Types of operating systems. Operating systems function largely under an automated programme but the building/ facilities manager can choose from

a number of options:

 out-source BEMS

 in-house BEMS

 a combination of the two.

The scale and complexity of the control system must be appropriate to the building and its operation, noting that staff costs are also an important factor in this.

(CIBSE-H 2000)

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2.9.2 Control of natural ventilation systems. Complete control of natural ventilation by opening windows alone is only possible in smaller buildings. For larger buildings with natural ventilation strategies, some form of mechanical

operation is essential, linked through to a BEMS control strategy. Controlled devices for natural ventilation consist principally of modulating inlet and outlet vents through the use of actuator operated dampers; in some cases low energy extract fans might be used to assist in auditorium spaces in stagnant wind conditions. CIBSE AM10 (2015) gives the following points for design consideration:

 Work through all normal operating modes – winter, summer and night time and emergency including such as smoke control and fail safe modes in the event of power loss. For example, when the building is in heating mode then occupancy sensors would control the ventilation rates in response to CO₂ levels. When the building is in cooling mode ventilation is adjusted, guided by internal temperature and sometimes external temperatures too. If the internal temperature is high in the evening and the external temperatures are lower, then night cooling is initiated – usually automatically.

 Vents should open not just on the basis of internal conditions but also when external conditions demand an override to protect against excessive wind speed.

 Occupant control should be territorial, be intuitive and in obvious positions.

 Consideration must be given to rain and wind control and security – particularly in relation to night time venting.

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2.9.3 Components of a ventilation system. These include the following:

Sensors:

 Temperature sensor – for example room or zone thermostats

 Slab temperature or internal air temperature – slab temperature is a better indicator if using thermal mass as temperature control – inserted 25 – 50mm into slab

 Humidity sensor – using for example the expansion of nylon film

 Air quality – the level of CO₂ in an internal space is most generally the accepted measure of the relationship between occupancy and ventilation.

CO₂ concentration is set at a level for initiating ventilation which is particularly useful in buildings with a varying occupant rate

 external weather sensors:

 External air temperature – mounted on north face out of direct radiation

 Rain intensity sensor – for closing or opening inlets and outlets

 Wind speed - for closing or opening inlets and outlets

 Wind direction – for selection of wind ward or lee ward vents

 Solar radiation – to increase ventilation rates if high solar gain or to operate sun screening

Actuators:

 respond to an output signal from a controller and provides the mechanical action to operate the final control device typically a valve or a damper

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Dampers:

 Used to control air flow at inlets and outlets Fans:

 required if additional power is needed to move air in cases of greater resistance (such as heat recovery or climate-induced lack of thermal buoyancy)

In naturally ventilated buildings – without the energy required for mechanical ventilation - lighting can account for 40% or more of the total electricity demand (CIBSE – H; 2000) and thus lighting design, fittings and controls are all important factors as part of a BEMS in order to maximise daylight and reduce lighting (and hence energy demand) of unoccupied spaces to a minimum.

2.9.4 BEMS user interface. CIBSE-H, (2000) recommends the following strategies for communications within a BEMS:

 management level for supervisors;

 automation level for controllers

 field level for sensors and actuators

BEMS systems are largely controlled by software apparatus and are operated by a range of users with different levels of responsibility, qualifications and

experience; therefore controls need to be set at a level which can be operated appropriately.

Adaptive building ventilation strategies for thermal comfort involve enabling a certain element of occupant control. Except in small building, it is not reasonable to

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expect occupants to take responsibility for overall running of a building and hence the need to incorporate an overall building energy management strategy.

“full occupant control is only feasible in small cellular offices where the control choices have limited effect on the rest of the building”. (CIBSE-H, 2000, p145)

CO₂ measurement is often used to assess occupancy and resultant ventilation demand with actuator controlled dampers to modulate external wind variables.

Strategies are usually divided into two parts: the occupancy period and night time ventilation strategies to cool the building. During warm weather in the occupancy period there will be no heating requirement and the BEMS is used to modulate the air inlets in order to control internal temperatures. During cold weather, when heating is required, the BEMS sets the inlets and outlets to provide for minimum ventilation to maintain air quality typically via CO₂ measurement at a set point below 1000ppm.

CIBSE note that a night cooling strategy requires temperature monitors to respond to an outdoor mean ambient temperature (typically 20°C ) and one of a daytime internal mean ambient temperature (22°C) or internal slab temperature (typically 23°C) or peak zone temperature (23°C). The BEMS will subsequently activate inlets and outlets in order for ventilation to flush the building and cool it prior to re-occupancy the following day.

2.9.5 Cost. The cost implications of the chosen ventilation system require a detailed life-cycle cost evaluation. In early design phases, a cost comparison of design features and ventilation system elements should be assessed and include:

Initial capital costs

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 The cost of building elements associated with natural ventilation, which includes operable windows, vents, chimneys, screens, louvers, etc.

These also include the cost increase associated with building components such as an atrium, if its inclusion is predominantly for natural ventilation purposes.

 Devices such as monitoring and control system

 Mechanical assistance system – if required

The capital cost of natural ventilation is generally 15% cheaper than mechanical ventilation but also note that building envelope costs may increase as a result of shading requirements to reduce passive solar gain and the need for better quality vents and damper systems. The net to gross area of the building can also be smaller because of plan depth restrictions in natural ventilation (CIBSE AM10 2015).

Operating and replacement costs include:

 Maintenance costs.

 Energy consumption if required

 Energy savings against a mechanical alternative.

 The replacement cost of those components with short lifetimes.

Utility costs typically in naturally ventilated office buildings are 40% less than that of an air conditioned building and maintenance costs can be up to a quarter of those incurred by an air conditioned building. (CIBSE AM10, 2105)

The investment in natural ventilation is both in the design coordination and

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then in the capital cost of the supporting building elements for example: stacks and louvers. Brand S. (1994) suggests that the skin or envelope of a building has a life span in excess of 20 years whereas services can be in need of replacement within 15 years. One of the important design considerations of natural ventilation is that the components that make the system work – windows, louvres, atriums, etc. - form part of the more durable elements of a building compared with the more quickly

redundant mechanical services.