Are Ground Source Heat Pumps Good For My Home?
What are Ground Source Pumps?
The technology used is the same as that used in refrigerators. Just as a fridge extracts heat from the food and transfers it into the kitchen, so a ground source heat pump extracts heat from the earth and transfers it into a building.
Radiation from the sun heats the earth. The earth then stores the heat and maintains just two meters or so down, a temperature is around 8-10°C even throughout the winter. A ground source heat pump uses a ground heat exchange loop to tap into this constantly recharge heat store to heat buildings and provide hot water.
The Advantages of Ground Source Heat Pumps
Ground Source Heat Pumps save money. Heat pumps are much cheaper to run than direct electric heating systems. GSHPs are cheaper to run than oil boilers, burning coal, LPG or gas. This is before taking into account the receipt of RHI, which amounts to over $3,000 a year for an average four bedroom detached house – larger than for any other technology under the RHI.
Because heat pumps can be fully automated they demand much less work than biomass boilers.
Heat pumps save space. There are no fuel storage requirements.
No need to managed fuel deliveries. No risk of fuel being stolen.
Heat pumps are safe. There is no combustion involved and no emission of potentially dangerous gases. No flues are required.
GSHPs require less maintenance than combustion-based heating systems. They also have a longer life than combustion boilers. The ground heat exchanger element of a ground source heat pump installation has a design life of over 100 years.
Heat pumps save carbon emissions. Unlike burning oil, gas, LPG or biomass, a heat pump produces no carbon emissions on site (and no carbon emissions at all, if a renewable source of electricity is used to power them).
GSHPs are safe, silent, unobtrusive and out-of-sight: they require no planning permission.
Heat pumps can also provide cooling in summer, as well as heating in winter.
A well-designed ground source heat pump system is likely to increase the sale value of your property.
Installing a winding horizontal ground array for a ground source heat pump system
How to Install Ground Source Heat Pumps
You will either have a horizontal ground array installed, or boreholes can be dug for a vertical installation. The choice will depend on the space available and your ground conditions.
Horizontal arrays can come with straight or slinky pipes. People who advocate the use of straight pipes don’t tend to like slinkies, and vice versa. The reality is that both work, as well as each other and whatever your chosen supplier is happiest with, will be fine.
The key is ensuring there is enough ground to allow pipes to be properly spaced. The calculation for clay soil is 50m2/kW output, so a 14kW heat pump needs 700m2 of unshaded land. Bear in mind that the array needs to be kept at least 5m from any boundary, and that straight pipes need to be 3m apart and slinkies 5m apart.
Cost for a horizontal ground installation will vary with the size of the array, but for a typical domestic property, it will be around $4,000-6,000.
The alternative to a horizontal ground array is boreholes. The cost will be in the region of $4,000-$7,000 per hole. As with horizontal arrays, the amount of heat that can be extracted will vary with the geology — loose stone will have about 20W/m and granite 50W/m.
On the upside, boreholes do not affect the whole garden as a horizontal array will, and there is an efficiency advantage: boreholes drilled 100m deep will deliver up to 5˚C more heat to the pump than a horizontal array. This gives a potential efficiency improvement of 20%.
Running Costs of Ground Source Heat Pumps
A house with a heat load of 20,000kWh will need to buy 5,000kWh of electricity to run the heat pump (assuming a Seasonal Coefficient of Performance of 4.0, which indicates that the heat pump will produce 4kW of heat for each 1kW of electricity it uses).
The current average price of mains gas is about 0.065/kWh (including VAT), so heating the same house on gas will cost £1,080. The heat pump saves just $230 per year — but don’t forget the Renewable Heat Incentive payment, which is currently $0.23/kWh.
What ground conditions do I need?
Heat pumps used to be called (and still are, in some cases) geothermal energy, though this is a misnomer. A ground array (either installed horizontally or vertically via boreholes) collects heat that is introduced to the ground by the sun. It is therefore finite and quantifiable. The amount of heat available to be collected will vary with the type of soil — for example, clay holds more heat than sand.
A good installer will check the ground conditions before sizing and pricing the installation. People who do not check may not be considered a ‘good’ installer.
How can I make a heat pump more efficient?
Heat pumps are all about efficiency — efficient use of energy and operating an efficient heating system. Heat pumps run on electricity and that will always be an expensive form of energy. System efficiency, therefore, starts with minimizing the amount of heat required and the amount of electricity needed.
A well-insulated house will need a smaller heat pump, a smaller ground array (or fewer boreholes) and less electricity, reducing capital and running costs.
You can also buy an inverter, or modulating, heat pump. These pumps vary the speed at which the compressor operates to vary the heat output. A house with a heat load of 20,000kWh would have a peak load of 14kW, so we need a 14kW heat pump.
That peak heat load is calculated to deal with an outside air temperature of (usually) -2°C. Obviously, it is not always -2°C outside so we do not always need 14kW. An inverter heat pump varies the heat output to suit prevailing conditions, saving electricity and improving efficiency.
Also, don’t over specify your heat pump. Over-sized systems are more expensive and, perhaps counter-intuitively, one that is too big will be less efficient.
To get the full benefit of a GSHP installation you will need to employ someone with design and installation experience. A ground source heat pump may not perform well unless it is incorporated in a good design by someone who understands the needs of the building and the local geology.
A well-installed ground source heat pump installation can provide sustainable energy over the whole life of a building.
What heating system is the best pairing for a ground source heat pump?
Underfloor heating is the perfect choice of the emitter for a heating system powered by ground source heat pump. This is because underfloor heating requires much lower water temperatures than radiators and a boiler could be said to overheat the water needed for UFH.
Is a ground source heat pump right for my home?
If you are considering a ground source heat pump, you need to ensure the following:
- Your home must have enough garden space for a borehole or horizontal array. You will also need to consider access to the site for excavation machinery.
- There is no point installing a heat pump of any sort in a home that is not well-insulated. A home with a high heat demand will need a larger system which will be expensive to buy, expensive to install and very expensive to run.
- It will need to be paired with a suitable heating system and as mentioned above, is most efficient with underfloor heating.