What is a ground source heat pump?

A ground source heat pump is made up the network of underground pipes and the heat pump itself. The pipes, known as a ground loop, can be either laid horizontally in a trench or vertically in a borehole. The only bit that is visible is the heat pump which performs the same role as a boiler in a central heating system.

How does it work?

The Sun heats our Earth through radiation. This energy is then absorbed into the ground below our feet.  In the UK, the temperature of the ground a metre or so down is constant temperature of around 11°C even in winter. The aim of a ground source heat pump is to transfer this thermal energy into your home.

Most pumps circulate a mixture of water and antifreeze around a network of pipes, known as a ground loop, buried in the garden.  The fluid absorbs solar energy stored in the ground. The fluid passes through a compressor that raises it to a higher temperature and turns it into gas. As the high temperature gases pass the heat exchanger, this heats water for the radiators or underfloor heating system and hot water circuits.

Ground source heat pumps deliver heat at lower temperatures and over longer periods than gas or oil boilers. They may need to be on constantly in winter to heat a home efficiently.

Pros and cons

Pros

• Eco-friendly  -  Low environmental impact as they reduce carbon emissions compared to conventional heating systems which burn fossil fuels. They need electricity to run, so can’t be considered carbon-neutral unless a renewable source, such as solar panels, is used to power them.
• Energy efficiency – For every 1kW of electricity used to drive the pump, it produces up to 4kW of heat, making it an efficient way to heat a home. The efficiency of a ground source heat pump is referred to as the Coefficient of Performance or CoP. The average UK winter heating temperature has an average CoP of 3.4:1, providing 3.4 units of heat for each unit of energy consumed, resulting in significant energy savings.
• Cut fuel bills – They can reduce your annual heating and hot water bills by up to 50%, say GreenMatch, a renewable energy company. According to the Energy Savings Trust, it’s possible to save up to £1,500 a year, depending on the heating system it’s replacing. Savings will be greatest if you switch from electric, coal or liquid petroleum gas (LPG) systems. But it’s still possible to reduce gas bills.
• Cash back – Government grants are available through the Renewable Heat Initiative to help recoup some or all the costs of installation. In addition, there is a cashback subsidy for the generation of heat from renewable sources. The current rate for a ground source heat pump is 20.46p/kWh (prices correct in February 2019). Based on this rate an average four-bedroom house could receive up to £2,750 per year for seven years, according to Which?
• Easy to use – no fuel deliveries, such as oil, are necessary, so no risks of running out and no need for storage, such as oil tanks in the garden.
• Low maintenance - Ground source heat pumps can be expected to provide reliable heating with minimum maintenance requirements for well over 20 years, with the ground loop lasting 50- 100 years, say suppliers.   
• Safe – heat pumps are safe. There is no combustion or burning of fuels involved and no emission of potentially dangerous gases.

Cons

• Expensive to install – typically ground source heat pumps cost more to install than conventional heating systems. The average cost is around £10,000 to £18,000, depending on the size of the system, according to the Energy Savings Trust. This doesn’t include the cost of fitting underfloor heating. The installation costs will vary significantly depending on whether the pipes are to be laid horizontally or vertically and the ground conditions.
• Disruptive groundworks – It may require a large portion of your garden to be dug up. If there isn’t enough space, you can drill a borehole for vertical pipes. But this can be prohibitively expensive. Air source heat pumps are easier to install as they don’t require any trenches or digging and require little space as they absorb heat from the air.  
• Limited suitability – Heat pumps perform better with underfloor heating systems than with radiators because of the lower water temperatures required, says the Energy Savings Trust.  They also require homes with outdoor space for pipes to be laid horizontally or suitable ground conditions for drilling.

Is your home suitable?

Heat pumps are only suitable for well-insulated homes. They work best in buildings that require little energy to maintain a temperature once it has been reached. Heat pumps produce heat at a lower temperature than conventional boilers and so a larger area is required for heat distribution. Underfloor heating is ideal, but heat pump specific larger radiators can also be used. 

Properties need to have the outdoor space for a ground source heat pump. They are most common in rural areas where more land is available. The average 150m2 home needs an area between 300 and 700m2 , according to GreenMatch. A vertical borehole requires less horizontal space and is more suited to suburban homes but drilling 50-150m deep makes it an expensive option.

Heat pumps may not be the best option for homes with a gas heating system, according to the Energy Savings Trust. The system will pay for itself much more quickly if it’s replacing electric heating. But you might want to think twice before choosing a heat pump instead of gas boiler because of the higher initial costs.

Due to the disruption involved in digging trenches, it may be best to install a heat pump when remodelling the garden or, if it’s a new-build home, during construction.

Installation

Ground source heat pumps are expensive to install but are highly energy -efficient and champions when it comes to CO2 emissions.  Systems are typically long-lasting and reliable.

A qualified installer is key to making sure the heat pump, collector and distribution systems (underfloor heating or radiators) are sized correctly to effectively heat your home.

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