A helping hand

17 May 2012

While a BMS may provide a high level of overall control of a building’s energy performance, there are times when it may need a ‘helping hand’ to achieve additional available boiler efficiency, says Sabien’s Tony Willis

For many years now it has been standard procedure to control a building’s services through a building management system (BMS) or building energy management system (BEMS). And when they are commissioned effectively, and correctly maintained, a BMS or BEMS will help to maintain the overall building efficiency and prevent energy being wasted.

Consequently, it is quite natural to assume that a building with an existing BMS/BEMS is operating to maximum efficiency in all areas and there is no need for additional ‘fine’ control. However, the majority of such systems are not typically programmed to control each item of plant down to the finest detail.

Their primary role is to optimise a building’s energy requirements rather than to optimise the specific plant components that make up a building’s energy-consuming services. So there can be times when a building’s energy performance can benefit from enhancing the operation of the boiler plant- for example, perhaps by retrofitting additional controls like Sabien’s M2G boiler load optimisers that address specific missed areas of boiler energy consumption.

In such cases it is vital that any such retrofitted controls do not conflict with the BMS/BEMS or any other heating / boiler controls. Their role is to add to what is already being achieved, building on the work of the core control system. In fact, this is a key consideration when evaluating additional controls and ensuring full compatibility and harmonisation.

Consider the operation of the boiler plant in a commercial environment. This can typically comprise a number of boilers or modular boilers working together. The usual configuration will have boiler sequencing employed where a lead boiler meets the base loads for heating and hot water, with one or more additional boilers (lag boilers) being brought into operation only when thesystem loads increase.

Also, there will usually be some rotation of lead and lag boilers to enhance boiler longevity and serviceability.

As noted above, these boilers can typically be controlled through a BMS/BEMS that monitors the water temperatures in the common header/s flow and return circuits. Thus, this would normally be a blended temperatures from all of the boilers that the BMS/BEMS is measuring and responding to. Under most circumstances it will not be monitoring the load profiles of each individual boiler under variable load conditions.

This is where problems can occur as individual boilers may be wasting energy through boiler ‘dry cycling’ without this being typically detected by the BMS/BEMS.

Unless retrospective measures are taken to control it, boiler dry cycling happens with the majority of boilers. It occurs when a boiler loses heat to its surroundings (standing losses) so that the water temperature in the boiler can fall below the current set-point of the boiler’s internal thermostat / load control. When this happens, the boiler/s may fire simply to recover the boiler’s standing losses, even though this energy is not being used to meet the current system demand for heat from the building.

For instance, the lead boiler may be meeting the building’s demands for 80% of the time and, due to transient loads, the lag boilers can continue to fire sporadically simply to replace the heat they are losing (standing losses) if not automatically hydraulically isolated. This heat loss occurs without making a useful contribution to the building’s heating or hot water systems demands.

Most BMS/BEMS are not typically programmed to deal with individual boiler dry cycling at this level. The BMS may attempt to address this problem with ‘demand control’, but this also has some inherent limitations.

Demand control will enable the boilers to fire until all of the system’s demands – e.g. domestic hot water, radiators, and air handling units etc. – are all satisfied. Unfortunately, in a typical building such loads are not typically satisfied simultaneously for any length of time. For the rest of the time there is a strong likelihood of boiler dry cycling occurring during transient loads periods.

Harnessing modern technologies

Dry cycling has been a industry-recognised problem for many years and early attempts at retrofit solutions were an abject failure (sadly, some have recently reappeared, in an attempt to exploit the ‘green pound’). These products can either delay the boilers’ firing under true load, or artificially reduce the boiler set points, both of which allow temperatures to fall in the spaces being heated. In addition, they can cause direct conflicts with the BMS strategy by artificially changing the boilers’ common set point temperatures.

An alternative, using Sabien’s patented M2G boiler load optimisation technology developed in the last few years, is to carry out real-time analysis of each boiler’s flow and return temperatures every 10 seconds and measure the decay of the flow and return every second (direct boiler load profile). This provides a true boiler load profile for each boiler and can therefore differentiate between a genuine call for heat and one that is simply firing due to overcome the standing loss.

The advantage of this approach is that it recalculates the values every time the boiler reaches its required set point temperature, so it adapts to variable set-points and does nothing to conflict with the BMS/BEMS operation or other controls such as weather compensation or sequencing. It also helps to mitigate the energy wastage caused by short-circuiting and short-cycling.

In this way, retrofitted boiler load optimisation is an example of how the right additional controls can work in harmony with other controls and increase the energy savings that can be achieved. Indeed, over 5,000 of our M2G units have now been installed, the majority working in conjunction with a BMS/BEMS to deliver energy savings over and above those provided by the BMS/BEMS.

So if you are considering retrofitting additional controls, the key is to understand how they work (don’t be fooled by manufacturers’ jargon), talk to others that have used them and be absolutely certain they won’t compromise comfort or your existing control strategy.

Sabien’s M2G boiler optimisation controls have been independently evaluated by organisations such as Serco, Jones Lang LaSalle, BT, Aviva, Royal Mail, central government departments such as Defra, and CLG, along with many local authorities and universities.

Tony Willis is Technical Sales Director with Sabien Technology