Bath Abbey’s geothermal floor

The collapsing floor of Bath Abbey, with more than 8,500 bodies buried beneath it, is being repaired, stabilised and provided with a geothermally sourced underfloor heating system.

Bath Abbey, one of the most visited churches in England (Photo: Jonathan Taylor).

Bath Abbey is well known as one of the most important buildings in the Unesco world heritage site, standing just metres from the Roman Baths and Georgian Pump Rooms. Its west front, with its ladder-of-angels relief, is a must on the tourist trail, and it is one of the most visited churches in England. As such it needs to withstand a high level of foot traffic, but the floor is not solid. More than 8,500 bodies are buried beneath it, and it is paved with 890 ledger stones. This has created longstanding structural and archaeological problems.

Between 1863 and 1874 a major restoration by Sir George Gilbert Scott included works to stabilise the floor, laying a lime-based concrete float. Scott dealt with settlement issues, but recent investigations have shown that these works took little account of the underlying archaeology, as the distribution level in which the ledger stones were sitting was a lightly compressed granulated course which contained bone fragments. This layer extends a little over a metre below the floor level. Beneath it are the burials and failing voids.

Scott had the current underfloor heating installed. This was a typical late-Victorian hot-water trench system of metal gratings releasing warm air which rises gently up to the high roof, and descends again once it has lost its heat to lightly chill the heads of the congregation in their pews.

The Abbey is an important space for the modern city. Religious use is but a part of the many activities that take place in the building. Open to tourists daily except for services and special events, it is used for concerts, organ recitals and choral events, and for hosting ceremonies such as university graduations, and music and singing sessions for children. These uses put a strain on the internal configuration of the furniture and the strength of the floor, and illustrate the ineffectiveness of the heating system.

The Abbey authorities have a plan: the Bath Abbey Footprint. Its aim is to modernise the welfare and leisure facilities; expand the service and interpretation space; update the rehearsal space for the choir; house the archives; highlight the archaeology; and upgrade the floor and the heating system.

Resolving the structural weakness of the floor will require disturbing the underfloor archaeology and human remains, but it also provides justification for introducing a geothermally sourced underfloor heating system. This would bring huge benefits to the Abbey by improving its viability as a performance venue, and reducing its heating costs and carbon footprint.

The current Abbey was started in 1499 by Bishop Oliver King, he who supposedly dreamt of angels ascending to and descending from heaven, inspiring the wonderful facade of the west front. It replaced an earlier, much larger, Norman cathedral, begun in 1099 by Bishop John of Tours. Bishop King’s monastic abbey was still unfinished by the Dissolution of the Monasteries in 1539. The city authorities refused to buy it from the crown, so it was stripped and left for ruin, until Queen Elizabeth I supported a project to raise money to repair and rebuild it as the city’s parish church in the 1570s.

The Norman foundations remain under the surrounding courtyards, where to the south also lies the Roman Great Drain and archaeology associated with the Roman temple and baths complex of the 1st century AD. A separate project from the city heritage services, the Archway Centre, will provide underground interpretation space for the Roman archaeology.

The Roman Great Drain is a key to the Footprint project. It carries waste water from the hot springs servicing the Roman Baths complex to the River Avon nearby, passing right through the Abbey’s south courtyard. This provides an opportunity for the Abbey to use a natural energy source that will reduce its energy cost and carbon footprint.

The drain has a flow rate of 14–15 litres/second of natural hot water at a constant temperature of 45°C. This is well within the 35–55°C for a typical modern underfloor heating system of the low-temperature hotwater type, producing an output temperature of around 23°C on the floor, and an inside air temperature of 20°C. Unfortunately, the thermal spring water is far too caustic to use directly in the underfloor heating system, as it contains over 42 different minerals and particularly high levels of sulphates and chlorides.

