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Domestic Heat Pumps

This section of our website provides guidance notes on the design and installation of domestic closed-loop ground source and air source heat pump systems.

It is aimed at both specifiers and their advisors but much of the information could be of interest to a more general reader. It provides outline rather than detailed guidance, focusing on the issues to be considered when selecting systems and components and estimating system performance. A useful list of Do's and Don'ts can be found at the end.

Types of system

A heat pump system consists of a ground heat exchanger, a water-to-water or water-to-air heat pump, and a heat distribution system preferable underfloor heating. Until recently, due to availability of grants distorting the market, heat pump systems using closed-loop, where the ground heat exchanger consists of a sealed loop of pipe buried either horizontally or vertically in the ground, have been the most popular.

Applications

Ground source and air source heat pumps can be used to provide space and domestic water heating and, if required, space cooling to a wide range of building types and sizes.

The provision of cooling, however, will result in increased energy consumption however efficiently it is supplied. Ground source and air source heat pumps are particularly suitable for new build as the technology is most efficient when used to supply low temperature distribution systems such as ecotherm underfloor heating.

They can be particularly cost effective in areas where mains gas is not available or for developments where there is an advantage in simplifying the infrastructure provided. This section will concentrate on the provision of space and water heating to individual dwellings but the technology can also be applied to blocks of flats or groups of houses.

Monovalent and bivalent system design an explanation

The most important first step in the design of a heat pump installation is accurate calculation of the building's heat loss, its related energy consumption profile and the domestic hot water requirements.

This will allow accurate sizing of the heat pump system. This is particularly important because the capital cost of a heat pump system is generally higher than for alternative conventional systems and economies of scale are more limited.

Over sizing will significantly increase the installed cost for little operational saving and will mean that the period of operation under part load is increased. Frequent cycling reduces equipment life and operating efficiency. Conversely if the system is undersized design conditions may not be met and the use of top-up heating, usually direct acting electric heating, will reduce the overall system efficiency.

A heat pump system can be designed to provide all the required heat (a monovalent system). However, because of the relatively high capital cost, it may be economic to consider a bivalent system where the heat pump is designed to cover the base heating load, while an auxiliary system, gas boiler or electric, covers the additional peak demand e.g. if the savings in capital cost offset any increase in running costs.

Reducing the output temperature required from the heat pump will increase its performance. Heat pumps have an operating temperature limit of 50°C to 55°C in most applications and are not suitable for monovalent operation in combination with traditionally sized wet radiator distribution systems.

The performance of the heat pump depends on the performance of the ground loop and vice versa. It is therefore essential to design them together.

Closed-loop ground source heat pump systems will not normally require permissions/authorisations from the environment agency. However, the agency can provide comment on proposed schemes with a view to reducing the risk of ground water pollution or derogation that might result.

The main concerns are:

• Risk of the underground pipes/boreholes creating undesirable hydraulic connections between different water bearing strata
• Undesirable temperature changes in the aquifer that may result from the operation of a GSHP
• Pollution of groundwater that might occur from leakage of additive chemicals used in the system

Where there is a risk of or actual releases of polluting matter to groundwater the agency can serve statutory notices to protect groundwater.

Types of ground heat exchanger

An indirect circulation system is the most common, where the ground heat exchanger consists of a sealed loop of high-density polyethylene pipe containing a circulating fluid (usually a water/antifreeze mixture) which is pumped round the loop.

Energy is transferred indirectly via a heat exchanger to the heat pump refrigerant. Alternatively the refrigerant can be circulated directly through a copper ground heat exchanger (this is called a direct expansion (DX) system).

Good thermal contact with the ground, the elimination of a heat exchanger between the ground coil circulating fluid and the refrigerant and the fact that no circulation pump is required, means that direct circulation systems are more efficient than indirect systems.

A shorter ground coil is required and the saving on installation cost helps to offset the higher material cost, but more refrigerant will be required and there is a greater potential risk of refrigerant leaks.

DX systems are most suitable for smaller domestic applications. The majority of systems are indirect. The ground heat exchanger is buried either horizontally in a shallow trench (at a depth of about 1m) or vertically in a borehole. The choice of horizontal or vertical system depends on the land area available, local ground conditions and excavation costs.

The collector coil can also be laid under water, for instance in a pond, but system efficiencies are likely to be lower because of seasonal variations in the water temperature. As costs for trenching and drilling are generally higher than piping costs it is important to maximise the heat extraction per unit length of trench/borehole.

Horizontal collectors require relatively large areas free from hard rock or large boulders and a minimum soil depth of 1.5m. They are particularly suitable in rural areas where properties are larger and for new construction. In urban areas the installation size may be limited by the land area available.

Multiple pipes (up to six, placed either side by side or in an over/under configuration) can be laid in a single trench. The amount of trench required can also be reduced if the pipe is laid as a series of overlapping coils (sometimes referred to as a SLINKYTM), placed vertically in a narrow trench or horizontally at the bottom of a wider trench.

