly with insulation improvements. GSHP will only be able to work efficiently and be more cost effective (reducing your payback) with your house being properly insulated. If you house isn’t properly insulated you may find that the GSHP system does not provide enough heat to meet your needs.

Do you have the garden space needed to install the pipework loops? This form tends to be cheaper than bore drilling.

Summary

In summary, GSHP is a credible alternative to common heating systems. It is improving in its efficiency and becoming cheaper. The RHI will reduce the cost further in years to come. GSHP will reduce your carbon emissions, on varying levels depending on what fuel is being replaced. Replacing electric heating is the most rewarding in terms of cost and CO₂ emissions, with natural gas being the least.

There is no maintenance (apart from service checks) involved with a GSHP system once it has been installed, and similar to certain other Renewable Technologies, it is a ‘plug and play’ technology. GSHP requires no fuel and also can provide space heating and hot water.

Air Source Heat Pumps (ASHP)

Air Source Heat Pumps work in a similar way to Ground Source Heat Pumps in that they extract a low grade heat from air rather than the ground. An ASHP system works again, similarly to a refrigerator, however what it can do is reverse itself, in the sense that it can provide heat but also be used to cool down a building. It does this my expanding or compressing a refrigerant. It can extract low grade heat from as low as -15⁰C!

Just like GSHP, it requires an electricity supply to run the system; however it is a much smaller system in terms of area that it needs, whereas GSHP often requires a large garden area.

Suitability and Costing

ASHP can work in two ways; air to air or air to water. Air to air uses fans to circulate warm air around your house, and air to water system a houses central heating system. Whereas air to water can be used for hot water and heating, air to air is used for heating only.

Like GSHP, ASHP is most efficient in under floor heating, large radiators as opposed to conventional radiator heating. However ASHP is less efficient generally as a system compared to GSHP.

The unit should be installed where it has access to warm air. Having it placed on a wall on the north facing side of a house will mean it will have to work harder, and so consumes more electrical energy. Whereas if you live in a warm climate, it should ideally be placed where it is cooler, in the UK this is obviously not the case.

ASHP has the added benefit of being able to be installed easier than GSHP and is also far cheaper, with installations costing between £6000 and £10,000 (Energy Saving Trust).

The table below is taken from the Energy Saving Trust website on Air Source Heat Pump savings.

The Renewable Heat Incentive (RHI)

As GSHP and ASHP won’t be producing electricity they are not eligible for the Feed-in-Tariff, but the government are currently in the process of finalising the Renewable Heat Incentive for technologies that produce heat and not electricity. This is due to be introduced in April 2011 and rather than measuring the heat generated by thermal technologies payment will be based on estimation. The estimation will be derived from the type of technology used, the age and size of the house – in essence the amount of heat needed to heat up your house.

Table 1. Energy Saving Trust, Ground Source Heat Pumps Savings Table. Available [Online] at: http://www.energysavingtrust.org.uk/Generate-your-own-energy/Ground-source-heat-pumps

Table 2. Energy Saving Trust, Air Source Heat Pumps Savings Table. Available [Online] at: http://www.energysavingtrust.org.uk/Generate-your-own-energy/Air-source-heat-pumps