Taking full control of boiler plant will reduce energy consumption and CO2
Optimising the performance of boiler plant requires a holistic approach that takes account of every aspect of the system’s performance. Tony Willis, Technical Sales Director with Sabien Technology Ltd, explains
The result of initiatives such as the Carbon Reduction Commitment and the Energy Performance of Buildings Directive (EPBD) means that every building operator and energy manager is under pressure to reduce energy consumption and CO2 emissions – even if they have already introduced energy saving measures.
The need to control boilers to achieve optimum performance with minimum energy consumption plays a key role in meeting these demands. However, there are various techniques for controlling boilers – often in parallel with each other – and how these differ is not always so well understood. Furthermore, it is often assumed that a building management systems (BMS) or building energy management systems (BEMS) will take care of all control requirements.
In fact, while BMS and BEMS controls can be very effective in managing the on/off switching of the boilers in response to heating demands in the building they will not control every aspect of the boilers’ operation. Similarly, weather compensation controllers will introduce an additional level of control but, again, they do not eliminate all energy wastage from boiler plant.
A case in point is the common, yet infrequently addressed, problem of dry cycling, where the boilers are firing but are not actively heating the water in the system or contributing to the building’s space heating. Dealing effectively with dry cycling in low and medium hot water systems requires another level of control, using boiler load optimisation. This can be used in conjunction with other control strategies and has been shown to yield energy and carbon savings of as much as 25% over and above the savings achieved by other controls.
Surprisingly perhaps, dry cycling is a problem that is often overlooked. For example, when Norwich Union decided to take a closer look at its boiler performance dry cycling was identified as a potential source of wasted energy. Pilot trials using intelligent boiler load optimisation on three of the company’s sites reduced gas consumption and associated carbon emissions by an average of 15% across all of the boilers tested, giving an average payback of less than a year.
The causes of dry cycling, the principles of intelligent boiler load optimisation and the results of the Norwich Union pilot are explained in more detail later in this article.
What Building Management/Energy Systems does
To understand how the various control elements for boiler plant fit together it’s necessary to consider the contribution of each control strategy, starting with BMS/BEMS.
Building management systems control various and well defined strategies for heating, cooling and lighting etc. In many cases the BMS/BEMS will have a central control and a number of outstations connected to all the required elements of the building facilities. In the case of boiler house plant control, the BMS outstation may have direct control of the operation of boilers and pumps etc.
To ensure that required comfort and occupancy times are maintained with the maximum fuel efficiency, the boiler/burners will be enabled or disabled, based on bespoke strategies that are written specifically to the building operator’s requirements and boiler plant design during the commissioning process.
The boilers are normally connected to the heating system via common flow and return headers; in this case the BMS will have temperature sensors installed to measure the combined common flow or return from all of the boilers.
The boiler set point temperature will be maintained by the boiler’s own thermostat or temperature control connected directly to the boiler flow pipe work. In most cases, the BMS will turn off or on the boiler/burner based on the combined header temperature set point.
Crucially, the BMS does not typically measure the direct flow and return temperatures of each individual boiler and therefore cannot monitor the current boiler load profile.
Weather compensation, also known as weather optimisation or variable temperature control, seeks to modulate the performance of the heating system in relation to outdoor temperatures. At its simplest, such a system will measure the outdoor temperature and the flow temperature of the water serving the heating system – typically radiators. Some systems will also incorporate measurement of indoor temperatures and assess whether the indoor temperature set point is being maintained
The principle is very simple. For example, if the outdoor temperature is 0°C the flow temperature of water to the radiators may be 80°C. However, if the outdoor temperature rises to 15°C, the flow temperature to the radiators may be reduced to 55°C, from a predetermined heating curve adjustment. Typically a three-port mixing valve to blend the required flow temperatures to the heating system.
Such systems may also include night set-back to reduce temperatures while keeping the fabric of the building at a reasonable temperature. In addition, timers may be used to match space heating to occupancy patterns.
This approach can be very effective in reducing energy consumption but it does have some limitations. Many buildings have constant temperature systems such as fan convectors or air handling units where this variable temperature strategy is not applicable. Equally, the majority of buildings will have domestic hot water requirements, again requiring constant temperature water.
Consequently, many buildings will have separate heating circuits for variable temperature, constant temperature and hot water systems and the boilers serving each will need to be controlled separately.
Neither a BMS nor a weather compensation controller will address the issue of dry cycling. In fact, weather compensation can increase the frequency of dry cycling by reducing the system load of the boilers.
Consequently, addressing the issue of dry cycling requires an additional level of control through intelligent boiler load optimisation. Very often, this can be combined with a BMS and weather compensation to provide a comprehensive control strategy that will maximise savings.
When a boiler is standing idle it acts like a giant radiator, losing heat to its surroundings – even a modern, well insulated boiler will typically lose 1-2% of its heat through radiated/standing losses. In addition, draught within the flue system may draw cooler air through the boiler, potentially adding a further 2-3% to standing losses. This is further exacerbated by the need to purge the combustion chamber each time a pressure jet burner fires (cooling down something you are trying to heat up!) which creates further boiler heat losses.
