Use of night-time cooling can help to minimise the risk of non-compliance with overheating criteria. It also reduces the energy consumed for comfort cooling during the day.
Night-time cooling, sometimes known as night purge ventilation, describes the use of cool night-time air to reduce the internal temperature of a space or building and hence the temperature of the building fabric. This has the effect of slowing the warming of the space(s) during the following day and can prevent excessive temperatures being reached, without the need for additional cooling.
This has particular significance for schools, where overheating criteria are set out in the latest version of Building Bulletin 101: Ventilation for School Buildings (currently out for consultation). The main criterion states that, from May to September, the room operative temperature should not be more than 1°C higher than the maximum acceptable temperature for more than 40 hours.
The maximum acceptable temperature for typical school construction is given by:
Tmax = 0.33Trm + 21.8, where Trm = Running Mean Temperature.
This requirement in the recent update of BB101 follows the guidelines as set out in CIBSE TM52: The Limits of Thermal Comfort: Avoiding Overheating in European Buildings.
Use of night-time cooling to pre-cool a space overnight increases the time required for the internal temperature to overtake the maximum acceptable temperature by 1°. It may even avoid this temperature being reached at all. It follows that the number of hours which count towards the allowable annual limit of 40 is reduced, thus improving the chances of avoiding overheating.
The success of night-time cooling is determined by how much heat energy is removed from the building during the night-time period. It is important that the method of distribution of the night time air enables it to pass over the maximum surface area within the room. This allows the room thermal mass to make the greatest possible contribution towards night cooling.
The performance of night-time cooling is also influenced by the type of exposed thermal mass. The more concrete surface that remains exposed, the more effective its contribution will be to night cooling. Bare soffits would thus contribute more than suspended ceilings.
Mechanisms for night-time cooling
Night-time cooling works by introducing cool external air to the internal spaces of the building through the night. This may be achieved by opening windows or through passive ventilation systems. However both methods are difficult to control accurately and leaving windows open can pose a security risk.
Another option is to use a mechanical ventilation system to introduce a controlled and predictable amount of outside air to each of the spaces through the night. SAV’s AirMaster mechanical ventilation with heat recovery (MVHR) units incorporate a night-time cooling function as standard. In night-time cooling mode, the heat exchanger is mostly bypassed so that external air is introduced directly to the space. Inlet temperature is not allowed to fall below 16°C, to avoid thermal stress to the fabric or contents. Air distribution is by Coanda effect across the major portion of ceiling surface, which maximises the effectiveness of heat exchange.
AirMaster’s use of EC inverter control of fan motors ensures that this night-time cooling is achieved with minimum energy consumption. The AirMaster bypass can also be used to provide ‘free cooling’ during the day, so long as external temperatures are lower than internal temperatures.
Night cooling is set up by simple timers built in to AirMaster local control panels. During periods of heat waves, efficient night cooling helps to give classrooms a fresh feel at the start of each day.