A Ground source heat pump works by using pipes buried in the ground to extract heat. Underground, the temperature remains constant - summer and winter - and as most heat is required in the winter, this temperature differential is used to warm water, usually for underfloor heating. It can also be used to raise the temperature of the incoming water to a conventional boiler so that less energy is used in heating it to a required temperature.
What is certain is that GSHP systems work best for low temperature underfloor heating methods. They cannot supply water at the temperature needed for normal radiators and additional boilers are required.
A ground source heat pump pipes a mixture of water and antifreeze around a loop of pipe - called a ground loop - which is buried in the ground. When the liquid travels around the loop it absorbs heat from the ground. There are three major types of closed loop geothermal systems: horizontal loops, vertical loops, slinky coils. Normally the loop is laid flat, (horozontal loop) or coiled in trenches about two metres deep (slinky loop - see photo) but if there is not enough space in your garden you can install a vertical loop to a depth of up to 100 metres - see image on the left.
The length of the ground loop depends on the size of your home and the amount of heat you need - longer and deeper loops can draw more heat from the ground, but a lot dependes on what type of ground you have. The photo on the right shows slinky coils laid horizontally.
The liquid flows into an electrically powered heat pump, comprising a compressor and a pair of heat exchangers before discharging back to the underground loop.
The pumps are required to draw the heated water through the system - it cannot be operated by gravity as it is not a continuously upward flow of heated water.
|System||Primary Energy Efficiency (%)||CO2 emissions |
(kg CO2/kWh heat)
|Oil fired boiler||60 - 65||0.45 – 0.48|
|Gas fired boiler||70 - 80||0.26 – 0.31|
|Condensing Gas Boiler + low temperature system||100||0.21|
|Conventional electricity + GHSP||120 - 160||0.27 – 0.20|
|PV panel + GHSP||300 - 400||0.00|
(Source: Sustainable Energy Ireland)
This is a good question.
If the electricity used to power the system is from a solar PV panel, then the answer is yes, however, if you are using electricity from the grid, then the answer is no.
If you look at the table above you will see that when a GSHP is using conventional electricity, then it produces more emissions than a condensing gas boiler.
GSHPs can only raise the temperature to around 40°. For this reason they are most suitable for underfloor heating systems or low-temperature radiators, which require temperatures of between 30° and 35°. Conventional radiators need temperatures of around 60° to 80° and this can be only obtained through use of the GSHP in combination with a conventional boiler or immersion heater.
GSHP systems cannot directly supply hot water. Hot water for taps is at around 60°. The incoming water supply can be preheated by the GSHP before reaching the boiler or immersion heater. but it is possible that an immersion heater working off off-peak electricity is more economical.
Refrigerants are present in GSHP systems and so present the threat of HCFCs and toxicity. However, new types and blends of refrigerant (some using CO2) with minimal negative impacts are being examined.
GSHP installed prices ranges from about £900-£1,300 per kW of peak heat output, excluding the cost of the distribution system. Trench systems are cheaper so tend to be at the lower end of this range. As a normal house requires betwwen 8 to 12 kW at peak times, it is expensive to install.