Ground Source Heat pumps

A Ground Source Heat Pump collects heat from beneath the ground (ground source) and convert this energy into usable heat which can be pumped around a heating system to provide warmth and comfort to any property.

A ground source heat pump will have 2 systems running alongside each other – one for the collection of the heat (from the ground) and one for the distribution of the heat (to your home/property).

Heat is collected in a ground source heat pump system by a pressurised network of pipes running through the ground and absorbing the heat stored in the ground as it passes through. Special heat transfer fluids are used for this and the heat pump will manage and control the amount of heat being collected to supply the quantity of heat need by the property it is heating.

It is fairly obvious that the amount of heat needed for the heating of an average sized house cannot be extracted from a few metres of pipework laid around your flower beds. Depending on the amount of heat you need, you will probably looking at several hundred metres of pipework that need to be buried in order to allow them to collect sufficient heat energy to enable a ground source heat pump system to operate effectively.

Your first questions when looking at a ground source heat pump is therefore, “Where are you going to put your collector pipework?”. You can choose one of 2 options and the decision will usually be made for you depending on the size of plot you have available and the funds available.

Boreholes

Boreholes or vertical ground collectors run a series of pipes that are drilled down into the ground using specialist equipment. This is a job performed by a specialist drilling contractor as the boreholes are typically 80- – 120m deep and involve drilling through rock. mud and other geological matter. They will feed down the pipework and connect it, along with any other borehole pipework, to a central manifold which then takes the energy in the collected fluid to the ground source heat pump itself where the heat is transferred to a higher temperature using a compressor and then pumped around your heating system to warm your home.

Boreholes are ideal where space is limited and you are prevented from laying horizontal collector loops due to lack or space or difficult geological conditions. The temperature of deeper boreholes is marginally higher than that for collectors situated just below the surface, meaning that you can usually extract more heat per metre from a borehole than a horizontal collector (ground loops). The biggest disadvantage to boreholes is the cost, as a typical house may require 3 or even 4 boreholes to satisfy its heating demand, with each one costing several thousand pounds.

Ground Loops

Ground loop or horizontal ground collectors are therefore a cheaper option in most cases and make most sense for very rural locations where access to large spaces of land is easier. With a ground loop system, instead of drilling vertical holes in which to lay the collector pipe, the pipework is laid horizontally in “loops” of 100-200m. ideally these are dug in 1m wide trenches at a depth of around 1.5m with the pipework running out along one side and returning on the other side of the trench. Again, several trenches will be needed for each home and they are connected up using an external manifold from which a main header pipe will deliver the heated collector fluid to the heat pump.

The actual length of pipework you require will need to be calculated taking into account the amount of heat your house needs, the efficiency of the heating system and the geology of the ground on which you are laying the pipe. You may require a contractor to excavate the trenches needed for this type of system, though this is still a much cheaper option than a borehole and can often be done with little technical knowledge or experience.

The Heat Pump

So we have enabled a collector system with which we can extract the heat from the ground successfully; the next stage is to transfer this heat, using the ground source heat pump, into a usable heat for your property. This is done using a compressor, much like your freezer extracts the heat from the colder area inside it and transfers it to a heat exchanger at the back of the unit. Here the heat pump will make the heat available to the pressurised heating system via a heat exchanger, with a flow and return, as with any heating system.

This heat is then pumped around the home to be used to heat the home itself and the Hot Water that is required. Space heating is done using radiators or underfloor heating, similar to any other wet system. The type of system will determine how efficient your system will perform, as by designing the system to use lower flow temperatures for the heat pump (<45°C), the efficiency will rise and thus running costs lower and incentive payments rise. Underfloor systems are ideal as they have the largest surface area of any emitter type, the same amount of energy can be transferred (emitted) using a larger volume of water at a lower temperature. Similarly, if possible, radiators can be oversized to what would normally be required by a conventional system (operating @ 70°C flow temperature) to allow lower temperatures to be used.

Heat Emitter Design Considerations

A ground source heat pump doesn’t tend to work at flow temperatures above 55°C as they tend to lose their efficiency. For this reason, it may be difficult to install a heat pump as a sole heat source in a very badly insulated property that has small/std size radiators. In such a situation we would always advise looking at increasing the insulation properties of the building and replacing the emitter system with one that enables the pump to perform to a higher level.

