Dry cycling control achieves major energy savings for IMechE

Case studies

Published: 07 April

Author: Alan O'Brien

Type: Case studies

Year: 2010

Intelligent boiler load optimisation, working in harmony with a BMS, has reduced gas consumption for heating and hot water by 17% at the headquarters of the Institution of Mechanical Engineers

Measures by the Institution of Mechanical Engineers (IMechE) to reduce energy consumption and carbon emissions have included installation of Sabien Technology’s M2G intelligent boiler load optimisers. Fitted to each boiler, the M2G units are designed to reduce wasted energy caused by dry cycling of the boilers, a phenomenon that BMSs do not typically address.

Brian Robinson, who was Head of Energy at IMechE at the time of the installation, explained: “The Institution is very focused on the energy consumption of its buildings and we reduced our carbon emissions by over 25% over a two year period. We initially asked Sabien to pilot the M2Gs and demonstrate the savings that would be achieved. Following the success of the trial, the units are now playing an important role in minimising energy consumption.

“We were very pleased with the results of the pilot and the ongoing energy and emissions savings that will be achieved for the Institution. We were also very pleased with the support we received from Sabien, which ensured that the project ran smoothly,” he added.

Fitted to each gas-fired boiler, the M2G units use a patented design that was launched in 2003 to monitor each boiler and provide very precise control of the boiler plant at individual boiler level. In this way, the dry cycling by each boiler, caused by standing losses (see below), is monitored and controlled.

Consequently, intelligent boiler load optimisation is designed to enhance and augment the performance of the BMS, to achieve greater energy savings. This is because a BMS is to designed monitor and respond to the common flow and return blended temperature from all boilers and therefore cannot identify dry by monitoring the performance of each individual boiler temperature profile under variable load conditions dry cycling can be identified and prevented.

At IMechE the performance of the BMS was optimised and the gas consumption was measured and corrected to allow for seasonal variations before the M2G units were installed. Subsequent measurements showed a reduction in gas consumption for heating and hot water of 17% - over and above the savings achieved by the BMS. This is equivalent to a reduction in carbon emissions of 25 tonnes per annum – with a payback of around 2.7 years.

Sabien’s CEO Alan O’Brien commented: “In the last five years we’ve carried out over 90 projects and consistently found that the problem of dry cycling of boilers often goes unnoticed. And it’s also clear that significant savings in energy and carbon emissions can be achieved by addressing this problem. At a time when the Carbon Reduction Commitment Energy Efficiency Scheme and other initiatives are creating an imperative to look at every aspect of energy consumption, dry cycling is something that needs to be looked at more closely.”

When a boiler is in standby, or off at set point, it acts as a large radiator and loses heat to its surroundings (a typical temperature differential, or ΔT, between a boiler and the surrounding air could be as high as 60°C). Even a modern, well-insulated boiler will experience 1-2% standing losses, with additional losses from the flue and, in some cases, from purging the combustion chamber before re-firing.

As heat is lost from the boiler (standing losses) the boiler’s temperature sensors/ temperature controls detect this and the boiler fires to compensate even when there is no requirement for space heating or hot water. Consequently, this problem is at its worst during spring and autumn when boilers tend to switch on and off or modulate, particularly when boilers are oversized (which accounts for around 80% of the UK’s boiler plant). Contrary to the beliefs of some, dry cycling also occurs with modulating boilers.

In a typical boiler room, therefore, as exemplified by IMechE, there may be one or two boilers running at full capacity to meet heating loads for most of the year, with other boilers either firing to top up the lead boilers at peak times or remaining on standby. The top-up and standby boilers, will be experiencing increased dry cycling and therefore unnecessary energy consumption. As the BMS is monitoring the blended temperatures from all boilers this dry cycling often goes unnoticed by building operators and their maintenance contractors.

In contrast, intelligent boiler load optimisation constantly monitors each boiler’s thermal response to changing loads, calculating the temperature gradient over time and determining when the boiler should fire and when firing should be inhibited. Consequently, it is fully adaptive to changing boiler load/heating demand and ensures that set point and comfort levels aren’t compromised.

Featured in Heating Ventilation Review Apr 2010