Specifier beware

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 widely adopted is boiler load optimisation via discrete controls retrofitted to each boiler. However, careful consideration is required.

The primary reason for installing boiler load optimisation is to prevent the inherent inefficiency of boiler dry cycling which is found 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 controls have resulted in technologies being implemented with flawed control strategies that conflict with existing controls and compromise ambient room and hot water temperatures. It is only recently that new 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 solution and the first thing is to scrutinise the control strategies used in the product.

A historical approach is to lower the boiler’s designed boiler thermostat/set point based 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 pipework to prevent the boiler temperature dropping below a certain temperature value. 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.

A control that overrides 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.

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 been to use predictive (time delay) boiler firing control. The control analyses the historical firing pattern of the boiler and then physically reduces the number of firings over a predetermined period of time.

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 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 nonexistent as the technology is self-learning and requires no calibration or seasonal commissioning.

Building services engineers play a key role in securing the best solution for their organisation and their clients. I would argue that part of that role is to determine whether the technology under consideration is a time delay or suppresses the boiler’s set point. Above all, I urge all building services engineers to avoid any solution that lacks real-time control and compromises ambient temperatures.

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