Of course, if you are undertaking a new build or renovation to a property the chances are that building control will have already ensures that your levels of insulation are of the highest level and you

Ground Source Heat Pump Design

may already have considered fitting Underfloor heating at least in the downstairs of the property as it offers comfortable and efficient heating without loss of wall-space. Typically, UFH system work with flow temperatures around 35ׄ – 40°C which complements a heat pump system ideally. Where UFH system are not possible, the flow temperature can be lowered by oversizing radiators. Basically, as the quantity of heat energy is proportional to the temperature of the water, by increasing the quantity of water you can off-set the temperature to a much lower requirement.

Typically, radiators can be oversized by around 3 times the “standard” requirement for any said room or space. The oversizing doesn’t have to be done using a simple size factor, however, as different types of radiator can offer increased efficiency and ratings (e.g. changing from standard to fan assisted or from single convector to double or even triple convector models).

Hot Water Provision

With a Ground Source Heat Pump, you will need to have a special hot water cylinder to produce the hot water for the property. These are more efficient versions of a standard DHW cylinder so that they can operate at slightly lower temperatures and have the ability to work at slightly lower operating temperatures. As with modern DHW cylinders, the Hot Water can be stored until such a point that it is required and the volume of the cylinder is calculated so that it can offer adequate quantities of hot water whilst having sufficient time to recover (i.e. be in a fully charged state). AS Heat pumps operate more gently than conventional boilers, the controls and settings for the HW can be crucial to providing adequate heat and hot water at the required times.

Benefits of a Ground Source Heat Pump

A properly designed and installed pump should give you years of faultless heating and hot water for your property. The design life of a good system will be in excess of 15 years and we know of many systems still operating well into the 25+ year category.

One main advantage of a ground source system is to have a reliable and cheap form of heating. In comparison with other heating sources (oil, LPG, mains gas boilers, storage heaters etc.) the cost per kWh of your heat is very low. We all know that fossil fuel derived heating systems will also increase in cost over the coming years as their supply falls and methods of obtaining them get more expensive.

Another major advantage is the green credentials you have from installing a renewable heating source in your home. As we become ever more aware of the impact of carbon fuels on the environment as a whole and the increasing responsibility to look after our planet for future generations, the choices we make become more critical. As domestic heating represents a large portion of the carbon dioxide emitted from your home, you can make huge reductions in your carbon footprint by actively choosing renewable technologies such as Ground source heat pumps.

The RHI – Renewable Heat Incentive

Perhaps the most compulsive reason for choosing to install a ground source heat pump system for most people will be the benefits available from the government run incentive schemes. The appropriately named Renewable heat Incentive (RHI) offers a quarterly financial return for anyone who has an eligible system fitted and using it to heat their home.

An eligible system is one of the specified technologies (Ground source heat pump, Air source heat pump, Biomass boiler & Solar Thermal collector) that is an approved and MCS registered product (checking it is manufactured to a standard and of sufficient efficiency) and installed to MCS standards by an accredited installer (to ensure the system is both effective, reliable and fit for purpose).

The RHI scheme will pay you for 7 years (on the domestic scheme), and the amount will depend on the quantity of heat your property requires and the type of heating you have. A Ground source pump currently has excellent rates of return, with many customers receiving £3000 – £4000 per annum, tax free. Visit our RHI page for more information on the scheme and its requirements.

The returns are therefore significant, remembering that ground source is much more specialised than other technologies and normally requires higher levels of investment in the first place due to ground loops or boreholes being needed. It is easy to see, however, that the returns available from the RHI alone, let alone any cost savings on your fuel bills, would pay back with interest any initial installation costs and therefore make a very attractive proposition for those building or renovating their homes.

For those wishing to operate a ground source heat pump for non-domestic purposes, there is an equivalent non-domestic RHI scheme which operates slightly differently and with lower tariffs, though for a 20 year period.

Conclusion

Ground source heat pumps therefore offer ideal heating solutions for many homes and can give you incredibly efficient heating with attractive investment returns as well as the peace of mind that you aren’t using precious and damaging fossil fuels to heat your home.

We have extensive experience in the design and installation of ground source systems which enables us to offer you a fully supported service and give you the best advice, products and workmanship available.

If you are considering a heating solution for a new build or an existing property, give us a call or drop us an email so we offer you our extensive knowledge and experience to enhance your project.