Bath Abbey: Moving Heaven and Earth

Installers are used to problem solving, particularly when working in older buildings, but navigating 8,000 burials to install underfloor heating isn’t usually on the list of concerns.

For the contractors working on a multi-million pound project to bring Bath Abbey up to date, though, the presence of so many ancient graves on the site was the driving factor that allowed this innovative scheme to come to fruition. As bodies decomposed and burials collapsed, voids had formed under the floor which meant that remedial work was essential, and the Abbey team saw an opportunity to bring the complex up to date for its 600-strong congregation and thousands of annual visitors.

The Footprint project was conceived: as well as excavating and repairing the floor, the Abbey would have a new audio-visual system and flexible seating; a song school for its renowned choir; a revamped museum; a modern education and visitor centre, complete with the site’s first public toilets; and a low-carbon heating system that would harness the heat from the city’s famous Roman baths (situated right next to the Abbey) and use it to warm the space.

The ambitious scheme was first discussed 15 years ago, but physical work didn’t begin until 2018. Project director Nathan Ward came on board in March 2020 as work to install the final pieces of the heating system began. His previous experience working with the National Trust meant that he appreciated the scale of the challenge at the Abbey.

“I have fitted sustainable heating systems in many historic properties at the National Trust,” he says. “It is workable, and should be done where possible, but generally not without there being huge compromises.”

Nevertheless, engineering firm Buro Happold had demonstrated that it would be an effective solution in the Abbey. Like many historic churches in the UK, it had been warmed with Victorian trench heating. The temperature was uneven, with hot air rising from the trenches and cold draughts being pulled down from the huge glazed areas. Low-temperature heating under the floor could be used to maintain a comfortable temperature in the first seven feet above the floor, with upgraded trench heating acting as a top-up from repositioned trenches, moved to minimise convective currents in the space.

The scheme makes use of the waters flowing underground past the Abbey. Image: Buro Happold

Drain of resources

With the Abbey’s team keen to harness the natural resource of the hot spring waters flowing right past the building, Buro Happold looked for the right means of capturing that energy and began discussions with heat pump specialist Isoenergy.

The firm proposed a number of solutions, including EnergyBlades placed in the 2,000-year-old Roman drain that carried the spa water from the Roman Baths out to the River Avon.

“The EnergyBlades are a closed-loop solution, so we circulate fluid through the blade,” says Edward Levien, commercial director at Isoenergy. “The other options we explored were open loop, so sucking the water out of the drain and feeding it through the heat pump itself. The problem with that is the water quality isn’t great, it’s got very high levels of chloride and other things which tend to block filters and create a maintenance nightmare.”

The work went out to tender and, having won the contract, Isoenergy began by installing pipework in the plant room. “Buro Happold did a lot of the heavy lifting in terms of scoping it all out,” says Levien. “The route connecting the pipework from the great drain all the way through Kingston Buildings [next to the Abbey] and into the plant room was probably the biggest head scratcher, and that part of the pipework was done quite early on so that we could get it out of the way before most of the work on the building started.”

Then began the difficult task of excavating the Abbey’s floor and some of the 8,000 burials underneath it – many of which were carefully removed to the Abbey cemetery a couple of miles away. The 2,000 stones across the floor, including 891 ledger stones, had to be removed and catalogued, to be replaced afterwards, and the team had to pick through the legacy of bullish remodelling work carried out by the Victorians (“They were a law unto themselves”, according to Ward). Unsurprisingly, it was this stage that threw up some of the nastiest surprises.

The hole story

“In the south transept, there was a void underneath a four-tonne marble monument that just kept going,” explains Ward by way of example. “We had to stop, design a steel framework to hold it in place and then excavate the floor and build it back up. That takes months.”

With the archaeological part of the work complete, the task of rebuilding the floor and installing the heating could begin. The pipes under the Abbey floor were installed by underfloor heating specialist firm UFHN. A bespoke clip-to-mesh system was used to prevent damage to anything of archeological value, with manifolds set into the heating trenches for easy access and maintenance.

The final piece of the jigsaw was installing the EnergyBlades in the Roman drain. The only access to the drain was seven metres below street level, through a manhole outside a busy ice cream shop. The confined environment, heat and humidity meant that the contractors needed special training for the job and could only work in blocks of 20 minutes at a time.

The EnergyBlades are usually an off-the-shelf product, installed in sets of four in rivers, lakes or culverts above ground, but they had to be adapted for use in the Great Drain. “We went down to the drain before the project went out to tender to help define the project and make sure it was possible to get everything in and out of the manhole,” says Levien. “So the EnergyBlades installed were custom made to fit through that cover and then in the physical dimensions of the drain itself. We had to use a higher grade of stainless steel for the drain due to the harsh conditions. That was the most challenging part of the project.”

The Great Drain dimensions and plans for the weir (Image: Buro Happold)

In March, the system was ready to be tested. “We had people coming in coats and scarves and then it was like walking into a sauna, because we’d ramped the heating up to max,” says Ward.

That’s not the end of the story – due to the truly unique nature of the system, it remains to be seen how much maintenance work will be required. “The water is highly corrosive in the drain, and we don’t know what impact that’s going to have on the blades, so that will be going on for years,” says Ward.

“The Roman Baths do a flush of their Great Bath four or five times a year, so they flush out all the silt and minerals down the drain, which then causes a problem for us. We don’t yet know whether we’ll have to maintain the EnergyBlades every six months, every two months, or every year.”

Despite the challenges of the project, it’s drawn attention from other churches and historic sites around the world. “The Church of England has a large programme to decarbonise its estate,” says Levien. “More generally we’ve had enquiries since this has come online from other spa towns that have thermal springs – there aren’t many in the UK, but there are globally. People are beginning to see that they have something coming out of the ground which is free and renewable. If they can use it to heat a building, it’s not just a tourist asset, it’s something more.”

On a mission

The Church of England has recognised the climate emergency and set out the aim of making its estate carbon neutral by 2030. It’s not just a matter of protecting the planet for future generations – the Church believes taking action against climate change is part of its Christian duty, and quotes its fifth mark of mission: “To strive to safeguard the integrity of creation and sustain and renew the life of the earth”.

The Church owns and operates around 40,000 buildings including 42 cathedrals, and many of these are listed or historic sites that don’t readily lend themselves to low-carbon heating solutions. It has formed a partnership with the Durham Energy Institute (DEI) to run an energy audit of its sites and then to develop a plan for decarbonisation.

Who’s who

Engineers: Buro Happold

Sustainable energy specialists: Isoenergy

Main mechanical and electrical contractor: Wheelers of Westbury

Underfloor heating pipework: UFHN

Building management system: Kinetic

Building and conservation contractor: Emery

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