On new build and major refurbishment projects it is quite normal for the BMS specification to change during the course of the contract. While many alterations are small and have negligible financial implications,others are very significant and can add 25 percent or more to the total system cost.

Changes often become necessary because the BMS specification does not state who is to supply the building services plant that the system will control and monitor.Though the make of very large items like chillers will probably be named,this is rarely true of most other plant.Since different makes of plant can have different control/monitoring requirements,what has been allowed for in the system supplier’s design may well have to be altered.

The impact on the controls will naturally be greater if the actual number of plant items is changed.For instance,the mechanical contractor may conclude that putting in two air handlers represents a more economic alternative than the single unit originally specified,as it reduces the amount of ductwork that has to be installed.Or,access restrictions might mean that boiler size becomes a problem and the only solution is to fit four rather than three units.Such decisions mean that the system strategy has to be adapted and more control and monitoring points added.

The most significant variations are generally the result of a building user’s needs changing - or simply not being clear.

Considerable time can elapse between a project’s conception and building work getting underway.During this period the client’s business may evolve,with the result,say,that the building requires more IT rooms – and consequently more air conditioning - than first envisaged.If it was drawn up some time ago,the controls specification may also not take account of the new and ever more complex regulations relating to building services and their energy performance.

If the client’s requirements are not even known – as is usually the case for speculative office buildings – the effect on the BMS’s design can be even greater.The shell-and-core services and controls will normally be designed with no knowledge of who the tenants will be and how the building will be divided up.This information may only become known near the end of the contract,with the BMS already partly installed.

Though it will not solve the problem of late client changes,a full and detailed controls specification will help to keep down other variation costs.This might seem blindingly obvious,but key BMS functions do get left out.Also,owing to the increasingly fast-track nature of many projects,the specifications that suppliers receive are sometimes very sketchy.Even when there is pressure to deliver quickly,cutting corners on this crucial task should always be avoided.Time spent developing a spec is never time wasted.

Fortunately,building management systems are considerably more flexible than they once were.The universal adoption of networkable, freely programmable BMS controllers,which are reengineered using graphical software tools,allows variations to be more easily accommodated.Other,more recent developments are helping to further simplify the re-design process and it is important that those who specify systems are fully aware of the options now available to them.

Sensors and supervisors

Radio-based temperature sensors are one such option.Though the technology is not new it is only very recently that wireless sensors have become economically viable for widespread use in HVAC control applications.Previously they have really only been found in places such as museums and historic buildings,where visible wiring is not acceptable.Now that they can cost the same or even less than the conventional wired alternative,they could be specified for a wide variety of premises.There is an added incentive to do so if they also offer maintenance free operation,which the latest,solar-powered models do.

Because wireless sensors can be easily re-located,their use can be very advantageous when late system changes are made.For example,a building user may decide that tighter space temperature control is necessary in areas designated as critical environments,which means that additional sensors have to be fitted.If ceilings are already up and wall surfaces finished the installation of wired sensors could well prove costly,and cause considerable upheaval.This would not be so with wireless devices.

Sensors may also need to be relocated when equipment is moved into the building.It is not unusual for them to end up with photocopiers and vending machines positioned very close by,which is clearly not conducive to accurate control.

Another decision that is frequently left until very near the end of a contract concerns the number and location of system supervisors (PC-based graphical operator interfaces).Often the building user may just opt for one,for use by maintenance or engineering staff.

However,it is increasingly common to give BMS access – albeit in a limited form – to a wider group of people,including financial and FM personnel and plant maintenance contractors.In a multitenanted office it may be desirable for all tenants to be able to manage and monitor their part of the system.

Traditionally,a separate(and expensive)supervisory software package had to be supplied for each individual user. However,with the advent of systems offering Ethernet/TCP/IP connectivity and supervisors that are able to display data on HTML pages,it has become possible for authorised users to gain access from anywhere using just an ordinary web browser such as Internet Explorer.This could be from a PC within the building or remotely,via say a laptop or PDA.

As a result of more recent developments,it is now feasible on many systems to do without proprietary supervisor software all together. Control settings and monitored data can be accessed by browser from web servers that are actually embedded within the controllers. Importantly the number of users has little or no affect on the system’s cost and consequently there is no reason for it to be specified in advance.

Modular control

The distributed intelligent controllers that are at the heart of every modern BMS have a particularly strong influence on system flexibility.These have become more and more modular in design,which allows systems to be sized to match a wide range of applications and easily expanded should their role be developed or some other change occur – such as the addition of a new piece of plant.

Modularity also facilitates highly distributed control, where each piece of plant has its own local,small controller.The obvious advantage of this arrangement is that it greatly reduces the amount of field wiring.Moreover,if the plant has to be physically repositioned, the associated wiring costs are minimal.

The soft configuration of intelligent controllers enables their firmware to be adapted to suit any change in control strategy.Using today’s advanced graphical tools a controls specialist can carry out such modifications very quickly.

However,when it comes to their input/output configuration, controllers have tended to be much less flexible.

Cylon has addressed this problem through its development of UniPuts, which can serve as inputs or outputs,either analogue or digital.A single controller channel can thus perform any i/o function;for instance,it can be configured to switch on a pump or control a valve,or accept a temperature sensor measurement or an alarm.It takes just seconds to configure(or re-configure)an individual UniPut to operate in the desired mode.

It is common practice for consultants to specify that the BMS controllers should have 10 percent or more spare i/o capacity to allow for any changes – both before and after hand over.

However,if this is fixed i/o,it is very unlikely to match unpredictable future needs.A design that is fully flexible will reduce or eliminate the requirement to fit further i/o modules or invest in additional controllers.

BMS technology has now advanced to a stage where flexibility is achievable at everym level of a system – from its architecture and controllers to its management interfaces and peripherals.By making the right choices,system designers are thus able to avoid or minimise potentially large variation costs.

Just as importantly,they are able to create systems that can be readily adapted and extended long after installation has been completed.

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