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Fine-tuning boiler efficiency

14 November 2012

With fuel costs continuing to rise there are compelling reasons for fine-tuning the performance of boilers so that they deliver optimum efficiency. To gain maximum benefits it’s often advisable to introduce additional levels of control to what’s already there.

Many facilities managers and energy managers will have heard of ‘boiler dry cycling’. They may also be aware that it can occur in the vast majority of commercial boilers and, if not controlled, will lead to unnecessary energy consumption. In fact, our experience shows that taking control of boiler dry cycling can reduce energy consumption by 10-25%, depending on the site and application.

So clearly tackling boiler dry cycling is well worth it. But how to go about it? The common assumption is that the existing controls – BMS, boiler sequencing, weather compensation – will take care of it. But in most cases they don’t!

Indeed, typically the existing controls are not even designed to detect and control boiler dry cycling. They have a different role to play and, if properly configured and maintained, make a valuable contribution to energy efficiency. But to maximise the opportunities for saving energy, standard controls generally need a bit of help.

Additional controls that have been proven to prevent boiler dry cycling in thousands of boilers, work in harmony with other controls to maintain comfort levels, require a relatively low capital investment and deliver a fast payback are the obvious solution.

What the BMS does

One of things that a BMS can do very well is to ensure that hot water is delivered to heating systems at a temperature that ensures comfort conditions are maintained in the building. In a commercial boiler room with two or more boilers it will typically do this by monitoring the blended water temperatures from all of the boilers and responding accordingly. It generally won’t be monitoring the boilers individually.

This is where wasted energy occurs and goes unnoticed. Once the heating system is satisfied it will usually only require one of the boilers (the ‘lead’ boiler) to maintain temperatures, so the others (the ‘lag’ boilers) switch off. As these lag boilers cool down the temperature of the water in the boiler may fall below the setting on the boiler’s thermostat. The result is the boiler will then fire to heat up the water in the boiler – wasting energy because this heat isn’t being used to heat the building. This is boiler dry cycling. It happens within individual boilers so a BMS does not typically detect it.

Demand control

Sometimes a BMS will try to control boiler dry cycling through ‘demand control’, which enables the boilers to fire until the system’s demands e.g. domestic hot water, radiators, and air handling units etc. have all been satisfied. Unfortunately, such loads are not typically satisfied simultaneously for any length of time and there is a strong likelihood of boiler dry cycling during transient loads.

So while a fully functional, correctly configured and regularly maintained BMS can deliver good energy savings there is still scope for retrofitting additional controls that address specific areas of wasted energy consumption.

Tacking boiler dry cycling

These issues have been recognised for many years but early attempts to tackle boiler dry cycling (some of which have re-emerged) were, and continue to be, wholly unsatisfactory. Using time rather than temperature as the basis of their control they increase the normal time delay between firing cycles and artificially lower boiler set points. Savings are achieved but at the cost of conflicting with other controls and making workplaces colder - which can jeopardise the FM’s KPIs for heating requirements within the building.

Sabien’s patented M2G boiler load optimisation technology works in a different way. It analyses each boiler’s flow and return temperatures every 10 seconds, and measures the decay of the flow and return every second. This provides a true load profile of each individual boiler and differentiates between a genuine call for heat and one that is simply firing to replace the heat caused by boiler standing losses.

Just as importantly, it recalculates the values every time the boiler reaches its required set point temperature, so it adapts to BMS/optimiser variable set-points and does nothing to conflict with other existing controls such as weather compensation, demand control or sequencing. The boilers’ designed set points are not altered.

The result is that the M2G adds to and fine tunes what the other controls are already achieving, so that the energy savings are extra to those achieved by the BMS, optimiser, weather compensation etc.

Sabien’s M2G has been evaluated and deployed by a number of FM service providers and end users. These include Interserve, Vinci Facilities, EDF Energy, Schneider Electric, Babcock, Carillion, John Laing, G4S, Serco, Jones Lang LaSalle, BT, Aviva, Royal Mail, central government departments such as Defra, and CLG, along with many local authorities and universities.

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