The plan is to raise the water level in a section of the drain using an internal weir. Corrosion-resistant heat exchangers in the flow of the drain will extract the heat, using water-source heat pumps to warm the closed flow loop of the underfloor heating system. Although the output is not sufficient to run the whole system, it will enable a significant reduction in the current gas bill. Tests were carried out in February 2016 to ensure that the new weir would not have any adverse effects on the hot water supply to the Roman Baths complex.

Planning permission and listed building consent for the project were obtained in 2013 from Bath and North East Somerset Council, and work began on the floor of the north aisle as a test phase. The findings and methodologies developed there informed the consolidation and underfloor heating installation for the rest of the floor.

The pews were removed from the north aisle, exposing the ledger stones for the first time in 140 years. The stones were cleaned, recorded and photographed by volunteers from the Abbey and from the National Association of Fine and Decorative Arts (NADFAS), before being lifted and removed to storage.

The next step was to consolidate the ground underneath the north aisle which had been causing the floor to collapse. The only practical way to achieve this was to fill the voids with grout, a process that was not reversible. Over 2,500 litres were poured in using gravity, not pumped, with one void alone requiring 119 litres. Underfloor heating pipes were then laid over the consolidated floor surface and covered with a screed before the stone flooring was laid.

Two-thirds of the floor in the north aisle has now been re-covered with the original ledger stones, and the rest has been laid in new stone. The new heating system is being monitored to measure its effectiveness with different floor surfaces.

The test phase included laboratory investigations by physics students from the University of Bath to determine the most suitable flooring material to deliver effective heating. The thermal mass of the replacement stone is important here; it needs to offer a good performance, as the qualities of the ledger stones vary enormously. Those in the poorest condition may not be reused in the floor, but will be retained and displayed in due course. Some of the historically important stones have been sensitively repaired using a variety of methods, again in a test phase to establish the optimal repair solutions.

As well as installing the heat exchangers and pipework in the Great Drain, the project involves replacing the boilers in the plant room, using the adjoining former coal store for the heat pumps, and an extensive array of pipework, with filters, storage tanks, and absolute segregation of hot-spring water and heating-system water supply.

The Abbey’s interior was dressed by the Scott restoration with typical Victorian low pews over the ledgerstone- dominant floor in both nave and side aisles, and more decorative tiered pews in the chancel. The floor repairs and proposals for re-ordering the interior with stackable seating in the body of the nave have met with some opposition.

In particular, the Victorian Society has opposed the permanent removal of the nave pews as the loss of a key work from Scott’s church restoration period. In its campaign to preserve the interior dressings, the society describes Scott’s nave furniture as ‘intended to “complete” the church as it would have been if the Reformation had not stopped its construction’. It describes the pews as ‘modelled on those in other 16th-century Somerset churches’.

The Bath Preservation Trust (BPT), in contrast, has come out in support of the project, considering that ‘on balance [it] believes that the conservation and communal benefits achieved by the overall project outweigh the harm’. The trust believes that the interior space as revealed by the pew removal more closely represents the Georgian experience of the Abbey and of many of those buried beneath its floor. The BPT has called for the ledger stone records and the associated history to be publicly accessible.

Until the floor stabilisation works started, the Scott pews covered a large proportion of the ledger stones, both protecting them and obscuring them from view. The survey of ledger stones by Abbey and NADFAS volunteers was extended to the whole floor in the early spring of 2017, when more than 50 volunteers captured information on the condition, inscription and position of each stone. The Abbey proposes to use the stories collated from the inscriptions as part of its interpretation plans for the future configuration of the site.

In 2016 the project was awarded a grant of £10.7 million by the Heritage Lottery Fund. This put it well on its way to the project total of £19.3 million, to complete a range of modernisation works of which the floor repairs and underfloor heating represent only a part. Fund-raising for this ground-breaking, eco-friendly underfloor-heating challenge continues.

This article originally appeared in IHBC’s Context 152, published in November 2017. It was written by Sarah Richardson and Daisy Nelson, both completed the conservation of historic buildings MSc at the University of Bath in 2016.

--Institute of Historic Building Conservation

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