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City Academy , Bristol

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Key points concerning the design: + Good fuel economy as a result of distributed heating plant. + Underfloor heating provided for low return temperatures, allowing optimum operation of condensing boilers. + Ventilation control in classrooms based upon occupancy schedule and PIR detectors. The school has a high facade/floor area ratio, which can increase heat-loss, but usually facilitates passive design and natural cross-ventilation. All spaces within the building are heated by an underfloor system supplied by distributed plant with three 60 kW condensing boilers in villages A-D, and five 60 kW condensing boilers in village E. Each manifold is linked to a temperature sensor, which controls a bypass valve as room temperature increases. The heating circuits are compensated, supplying lower temperature hot water as external temperatures increase. The BMS system also has optimum start/stop controls, avoiding unnecessary heating when the building is unoccupied. In the zones exposed to external noise, incoming air is tempered by a low-pressure hot water heater battery whilst, for other zones, supply fans include an electrically heated element.

Acronym of the case

AdVent Case 13

Lessons learnt

The under-floor heating used throughout the building meant that a lower return temperature was achieved when operating the boilers in condensing mode throughout the year. The use of zone controllers, instead of TRVs meant that a higher degree of control could be achieved in avoiding overheating spaces. The ventilation air volumes in the building are linked to both occupancy and outside temperature, applying more control that can be achieved using natural ventilation, and defaults to of in a more effective manner than central AHUs. The zoning allowed the reduction of circulation lengths and enabled specific demands to be matched. The decentralized strategy also utilized much smaller individual boiler capacities, thus reducing the duration that boilers operate at part load.

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