PASSIVHAUS CASE STUDY BUSHBURY HILL PRIMARY SCHOOL
Ventilation of School Buildings that the air temperature in occupied spaces should not exceed 28°C for more than 120 occupied hours a year. In fact, modelling predicted that the maximum time any space would exceed 28°C was 35 hours a year, well within target. The ventilation solution is very much appreciated by the school’s head teacher Leigh Smith: ‘We can see the benefi t of the building’s ventilation design, the spaces are so comfortable and the pupils just love it,’ she enthuses. Like the ventilation system, the ventilation
plant is simple. A single MVHR unit consisting of a supply and extract fan and a heat-wheel, for heat recovery, supplies all the fresh air. The unit does not have a heater battery; instead the fresh air heating requirement is offset using a simple room- based, LPHW radiator system fed from a pair of condensing boilers. To meet the heat demand the school is
fi tted with conventional radiators run at low temperature, rather than expensive low surface temperature units. These warm the
Turning off the gas
As energy use is driven down elsewhere in the Passivhaus school, the power consumed in the kitchen becomes increasingly signifi cant. A major challenge at Bushbury Hill was to design a low energy catering kitchen. Even the relatively small kitchen at Bushbury is full of energy consuming, heat generating equipment, including a freezer, fridge and cooking ranges and ovens. In addition to the energy consumed by this equipment, the heat it generates can be substantial, particularly in a well- insulated kitchen.
Heat is usually removed from a kitchen by the kitchen hood. This performs a multiple role: in addition to removing heat from the cooking equipment, it removes vapour and other particulates and where gas is used for cooking. The hood also removes the products of combustion and helps keep the kitchen at a lower pressure relative to the rest of the building to stop the spread of smells.
Based on the proposed equipment, a conventional kitchen design would have required a cooker hood to extract 3,600 m3
/h from the
kitchen. It would also require the same amount of replacement air supplied to the kitchen; this would need to be heated to a minimum of 12°C in winter to ensure it does not cause discomfort to the caterers. Put simply, the energy demand for a conventional kitchen ventilation would have been 10kWh/m2
Institute’s 15kWh/m2
target for the school. At Bushbury, Passivhaus consultant Nick
www.cibsejournal.com
Grant, principal of Elemental Solutions, working with building services engineer Alan Clarke, set out to design a catering kitchen suitable for a Passivhaus school. The fi rst step in developing a low energy kitchen was to select low energy fridges and freezers to minimise their heat outputs. Next, Grant and Clarke set about minimising the heat output of the cooking equipment with an all- electric solution based on induction hobs. These only lose about 25% heat to the kitchen, whereas gas hobs lose almost 100%.
/h. In addition, the heat produced in the kitchen was used to preheat the incoming fresh air using an easy to clean run-around coil system. The supply air system also includes a heater coil from the school heating system for times when the heat output from the kitchen is too low to bring the supply air up to temperature. The solution worked well. In fact, the catering company is so impressed with the comfortable conditions in the kitchen that they are looking at using a similar system in other kitchens they operate.
The all-electric strategy enabled the total ventilation rate to be reduced by a third, to 2,400m3
/year, two thirds of the Passivhaus /year total heat demand
Alan Clarke presented a paper to the on Passivhaus School Kitchens at the 16th International Passivhaus Conference, to download a copy go to:
www.elementalsolutions.co.uk/ books-and-free-downloads/building-and-energy/
May 2013 CIBSE Journal 33
The interconnected system of grilles and louvres allows secure ventilation and enables the system to be used at night to pre-cool the building in hot weather
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