Christ's College Heat Recovery

A conventional “naturally ventilated” school building uses radiators or underfloor systems to provide heat to the classrooms, and opening windows to provide ventilation. Unfortunately, the heavy occupancy of school classrooms requires high levels of ventilation with the result that during the winter months energy is used to heat the classroom and literally thrown out of the window as warm stale air is swapped for cold fresh air. 

This system uses a very different strategy to avoid these energy losses while creating a comfortable internal environment. The “breathing wall” system does not require a radiator but integrates a local heat recovery unit into the perimeter of each classroom, using warm stale air to heat cold fresh air, effectively recycling the heat energy from the classroom. The heat exchanger enables the heat created by people (approximately equivalent to a 3kW heating element for 30 pupils) and electrical equipment to be retained in the room with a high level of efficiency. Each unit incorporates either a small electric heater battery to provide a boost on cold mornings.

This heating strategy is augmented by the introduction of zoned underfloor heating in large assembly spaces and other spaces that have less constant operating hours, to allow local heating control without needlessly heating the air of other connected unused spaces.

Overall, the mechanical ventilation and heat recovery system strategy considerably reduces the heat loads required by the occupation density of typical teaching accommodation, allowing significant building and running efficiencies over a more conventional approach by reducing the size of the plant and hot water system; in addition, the low velocity air supply requires less energy and the localised units allow the units to respond to specific conditions on a room by room basis. 

In summer, the system is programmed to ventilate the rooms at night without jeopardising security, cooling the exposed thermal mass of the walls and slabs.