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Specifier beware

17 March 2011

Retrofitting boiler controls can deliver significant energy savings, when the right technology is chosen. Tony Willis, Technical Sales Director of Sabien Technology, provides some pointersto help avoid choosing the wrong technology

With growing pressure to reduce energy consumption and CO2 emissions,there is a renewed focus on the efficiency of boiler plant. It is now commonplace to find the existing building management systems (BMS) fully optimised and additional controls, such as weather compensation and sequencing, in place to deliver further control and savings. However additional savings can be achieved by focusing on the efficiency of each individual boiler directly.

One technology which is being widelyadopted is“boiler load optimisation” via discretecontrolsretrofitted to each boiler.However, careful consideration is required before implementation.

The primary reason for installing boiler load optimisation is to prevent the inherent inefficiency of boiler “dry cycling” which isfound in virtually all boilers regardless of age, size and existing controls. This is caused by boilers firing to compensate for standing heat losses when there is no true demand for heat from the building or hot water services.

Unfortunately, attempts to prevent boiler dry cycling via retrofit controlshave resulted in technologies being implemented with flawed control strategies that conflict with existing controls (i.e. BMS, weather compensation, sequencing) and compromise ambient room and hot water temperatures.It is only recently that newly developed technology has become available which analyses the boiler thermodynamics and controls the boilers in real time, working in harmony with existing controls and maintaining design set points with no impact on comfort levels.

So, in my opinion, it’s essential to determine which product will provide the best solutionand the first thing is to scrutinise the control strategies used in theproduct.

A historical approach is to lower the boiler’s designed boiler thermostat/set pointbased on the boiler return temperatures to establish the boiler load. However, this approach is flawed on systems that have constant temperature circuits or boilers with shunt pumps.

In many of these, the controls will have a temperature sensor on the return pipeworkto prevent the boiler temperature dropping below a certain temperaturevalue. So, clearly, lowering the thermostat of a boiler will result in savings – but in today’s modern buildings this approach can cause problems with the BMS and comfort levels.

For example, the BMS or weather compensationmay be applying a variable set point to the boilers i.e. in the morning the boilers will be operating at 80oC; during the afternoon, due to solar and internal heat gains, the BMS may reduce the set point to 65oC. The purpose of this is to provide energy savings and optimise the boilers/heating system to maintain the desired space and hot water temperatures.

A control thatoverrides the boiler’s set points will conflict with the BMS and the variable set point control. This could lead to the BMS bringing on additional boilers to compensate for the decreased temperature, resulting in unnecessary energy consumption. Furthermore, ambient room and hot water temperatures will be reduced resulting in potential complaints from the occupants.

In the above example the same results could be achieved by manually adjusting the boiler thermostat to a lower required temperature, but only at the cost of not meeting the boiler plant’s requirements.

Another approach has beento use predictive (time delay) boiler firing control. The controlanalysesthe historical firing pattern of the boiler and then physically reduces the number of firings over a predetermined period of time. This approach does not use real time data or temperature profiles of the boiler load,therefore the boiler will be restricted from firing under a true heating load. Again the boiler’s required thermostat settings will be artificiallylowered, resulting in comfort levels and hot water temperaturesbeing compromised.

I would suggest that a more desirable scenario is for the boiler load optimisation to integrate with and complement the existing BMS, something that is now possible using newer, patented technologies. By measuring the flow and return temperature of each individual boiler every second and analysing the data every 10 seconds,modern technologies can accurately determine if there is a genuine boiler load from the system or if the boilers are firing as a result of dry cycling.

By identifying and preventing dry cycling average savings of 12% across an estate can be achieved over and above the existing controls. Capital cost is low, typically giving a payback of two years or less, while maintenance costs are non-existent as the technology is self-learning and requires no calibration or seasonal commissioning.

So it’s very important to determine whether the technology under consideration is simply a time delay or suppresses the boiler’s set point.I would also recommend vetting the suppliers; asking probing questions about their track record and speaking to their clients to verify their bona - fides. And, above all, I urge all building operators to avoid any solution that lacks real-time control and compromises ambient temperatures.

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