42 BCD SPECIAL REPORT ON HISTORIC CHURCHES 31st ANNUAL EDITION AIR SOURCE HEAT PUMPS The pros and cons of heat pumps hit the headlines increasingly frequently and the received wisdom has been that they’re only worthwhile in modern wellinsulated buildings; can they work in a church? In fact, they could be ideal. Heat pumps use a similar technology to a household fridge but in reverse: rather than taking warm air and cooling it down they extract the heat from one source, such as the ground, a river, or the outside air and use it to warm the building. They are incredibly efficient because they transfer energy that already exists, allowing them to produce way more energy than they take to run. Heat pumps and fridges work on the principle that when a material changes phase from solid to liquid or from liquid to gas, it absorbs thermal energy: then, when it returns to the previous state it releases that energy. Just like a fridge, heat pumps contain a fluid refrigerant. In the case of an air-source heat-pump, the outside air is drawn into the unit and across the pipes where the refrigerant is in its gaseous phase, so heat is drawn out of the air into the pipes. This gas is then compressed, turning it to its liquid state and releasing the heat for use inside the building either directly, as warmed air, or indirectly through a warmed water heating system. AIR OR WATER – WHAT’S THE DIFFERENCE? The most common heat pump system in the UK is air to water. Unsurprisingly, they work by transferring the heat from the air outside to water, which is then distributed around a wet central heating system via radiators or underfloor heating, albeit at a lower temperature than traditional systems. Air to water systems can also be used to produce hot water. Air to air systems use the heat produced from the heat pump to heat air which is blown directly into rooms using fans which can be floor, wall or ceiling mounted. Some of these systems can also be used for cooling, much like an air conditioning system, but they can’t produce hot water. As the name suggests, air to air systems can’t make use of existing wet heating infrastructure. Unlike gas and oil boilers, heat pumps deliver heat at lower temperatures so they need to be operated over longer periods. This means that radiators might not feel as hot to the touch but will produce a more comfortable and consistent temperature. Generally there will not be enough radiators in a church to achieve the required temperature on a winter’s day. Various ‘hybrid’ heating systems exist which might use a fossil-fuel boiler to kick in during very cold spells or when more heat is needed over a short period. Less carbon-wasteful options include electric panel heaters, lamp-like infra-red radiant heaters, and a variety of personal heating devices, which are ideal when there is a low turn-out. These include electrically heated mats, cushions and blankets, and electric pew radiators. Running a pump with underfloor heating is particularly effective because the radiating surface is far larger than traditional radiators, so the place heats more quickly even though the water is less hot. While the disruption caused by the need to remove the pews and lift the floor can seem excessive, many churches are in the process of reordering, creating a great opportunity to install new heating. GOOD FOR CHURCHES Churches are not like homes; they only need to be heated around 10% of the time, and they don’t need to be as warm. The key point for church heating then, is the fact that air source heat pumps work differently to conventional boilers. They might distribute heat in the same way (via radiators for example) but they produce a more consistent, slightly lower temperature throughout the space, avoiding the need for bursts of higher heat for shorter periods of time, which are costly in terms of both bills and carbon. Furthermore, sudden bursts of heat also cause the surrounding fabric to expand and contract rapidly, and can cause distress in historic fabric. Air source heat pumps are not only very efficient to run but because they capture the heat that’s already in the environment, the system itself emits no carbon dioxide, which is also good news for air quality. Heat pumps typically last longer than traditional boilers and as they don’t burn fuel they are considerably safer, with no risk of gas or carbon monoxide leaks. The cost savings from heat pumps depend in part on what they are replacing – the more expensive the existing fuel, the more likely it is that a heat pump will be cheaper to run. This may not always be true in the short term while electricity prices remain high and unpredictable, but heat pumps are undeniably a futureproof choice and will become cheaper and greener as more renewable energy hits the grid. NOT ALL HOT AIR These major changes have already brought significant benefits to churches in other parts of the country. The Grade II*-listed medieval church of St Egilwin the Martyr in the Diocese of Leicester has installed a heat pump and air-to-air heating system, with air blowers in the aisles supplemented by four electric wall heaters in the chancel. This large church used to be very hard to heat but the systems now directs 50kW of heat directly at the congregation. Meanwhile St Anne’s, in the Diocese of Carlisle, was one of the first in Cumbria to install an air source heat pump with underfloor heating. The Grade II*-listed Georgian church is typically used on a Sunday and some weekday evenings, but falling congregation numbers, in part due to the cold and damp conditions, led to the decision to reorder to create a more welcoming and flexible space. The removal of the pews provided the ideal Air source heat pumps installed in the tower of St Andrews by the Wardrobe, a Wren church in the City of London which had been bombed in the Blitz. The post-war panelling now conceals new fan-coil heating units, and electric radiators provide supplementary heating. (Photo: Church of England)
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