A helping hand by Tony Willis

Industry comment

Published: 22 March

Author: Tony WIllis - Technical Director

Type: Industry comment

Year: 2017

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 maximum boiler efficiency. *Tony Willis explains, with reference to boiler plant.

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, particularly boilers, down to the finest detail. Their primary role is to optimise a building’s energy requirements rather than to optimise the individual 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 and optimising the boiler plant, by retrofitting additional controls that address specific areas of wasted energy consumption.

In such cases it is vital that any such retrofitted controls do not conflict with the existing BMS/BEMS control strategy. 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 ensuring full compatibility and harmonisation.

This principle can be illustrated by considering the operation of the boiler plant in a commercial environment, which will typically comprise a number of boilers or modular boilers working together. The usual configuration will have boiler sequencing 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 when the system loads increase. Also, there will usually be some rotation of lead and lag boilers to enhance boiler longevity.

As noted above, these boilers will typically be controlled through a BMS/BEMS that monitors the water temperatures in the building’s common flow and return circuits. Thus it is normally the 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’, ‘short circuiting’ or ‘short cycling’ (see box) without this being detected.

To look in more detail we can consider the boiler ‘dry cycling’ phenomenon. Unless measures are taken to control it, boiler dry cycling happens with the majority of boilers under part-load conditions. It occurs when a boiler loses heat to its surroundings (acting like a radiator) so that the water temperature in the boiler falls below the current set-point of the boiler’s internal thermostat / load control. When this happens, the boiler may fire simply to recover the boiler’s standing loss/temperature of the wasted heat – even though this energy is not currently being used to meet demand for heat to 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) caused by boilers short circuiting due to the lack of boiler hydraulic isolation (back end motorised valves). This heat loss occurs without making a useful contribution to the building’s demands for hot water.

Most BMS/BEMS are not typically programmed to deal with individual boiler dry cycling The BMS may attempt to address this problem with ‘demand control’, but this also has 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 – 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 dry cycling occurring during transient loads.


Harnessing modern technologies

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

An alternative, using 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. This provides a true load profile for each boiler and can therefore differentiate between a genuine call for heat and one that is simply firing due to standing losses.

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 10,000 of our M2G boiler load optimisation controls 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 BMS control strategy.