The trench lengths are likely to be 20% to 30% of those for a single pipe configuration but pipe lengths may be double for the same thermal performance. Vertical collectors are used where land area is limited and for larger installations. They are inserted as U-tubes into pre-drilled boreholes generally 100mm to 150mm diameter and between 15m and 120m deep.

DX systems are only suitable for shallow vertical collectors (maximum depth 30m). Vertical collectors are more expensive than horizontal ones but have high thermal efficiency and require less pipe and pumping energy. They are less likely to suffer damage after installation. Multiple boreholes maybe needed for larger residential applications.

Ground characteristics

It is important to determine the depth of soil cover, the type of soil or rock and the ground temperature. The depth of soil cover may determine the possible configuration of the ground coil. If bedrock is within 1.5m of the surface or there are large boulders it may not be possible to install a horizontal ground loop. For a vertical borehole the depth of soil will influence the cost as in general, it is more expensive and time consuming to drill through overburden than rock as the borehole has to be cased.

The temperature difference between the ground and the fluid in the ground heat exchanger drives the heat transfer so it is important to determine the ground temperature. At depths of less than 2m the ground temperature will show marked seasonal variation above and below the annual average air temperature.

As the depth increases the seasonal swing in temperature is reduced and the maximum and minimum soil temperatures begin to lag the temperature at the surface.

At a depth of about 1.5m the time-lag is approximately one month. Below 10m the ground temperature remains effectively constant at approximately the annual average air temperature (i.e. between 10°C and 14°C in the UK depending on local geology and soil conditions).

In order to determine the length of heat exchanger needed to meet a given load the thermal properties of the ground will be needed. The most important difference is between soil and rock as rocks have significantly higher values for thermal conductivity. The moisture content of the soil also has a significant effect as dry loose soil traps air and has a lower thermal conductivity than moist packed soil. Low-conductivity soil may require as much as 50% more collector loop than highly conductive soil. Water movement across a particular site will also have a significant impact on heat transfer through the ground and can result in a smaller ground heat exchanger.

A geotechnical survey can be used to reduce the uncertainty associated with the ground thermal properties. More accurate information could result in a reduction in design loop length and easier loop installation. For large schemes where multiple boreholes are required, a trial borehole and/or a thermal properties field test may be appropriate.

Design issues Sizing

The length of pipe required depends upon the building heating load, soil conditions, loop configuration, local climate and landscaping. Sizing of the ground loop is critical. The more pipe used in the ground collector loop, the greater the output of the system, but as the costs associated with the ground coil typically form 30% to 50% of the total system costs, over sizing will be uneconomic.

Conversely, undersizing, would lead to the ground loop running colder and could, at worst, result in ground temperatures not being able to recover and heat extraction from the ground being unsustainable i.e. the ground loop must be sized to meet the peak thermal power but also to deliver energy at no greater rate than the surrounding earth can collect energy over a twelve month period. If a system provides heating and cooling, energy transferred to the ground in summer can be stored and used in winter.

Assuming that other conditions remain constant, the specific thermal power that a loop can extract (usually measured in: w/metre pipe length for horizontal loops, w/m trench length for SLINKY's and w/m of borehole for vertical loops) will be dependent on the temperature difference between the circulating fluid and the 'far field' ground temperature (i.e. away from the influence of heat exchange with the collector coil).

The amount of energy that the ground loop can deliver is derived from the hours of use at particular temperature differences (and hence energy fluxes) over a given period.

Loop depth, spacing and layout

The deeper the loop the more stable the ground temperatures and the higher the collection efficiency but the installation costs will go up. Horizontal loops are usually installed at a depth of approximately 1m. Health and Safety Regulations do not allow personnel to enter unsupported trenches if they are more than 1.2m deep.

To reduce thermal interference multiple pipes laid in a single trench should be at least 0.3m apart and to avoid interference between adjacent trenches there should be a minimum distance of 3m between them.

Vertical boreholes should be at least 3m and preferably 5m apart. Careful consideration should be given to the pipe layout in order to keep the dynamic hydraulic pressure drop across the ground heat exchanger as small as possible to minimise the pumping heating energy requirement.

The efficiency of a heat pump is a function of the difference between the temperature of the source and the output temperature of the heat pump (i.e. the temperature of the distribution system).The smaller this temperature difference the higher the coefficient of performance of the heat pump will be. For example if the distribution temperature required falls from 60°C to 40°C the COP can increase by more than 40%.It is therefore important to use the lowest possible temperature distribution system.

Space heating Table 1 shows the supply temperatures required for a range of domestic heating distribution systems.

Table 1 Space heating temperatures required

Distribution System	Delivery Temperature °C
Underfloor heating	30-45
Low temperature Radiators	45-55
Conventional radiators	60-90
Air	30-50

Distribution System Delivery Temperature °C
Underfloor heating 30-45
Low temperature Radiators 45-55
Conventional radiators 60-90
Air 30-50

Heat pump systems in existing properties

Ground source heat pump systems may not be suitable for direct replacement of conventional water-based central heating systems because of the high distribution temperatures unless extensive measures are taken to improve the thermal insulation of the building.

