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Heat pumps

This page introduces how a heat pump may be employed for heating in your home, or in church premises. It gives an overview of the different types available; and should help you assess whether your building might be suitable for a heat pump.

Heat pumps include ground source and air source heat pumps. They work like refrigerators in reverse, cooling the ground or the air while warming inside the building.

The heat thus extracted is low grade, therefore it needs to be well deployed usually at low level, and in a well-insulated space.

They need to be powered by electricity, but take much less from the grid than conventional electrical heating – therefore they can reduce a building’s carbon emissions and contribution to global warming.

Heat pumps have been employed successfully on several sites in the Diocese, most notably the parsonage of St John Wembley.

This remains a fairly new technology which must be competently specified and installed.

See also Climate Action Projects and Generic Building Solutions. For overview of energy microgeneration generally, see Generating your own energy.

Generic Building Solutions

Ground source heat pumps were among the technologies discussed in Arup’s Generic Building Solutions report.

Much of the material for this page is drawn from that report, which should be consulted for further details and analysis.

Suitability

Ground source heat pumps are beneficial for new homes, and may be worth considering for some church halls and possibly churches especially new church buildings.

Air source heat pumps may also be beneficial in some cases.

The heat that is obtained from a heat pump low grade – in the case of a ground source system, at an average flow temperature of approximately 45 degrees C.

This is cooler than a traditional central heating system. A heat pump therefore needs to be matched to a heating system capable of working to this lower water temperature.

Heat pump systems are generally compatible with underfloor heating, because of this low grade heat.

Also it is important for successful functioning of a heat pump that the building should be very well insulated.

Many if not most existing churches are not readily suited to upgraded insulation. The exception is where a roof void can be packed with insulation. However this is unlikely to be sufficient for viable running of a heat pump – too much heat would still be lost through the walls before it reaches the ceiling.

For new churches or extensions however, a heat pump may well be worth considering. Any new building should be super-insulated; they are also likely to be smaller in height and volume than many historic churches.  Existing church halls (of modern construction) and also new church halls may also potentially be suitable candidates for a heat pump.

Research has not given consistent answers about the benefits of heat pumps in domestic premises. The new Vicarage of St John Wembley has a ground source heat pump which performs very successfully. Not all case studies are equally favourable – but a heat pump remains very well worth considering for many houses, and certainly for new houses.

Ground source heat pumps

Below 2-3m depth, the ground temperature varies very little over the course of a year, and is at the approximate mean annual air temperature. As a result of these constant temperatures, the ground has the potential (via a heat pump) to provide heating during the winter, and/or cooling during the summer.

Ground-source heat pump systems are best applied in buildings that have both a heating requirement in winter and a cooling requirement in summer. This ensures that there is a net thermal balance over the year, so that the heat removed from the ground in winter is returned to the ground in the summer.

Almost all churches are heated only, therefore the economics are poor for this approach. Some other buildings eg homes may need summer cooling (even more in the future if climate change takes hold).

Where an application is being considered with a heating load only, local geology and ground water movement need to be considered. Otherwise the ground may be progressively cooled from year to year. This reduces the efficiency of the heat pump.

Also the disruption of excavating for a ground source heat pump is also significant – especially for a church. See Archaeology, below.

Types of ground source heat pump

There are two basic types of ground source heat pump:

  • Closed loop systems. These circulate water through a series of coils buried in the ground. The coils may be deep vertical bores, or shallow horizontal coils;
  • Open loop systems use the ground water through a single borehole for abstraction, and return water to ground once heat has been removed.

An open loop system needs at least 100m separation between abstraction and water return, to prevent short-circuiting of the heat – this amount of space is a luxury that is not available on many church sites.

As stated, closed loop systems are in turn of two types:

  • Horizontal coil systems. These are suited to small installations of no more than 30 kW. A large extent of open ground would be needed for horizontal heat exchangers. This is beneficial for example in new housing estates. It might also be a way of heating a church hall, say when the church car park needs to be re-surfaced anyway;
  • Vertical heat exchanger loops are also sometimes described as ‘energy piles’. There is an excellent example in the new Church Centre at St Paul’s Hammersmith.

Vertical boreholes are typically 80 to 120m deep, and spaced on a 6m grid for each loop. Each metre of installed pipe generates from 30 to 70 watts. The viability of deep piles is improved when piling is needed anyway for new construction.

Archaeology

Excavating for a ground source heat pump on a church site may disturb archaeological remains. A desk-based archaeological assessment will be a pre-requisite. The Diocesan Advisory Committee will need to consult its Archaeological Advisor. See Permissions, below.

Ground water

Open loop systems may require permission from the Environment Agency, due to heat transfer and the risk of aquifer contamination. This applies especially in proximity to any river or water course.

Any physical barrier between the system and a neighbouring water course may not make much difference – for example concrete is a good conductor of heat.

A water source heat pump (not considered in this web page) also needs EA permission.

Air source heat pumps

An air source heat pump consists of an external heat extractor, preferably located on a hidden elevation at the back of the building, within shrubbery perhaps – where it will not be too unsightly – especially for a church building.

The extractor sucks heat from the air, which is then pumped through a heat transfer medium to the internal heating circuit.

An air source heat pump is cheaper to install than a ground source heat pump, as it does not involve excavation. However the heat produced is even lower grade, making super-insulation of the building even more necessary.

The economics are most likely to be favourable on a site without mains gas – the improvement relative to electricity is greater, and there may be no benefit over gas, economically speaking, in many cases. However this is more likely to be favourable for a new building, such as a new house, or a church extension.

Supplementary heating

Some heat pump systems can be sized to take a base load of the heating demand through the year, with a gas fired boiler used as a top-up for peak demand.

This arrangement is referred to as a bivalent system.

If one third of the peak load is carried by the heat pump then this will reduce the capital cost of the system, but over the course of a heating season will contribute 60% of the delivered heat.

The supplementary heat can be made up with a gas fired boiler.

This may improve economic viability. It may work well when a new heat pump is installed in series with an existing gas boiler which still has a few years to run – and whose life may thereby be extended.

Finance

The Renewable Heat Incentive (RHI)

The government’s Renewable Heat Incentive is a critical factor in making a financial case for heat pumps, especially for a ground source heat pumps providing heating only.

The Renewable Heat Incentive is a scheme to encourage the installation of any renewable or low carbon heating systems. Another such system is biomass heating. See Climate Action Finance.

Permissions

A faculty is also needed for any installation in a church, before which the Diocesan Advisory Committee (DAC) should be consulted.

Planning permission from your Local Planning Authority (Council) is also likely to be required, if there is a material change in external appearance, and/or if the building is listed or in a conservation area.

In the case of an air-source heat pump, it is especially necessary to check with the Local Authority at feasibility stage.

There is no automatic bar to making changes which affect a church’s appearance – but it must be done well. You are likely to need to make every effort to mitigate the visual impact of any new installation, in order to gain permission.

Engineers and consultants

Professional advice should always be sought.

It is important to retain the services of consultants and installers who are not only technically proficient (this cannot be taken for granted), but also well versed and practised in all matters of renewable heat and the RHI.

In addition to an experienced engineer, you may need an architect to coordinate your project – he or she could be your Quinquennial Inspector.

Installers should be certified under the Microgeneration Certification Scheme (MCS).

More information and advice

Advice can also be sought from the Parish Property Support Team and the Head of Environmental Challenge.


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