Last edited 19 Feb 2019

Night-time purging

It can be necessary to provide cooling to buildings during warm weather, or where there are significant thermal gains, such as solar gain, people and equipment. This cooling is sometimes referred to as comfort cooling.

Thermal mass can be used to help cool buildings, or to reduce the need for active cooling systems such as heating, ventilation and air conditioning (HVAC) systems. Thermal mass describes the ability of a material to absorb, store and release heat energy. It can be used to store high thermal loads by absorbing heat during warm conditions, to be released when conditions are cooler. This can be beneficial both during the summer and the winter.

Night-time cooling, or night-time purging uses the thermal mass of a building to absorb heat gains during the day, then cools the mass at night using external air and discharging accumulated heat to the outside so the temperature of the thermal mass is lowered ready for the next day. In the UK this reduces internal temperature rises during the day by around 3 to 6°C.

Night-time cooling requires that the construction of the building includes significant thermal mass which is exposed both to the occupied spaces of the building and to ventilation paths (either directly, or by indirect convection). This can mean that buildings have exposed concrete floor slabs, exposed brick walls, no carpets, no suspended ceilings and so on. Thermal mass for night-time cooling is most efficient in horizontal surfaces, in particular floors, as cool ventilation air will tend to fall to the floor level. NB the exposed mass can cause acoustic issues with high internal reverberation.

Night-time cooling is particularly effective in climates with a large diurnal temperature range (an absolute minimum of 5°C), where external air temperatures are too high to provide adequate natural cooling during the day, but where night-time temperatures are low enough to ‘pre-cool’ the building ready for the next day. NB Diurnal temperature differences may be lower in urban environments than rural environments.

It is also best suited to buildings are occupied during the day, but unoccupied at night.

Night-time purging can be passive, active or hybrid:

  • Passive systems rely on passive or natural ventilation to supply fresh outside air into the building and remove warm internal air, and in so doing, remove heat from the thermal mass. Natural ventilation can be wind driven or buoyancy driven. Buoyancy-driven stack ventilation can be particularly effective as a passive mechanism for night-time purging as this is generally the time of day when the difference between the internal and external temperature is at is greatest and so the stack effect is at its strongest. Passive systems have very low operational and maintenance costs. However, they require open air pathways within the building, which can be a security or privacy issue, and natural ventilation may not be possible because of local air quality or noise issues.
  • Active systems use fan assistance to help drive air across the thermal mass, for example by ventilating floor voids. Active systems can be more targeted and controllable than natural systems, and air duct sizes can be smaller. Fan operation consumes energy, but this will tend to be less than full HVAC systems.
  • Hybrid systems may only activate fan assistance when natural ventilation is insufficient.

The thermodynamics of night-time cooling is extremely complicated and requires careful analysis.

Correct operation may require staff training and fine tuning after occupation to ensure that the process is performing as expected. Careful control is required to ensure the correct level of cooling is provided. Building management systems (BMS) can use information about external and internal conditions to determine the level of cooling required and to activate systems.

NB it is important in the first instance that heat loads are minimised, for example by providing shading to reduce solar gain.

NB Approved document F, Ventilation, defines 'purge ventilation' as, ‘…manually controlled ventilation of rooms or spaces at a relatively high rate to rapidly dilute pollutants and/or water vapour. Purge ventilation may be provided by natural means (e.g. an openable window) or be mechanical means (e.g. a fan).

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