Case Study

The refurbishment of York Guildhall shows how a heat pump can be used to provide heat for historic buildings, even those from the 15th-Century, says Anastasia Mylona, Technical Director at The Chartered Institution of Building Services. The refurbishment of York Guildhall shows how a heat pump can be used to provide heat for historic buildings, even those from the 15th-Century, says Anastasia Mylona, Technical Director at The Chartered Institution of Building Services Engineers.

The use of heat pumps leant to an impressive refurbishment of York City Guildhall. The River Ouse runs adjacent to (and sometimes inside) York's 15th-Century Guildhall Building. The Guildhall is part of a historic complex of Grade I, II and II* listed buildings in the centre of the city. Its river frontage is one of the iconic views of historic York, which meant there were severe limitations on what modifications were possible in its refurbishment.

SGA Consulting's building services scheme for the refurbishment and extension of the Guildhall uses a heat pump to harness the river to provide the complex with 110kW of low carbon heating and cooling.

Refurbishing listed elements

To develop the scheme, SGA Consulting worked with architect Burrell Foley Fischer. York City Council's brief to the team was to refurbish the listed elements of the scheme to improve accessibility, occupant comfort and energy efficiency and to design a new office extension and restaurant at the rear of the complex to transform the complex into a new flexible hub for the city's digital and creative industries.

"Because the building is situated right next to the River Ouse, it seemed the obvious choice to make use of the river to provide ‘free' heating and cooling to the building," says Baert Stevens, a director of SGA Consulting. Permission to use the river was obtained from the Environment Agency and the Canal & River Trust and an unobtrusive route for the abstraction and discharge pipework was devised from the basement plantroom to the river.

The techy stuff

Heating and cooling are provided by a 2-circuit reverse-cycle heat pump, using R410A refrigerant. This enables reclaimed heat and coolth to be captured and used in the building, which helps reduce heating carbon emissions by a factor of three. To optimise its efficiency, the heat pump heating circuit runs at 50°C flow/45°Creturn, while the cooling circuit runs at 6°C/12°C return.

SGA Consulting's building services strategy was to adopt a fabric first approach. Servicing the new office extension and restaurant was relatively straightforward because these the fabric of these elements were designed to exceed Building Regulations minimum. However, the listed status of the existing elements of the complex meant that opportunities to improve fabric thermal performance were limited, which had an impact on how the heat pump derived heat could be used.

Extracting heat from the river

The heat pump takes heat from the river in winter by extracting water at 3°C and returning it at 1°C. The lower temperature of the heat pump heating circuit made it ideal for underfloor heating, where the large floor areas compensated for the lower water temperature. The heat pump heating circuit is also used to supply fan coil units, which incorporate oversized heating coils to compensate for the circuit's lower temperatures.

The heat pump also provides cooling to the fan coil units in south-facing rooms in the listed parts of the building, again using river water extracted at approximately 22°C returned at 25°C

Using the heat pump removed the need to house conventional chillers in the complex with the associated noise and challenge of concealing the air-cooled condenser externally in this historic part of York. Alongside the electric water source heat pump, the scheme also includes gas-fired boilers. These supply a conventional low pressure hot water heating circuit at 80°C f low/70°C return. This provides heat to the listed Victorian cast iron radiators in the Council Chamber as well as the domestic hot water calorifiers which provide hot water to the kitchen and toilet blocks. 

High heat losses

Heat losses in the 15th-Century Guildhall were particularly high. The building's Grade I listing meant that it was impossible to enhance the thermal performance of the walls and windows, although the team were able to add additional insulation to the roof as part of the lead roof replacement works. Bomb damage during WWII meant that the Guildhall floor had been replaced, so English Heritage permitted underfloor heating to be installed in 7m-high space. However, the hall's heat losses were so great that output from heat pump-supplied underfloor system was insufficient for the space.

SGA's Consulting's solution was to conceal trench heaters within the floor, connected to the higher-temperature gas-fired boiler circuit to supplement the underfloor system on very cold winter days. As a consequence, the trench heaters only supply 12% the Guildhall's annual heating demand. "This type of mixed use shows how heat pumps can be used to provide heating to old buildings where the rate of heat loss would be too high otherwise," says Stevens. Overall, the river-source heat pump delivers carbon savings of 8 tonnes per year.

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