Inevitably, these standing losses will cause the boiler to fire unnecessarily to overcome the temperature losses while the boiler is idle. However this firing, or dry cycling, does not contribute to the actual building’s heating demand; it is simply compensating for these ‘standing losses’ and thus wasting energy.
This situation is caused by the fact that over 80% of boilers in the UK are oversized to ‘play safe’ or to meet extreme weather conditions that are only rarely encountered. This exacerbates dry cycling by increasing the number of times the boiler will fire unnecessarily.
In the past, simple timed delay devices (which we do not advocate or recommend) have attempted to overcome ‘dry cycling’. They work by delaying firing for a pre-set time after the system calls for heat. Very often, though, that call for heat reflects a genuine requirement for heat. Consequently the delay will simply cause the boiler’s designed set point to be depressed, thus causing the boiler to fire for extended periods. The result is both an increase in energy consumption and a compromise on comfort levels in the building.
Intelligent boiler load optimisation is able to recognise and identify dry cycling by constantly monitoring the boiler’s thermal response to changing loads every 10 seconds. Two digital temperature probes measure and monitor the boiler flow and return temperatures. The onboard software calculates the temperature gradient over time and determines when the boiler should fire for true building/heating demand and when it should remove/inhibit unnecessary boiler firing and energy consumption.
Crucially any control system that inhibits the boiler from firing must be fail-safe and always ensure the boiler’s designed set point is met, therefore avoiding any additional servicing and maintenance requirements.
Boiler load optimisation in action
When Norwich Union (part of AVIVA) set itself a target of reducing gas consumption by 5% in 2008, one of the key areas the company looked at was the performance of its boiler plant.
“As part of AVIVA’s CSR policy we are committed to sustainability and one element of this is to focus on our use of resources,” explained Norwich Union’s Energy & Utilities Manager Gregory Luxford. “A number of technologies that claim to improve boiler efficiency were investigated and, following a pilot trial, Sabien’s patented M2G was selected for installation across Norwich Union’s UK estate.
The three sites for the pilot were chosen by Norwich Union as representing a cross-section of the company’s portfolio. The pilot, which was managed by Sabien, was conducted for 30 days at each site between January and March 2008. During this time gas consumption and associated carbon emissions were reduced by between 14% and 17%, with an average of 15% across the three sites. The average payback was only 50 weeks, with annual CO2 savings of 76 tonnes.
As the pilot was carried out during the colder months of the year, when boilers were operating at high capacity, Norwich Union expects to achieve even higher proportionate savings during periods of higher ambient temperature when the boilers will be subject to increased dry cycling.
During the pilot, M2G units were retrofitted to each of the pilot boilers where the flow and return water temperatures are monitored every 10 seconds. Each time a call for heat is made, the M2G automatically checks the latest data it has stored and decides whether it is a genuine demand from the building/BMS or whether the boiler is firing because of standing (heat) losses.
Crucially, M2G automatically references the thermostat set points to ensure that room and hot water temperatures are unaffected during pilot operation and that there was no compromise to the buildings’ comfort levels.
In order to create an accurate comparison, Sabien’s pilot methodology is to configure the M2G to operate in ‘Save’ mode on one day and ‘Bypass’ mode the next day. In Save mode the M2G is operational and makes savings. In Bypass mode, the M2G is bypassed and makes no savings.
Industry standard degree day calculations are then carried out to compensate for variation in daily ambient temperatures during the pilot period, so the comparison between Save days and Bypass days is accurate and meaningful.
“The pilot project delivered very impressive results and enabled us to make a strong business case for rolling out the M2G technology across a further 30 properties on the estate,” Luxford recalled. “The roll out was completed early in 2009 and we are now in the process of measuring boiler performance at all of the M2G sites to confirm the savings that have been achieved,” he continued.
Given the scale of the Norwich Union estate, Sabien’s ability to manage this £188,000 project, covering an area from the North of Scotland to the South West of England, was a key criterion. Working closely with the Norwich Union management team, a schedule was developed and communicated to building managers to arrange access and ensure there was no disruption to the staff working in the buildings.
“In a project of this nature a good working relationship is vital. We found Sabien to be very responsive and very practical in their approach and this was a major contributor to the smooth delivery of the project,” Luxford recalled.
Maximising savings with a quick payback
From our experience of fitting intelligent boiler load optimisation systems we have found that the savings achieved typically vary between 10% and 25% depending on the nature of the boilers and the use of the building(s), with payback often achieved in less than 18 months. So, while the standard levels of control such as building management systems and weather compensation have an important role to play, they are not the complete solution. It is only by taking a holistic approach that increasingly stringent energy targets for buildings will be satisfied.
As featured in Energy Performance Guide 2009