A wet radiator system usually operates at 60°C to 80°C and a drop in circulating temperature by 20°C would require an increase in emitter surface of 30% to 40% to maintain the same heat output.

For an air system reducing the delivery temperature to 35°C would require increasing the air change rate by up to three times to maintain the same output.

For new housing where high insulation levels result in low heating demand, ecotherm low temperature water based underfloor heating systems are the best option. Ideally the system should be designed to give floor surface temperatures no higher than 26°C and should be sized using a water temperature difference of about 5°C.

Heat pump short cycling

The thermal capacity of the distribution system is important. If it is too low the heat pump may suffer from artificially long off periods at times of light load. This effect is partly due to the presence of a restart delay (designed to reduce wear on the compressor by preventing rapid on/off cycling) in the heat pump.

To avoid it, sufficient non-disconnectable thermal capacity to compensate for the loss of output during the delay restart period needs to be provided. eco hometec recommend the installation of a buffer tank. eco hometec supply a range of buffer tanks with an integral solar coil that enables the connection of solar thermal panels. Solar energy input, to the buffer, will reduce the amount of energy needed from the heat pump and will further lower running costs, energy consumption and emissions.

The required buffer capacity will depend on the system and user requirements. eco hometec buffer tanks range from 100 litres to 700 litres.

Hot water is usually required to be delivered from the tap at temperatures in the range 35°C to 45°C and for domestic installations; the thermal power output of the heat pump will be inadequate to deliver direct heating of incoming mains water to this level so a storage system is required.

Heating is usually carried out via a primary coil or jacket to a storage cylinder. For most domestic heat pumps the maximum output temperature will be 55°C and the maximum water storage temperature achievable will be 50°C. An auxiliary electric immersion heater will be required to provide a 'boost' facility, and also to raise the water temperature so that it can be stored at 60°C in order to reduce the risk of Legionella.

Because the efficiency of the heat pump falls as the output temperature rises it may be more economic to use the immersion heater to heat the stored water at temperatures above 45°C.

The stored water volume should be sized so that virtually all the energy input could be supplied during the Economy 7 (or other reduced rate) electricity tariff period.

Another option is to preheat the incoming cold water in a separate preheat tank via an indirect coil at whatever temperatures are being used to perform space heating. eco hometec recommend the use of solar thermal hot water panels to further reduce the energy used for hot water heating.

Do's & Dont's - Concept Stage

Do
Prioritise the reasons for considering a heat pump system (you can then rank the principal benefits which can be quantified during the design process). These could include:

• Capital costs
• Running costs (fuel)
• Maintenance/servicing/inspection costs
• Lifetime costs
• Available alternatives e.g. do you have gas to site
• Is the property/application suitable

Primary Energy use & Environmental impact - CO2 emissions

• Check the suitability of the local soil and geology for an effective ground loop heat exchanger
• Consider carefully if air source is a better option
• Consider using solar thermal panels to contribute to central heating & hot water heating
• Check site access for equipment to install a ground heat exchanger e.g. digger/drilling rig
• Contact the electricity distribution network operator (DNO) to find out the maximum load that can be connected to the electricity network
• Heat pump location, is the space large enough for unit and associated pipe work etc
• Heat pump operational noises, heat pump systems are designed to run up to 24hrs a day. The location of the heat pump should be carefully considered and final location should not be where running noises could cause a nuisance especially during night time

Dont

Expect initial capital costs to be lower than that for a conventional boiler.

Do's & Dont's - Design Stage

Do

Recognise that a heat pump system needs to be sized not just to meet the peak thermal power requirements but also to deliver the annual energy requirements sustainably

• Output for ground source heat pump is limited to the amount of renewable energy that can be collected from the surrounding ground
• Calculate building heat losses accurately (the accurate assessment of infiltration rate is particularly important)
• Assess monthly/annual useful energy requirements based on actual anticipated occupancy and use
• Consider providing domestic hot water (DHW) (determine usage, loads and system type)
• Consider using solar thermal panels to contribute to central heating & hot water heating
• Consider the need for space cooling (if any) and quantify
• Decide on the need for supplementary heating/cooling (if any) and quantify
• Consider the lowest temperature possible heat distribution system ecotherm underflow heating (the lower the heat pump output temperature the more
• efficient the operation of the heat pump system will be)
• Take care over the design of the ground heat exchanger i.e. pipe length, diameter, configuration etc
• Wrong ground heat exchanger pipe lengths and diameters are costly errors
• Ensure that the ground heat exchanger and the heat pump are designed to operate efficiently together
• Consider whole house ventilation and heat recovery systems as the preferred method of ventilation

Dont

• Guess or use rules of thumb for heat loss calculations
• Assume there will be sufficient space for a horizontal ground heat exchanger without calculating the length required

Please browse our website for more information about eco hometec and our range of Ground Source & Air Source Heat Pumps then click here to submit your plans or contact us on Freephone 0800 8620278 to speak to one of our team or e-mail us at sales@eco-hometec.co.uk.