15 June 2005
Staff complaints and excessive energy bills are the outcome from poorly operated ‘mixed mode’ buildings. As Andrew Martin explains, they can offer comfortable conditions using less energy than fully air-conditioned buildings but they do require active management to achieve this
AS CORPORATE SOCIAL RESPONSIBILITY issues grow in importance FMs are in the front line to demonstrate that their buildings are operating correctly particularly to keep employees comfortable whilst ensuring that energy consumption is minimised. This will become all the more important when requirements for building energy certificates in public buildings are introduced in the revised Building Regulations expected at the end of this year.
Mixed mode buildings can give rise to particular challenges for facilities staff. They include combinations of natural and mechanical ventilation, and may also incorporate full air conditioning with mechanical cooling and dehumidification. Natural ventilation elements comes from windows that are opened manually or automatically at night during hot periods to provide cooling of the internal building fabric and fittings. This provides a heat sink to reduce the internal temperature during the following day. These buildings can significantly reduce energy use when operating correctly but it is also easy to allow them to deteriorate into a state of poor control and higher energy consumption.
The combination of both mechanical and natural ventilation in a building allows occupants control of their own ventilation whilst ensuring that internal comfort is not compromised, for example, due to poor ventilation levels in winter or high temperatures in summer (where air conditioning is provided in conjunction with natural ventilation). The approach is generally to let the passive systems control the base conditions particularly in mid season, and use mechanical systems to meet peak conditions. Variations in comfort level are acceptable to occupants with appropriate local controls such as blinds, windows, lights, heating and so on, and this allows more scope for more adventurous, less energy consuming ventilation
and cooling plant designs.
When moving into a mixed mode buildings it is important to ensure that they are fully commissioned and subsequently fine tuned to cover the variety of external conditions that occur over the course of a year. Automatic controls should operate such that there are no significant areas of overheating, no untoward draughts, minimum perceived air noise and reasonable metabolic CO2 concentrations.
It is also important to provide training for the FM and the occupants. This can range from a short written description regarding design intent and operating instructions, to a more comprehensive briefing for the FM. Everyone should understand the need for adequate air flow paths into the building, arranging partitioning to encourage air flow and minimising the effect curtains and blinds. The occupants should also be encouraged to open windows before the space overheats. Problems can arise because no-one is given responsibility for this task or because of the issues that opening a window serving an open plan office area can cause!.
Further problems can arise when the external temperature remains above the internal temperature. Do you try and maintain the lower but increasingly uncomfortable temperature inside, or do you increase ventilation by opening windows to maximise air flow but at the risk of increasing the internal temperature further? Generally, if keeping the windows closed does not aggravate the situation any more than having them open, then they are probably best left shut for as long as possible, with the expectation that they will be opened once the internal temperature rises above say 25°C. This particularly applies where the external temperature rapidly rises during the course of the morning. At this point it is probably more beneficial to maximise air flow over occupants (which provides some cooling effect) even if the outside air is significantly hotter. Obviously this does not apply where mechanical cooling systems are operating, in which case vents should typically remain shut (subject to maintaining sufficient outside air for the occupants).
Operating issues Vents (and doors) must close adequately and not allow uncontrolled air infiltration, particularly where they have actuators fitted under automatic control. Uncontrolled air infiltration will add to heating requirements in winter, and cooling requirements in summer. Consider installing air curtains or double sets of doors over entrances as well as minimising of the number of entrances to prevent rapid inward / outdoor air flows into building due to pressure differences between different areas.
Weather interlocks fitted as part of automatic control system are often a source of problems in mixed mode buildings. These include rain sensors (to close roof vents), solar sensors (to open vents) and wind speed sensors (to close vents in high winds). The operating parameters should be checked to ensure that they have not been overcautiously set as this will reduce performance of the natural ventilation system typically leading to energy consuming mechanical systems operating unnecessarily. Rain sensors in particular seem to be the source of operational problems: they either fail (safe) leaving vents permanently shut and thus potentially compromising comfort levels, or they are too sensitive, closing vents at the first sign of
moisture in the air and starting mechanical ventilation; or they take hours to dry out after a passing rain cloud has initiated closure of the vents. Rain intensity sensors are better, rather than capacitance type devices that activate as soon as wet and then take time to dry.
Solar sensors are sometimes used to detect heat gain and thus open vents before overheating becomes a problem, typically in glazed atria. They are also used in lighting control systems typically to control outside lighting on and off. These sensors rapidly get dirty and require regular cleaning to work effectively.
The use of internal lighting when there is ample natural lighting is a common sight in buildings. Not only does this waste energy but the heat gain from lights contributes to the overheating of the space in warm periods, which in turn adds to the energy consumption of any mechanical cooling systems. A further variation on this is when blinds are left down and the lights remain on, although the blinds may be required to reduce solar gain in summer which is likely to outweigh the heat given off by the lights on the zone affected.
Another aspect to consider is the availability of space heating systems during the summer. There should be an interlock to ensure that space heating does not activate following night cooling. It is surprising in how many buildings that the controls operate the heating in summer to ensure that 21°C is achieved by the start of occupation despite having cooled the space overnight!
DEFRA Building, Bristol
The new 5 storey DEFRA building was designed with an energy target of 120kWm2/year in line with the ECON 19 good practice benchmark for a naturally ventilated building. The building services design by WSP embraced mixed mode ventilation as well as maximum use of natural light thus reducing the reliance on mechanical cooling. It is naturally ventilated using a combination of automatic and manually controlled windows which use the atrium as an air extract path to exhaust air via roof louvres. Night cooling is provided by operating automatically controlled vents at night which allows the cool night air to flow over the exposed concrete ceilings slabs to provide a heat sink the following day. Mechanical ventilation and cooling is available via an underfloor air distribution system in hot weather, and all year round to ‘business zones’ where photocopiers and other heat producing equipment are housed. The building has been operating for four years and generally it has performed as expected. The BREEAM rating achieved for the building was ’excellent’ and this has manifested itself in the low running costs.
... Andrew Martin is Asset Services Manager at WSP Knowledge Solutions, part of WSP Group plc. Acknowledgement is given to the fact that some of the work described above was undertaken whilst the author was previously employed at BSRIA