On-site testing by BSRIA
To verify the CFD models, the testing specialist BSRIA was commissioned to carry out physical performance tests on the two potentially worst-performing classrooms in each school once they were completed, in January 2007. Temperatures and CO2 concentrations were found to be well within allowable levels, in line with modelled predictions. CO2 ranged from 900ppm to 1100ppm.
"What we predicted - and what we found - was that the classroom's occupants drive the circulation system. The column of warm air rising from each person draws fresh air to where it's needed," Murphy says.
"And the system is self-regulating. When a class is over and all the children have left, the buoyancy system slows down so in winter you don't come back in to a freezing room. By the same token, the greater the gain from solar heating, the greater the buoyancy driver that ventilates the room."
Applying the radically basic idea of using only windows to ventilate a room requires understanding from staff and some common sense. A simple user guide has been drafted, setting out the ventilation philosophy and giving examples of how much window to open in different kinds of weather. "Teachers need to ditch the idea that if it's cold outside, you shut the windows. They have to be open to a greater or lesser degree.
"Also, you need to leave a decent space between the window and the first desk so the child on the end doesn't feel a draught. Provided you make that allowance, cool air will drop to floor level and disperse." Any local chilling effect next to windows has been counteracted by putting radiant heating into the ceiling. "It feels like the warmth of the sun," Murphy says.
There is a danger with buoyancy ventilation that when outside air temperatures rise, the "motor loses power", he admits. The higher the temperature differential between air entering and leaving the room, the more effective the system is. "In the summer, as the outside air temperature approaches the same level as the inside air temperature, the convection current slows down and can stall. On a calm day, even with windows wide open, there may be relatively little air exchange.
"We were concerned, "Murphy says, "and modelled the number of hours where the system would fall over. Based on average annual weather patterns, it turned out to be less than one hour per year," he says. "So I don't see the value in spending huge amounts of money on fans or stacks to achieve the prescribed ventilation value for that one hour. This shouldn't be an M&E problem. It comes down to proper facade and window design."
The same approach is being applied by Mott MacDonald on current projects in Sheffield's Building Schools for the Future programme - where, once again, funds are tight.
BSRIA carried out tests at Meadowhead and Westfield schools in January 2007. In the two worst-performing classrooms at each school, BSRIA installed 32 metal cylinders, one at each desk place to represent a child, plus one at the front representing the teacher. Each cylinder housed a light bulb rated at 65W for children and 100W for the adult, giving off heat equivalent to a full class. Carbon dioxide was released from multiple points to simulate exhaled CO2.
CO2 levels were monitored in the classroom and in the corridor outside over the course of a full school day. Periodic checks were carried out and windows opened or closed to varying degrees to maintain a comfortable temperature.
Testing at each school lasted a week. The weather conditions were windy during the Westfield testing week and calm while tests were under way at Meadowhead.
BSRIA reported that there was no problem in meeting the minimum 3l/s per person ventilation requirement stipulated in Building Bulletin 101 Ventilation of School Buildings. At no time did CO2 concentrations exceed the maximum allowance of 1500ppm. Concentrations averaged 900-1100ppm throughout the week-long tests.
A note of caution was sounded by William Booth, BSRIA's head of site investigations and physical modelling, because the tests were carried out in winter. Cold outside air temperatures and windier weather promote a high air exchange rate. But feedback from Kier's facilities managers after a year of operation confirms that the ventilation strategy works in summer as well.