Getting to grips with boiler dry cycling

The increasing pressure on building operators to increase their energy efficiency and minimise carbon emissions necessitates a holistic evaluation of every aspect of performance, down to the finest detail. At the same time, there is a tendency to assume that sophisticated control systems will take care of all of this detail –this isn’t necessarily the case and is often missed or not recognised.

Without a doubt, controls such as building management systems (BMSs) and building energy management systems (BEMSs) make a vital contribution to optimising plant usage but they simply aren’t designed to do everything – even when they’ve been correctly commissioned and are regularly maintained. So, where there is scope for additional, affordable measures working alongside and in harmony with a BMS/BEMS, it makes sense to implement them.

Dry cycling

A case in point is that of boiler dry cycling, a phenomenon that a BMS or BEMS is not typically designed to address and one that many building operators and their maintenance contractors are often unaware of. Yet anyone who wants to optimise the performance of boiler plant needs to be aware of this. Our experience from over 90 projects in the last five years shows that boiler dry cycling can waste as much as 25% of the fuel consumed by the boiler plant.

As many readers of BSEE will be aware, there have indeed been attempts to prevent boiler dry cycling in the past but, to date, all of them failed. It is only in the last few years that newly developed intelligent boiler load optimisation technologies have been applied to boilers to take effective control of dry cycling.

At this stage it’s worth reviewing the causes of boiler dry cycling to avoid any confusion with terminology. When a boiler is in standby, or off at its set point, there can be a temperature differential of as much as 60°C between the boiler and the surrounding air. Inevitably, heat moves down this thermal gradient so that, effectively, the boiler experiences standing losses as it radiates heat to its surroundings (1-2% in a well insulated boiler). In addition, there will be further heat losses from the flue system and this will be exacerbated if the combustion chamber is purged with cooler air before firing.

The result is boiler dry cycling, where the boiler’s temperature sensors/ temperature controls detect this and the boiler fires only to compensate for these standing losses, irrespective of whether there is a requirement for space heating or hot water demand.

It is also worth noting that this dry cycling occurs most often when the boilers are under lighter load conditions, or due to existing building controls. Better insulated buildings with lower heat loads can also exacerbate this situation, especially when the boilers have over-capacity to meet extreme cold weather that is only experienced on a few days a year. This is a problem that occurs with both modern and existing boilers with and without modulating and non-modulating burners.

Working with the BMS

Unsurprisingly, many people expect the BMS or BEMS to take care of dry cycling as part of its routine activities but there is a fundamental problem here. The key role of a BMS is to optimise the whole building through the building services systems it controls. This means it typically looks at each system as a whole but not the performance of the individual plant items. In the case of boilers, it monitors and responds to blended flow and return temperatures from all the boilers common header, so unless there’s only one boiler the BMS gets an overview, rather than a detailed picture for each boiler temperature profile, as the heating loads on the boilers vary.

This was the situation at the headquarters of facilities management provider Serco, where Sabien’s M2G boiler load optimiser was used to address dry cycling issues. “Many people think building energy management systems can control every aspect of a boiler’s operation but this is not the case. The M2G interfaced very smoothly with our BEMS and the two systems now complement each other to maximise energy savings” explained Alan Taylor, Serco’s Technical Manager for Government Integrated Services.

A common scenario that exemplifies the occurrence of dry cycling is when a number of boilers are sequenced by the BMS, with one running at full capacity, a second firing intermittently and a third on standby. The blended temperature of the whole system that the BMS sees does not indicate that both the second and third boilers will be experiencing standing losses and dry cycling.

The fact that the BMS doesn’t see this is also why many building operators are unaware that wasteful dry cycling is occurring – and it’s the reason why dry cycling isn’t included amongst the standard control strategies that are included in the majority of BMSs.

Of course, it is technically possible to re-configure  a BMS using both hardware and software to include dry cycling within the strategy, but this is a time-consuming and costly process that eats up expensive programmer time (we’ve heard of estimates as high as 18 months to develop these programmes). It also necessitates additional sensors and, quite possibly, upgrading of the capacity of the boiler room’s BMS outstations.

In fact, when managing agents Jones Lang LaSalle (JLL) discovered dry cycling problems at some of their customers’ premises, re-programming the BMS was their initial consideration. Mark Bottriell, JLL’s Portfolio Energy Manager for Europe, Middle East and Africa explained: “In one case the BMS is very sophisticated and it would have been technically possible to have a special programme written to control the dry cycling, but it was more time-effective to use an off-the-shelf product, which has potential to integrate fully with the existing BMS, particularly when contemplating new programming requirements.”

So if the cost of re-configuring the BMS is prohibitive, it makes sense to use a retrofit solution that is specifically designed to remove dry cycling and complement the BMS and other controls.

Here, the main problem in the past has been that attempts to control dry cycling have not been able to differentiate between a genuine call for heat or that the boiler is just firing to overcome the standing losses. A key part to this method is to ensure that the boilers designed set point temperature is always maintained without compromise to comfort levels. The patented M2G intelligent boiler load optimisation technology is able to make this differentiation. This is done by constantly monitoring the 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.

As a result, this approach is fully adaptive to changing boiler load/heating demand, while temperature set point and comfort levels aren’t compromised. Crucially, this is a technology that is designed to work with existing control systems to enhance their actions and deliver additional energy savings that are not traditionally achieved.

While the additional energy savings and CO2 reductions that can be achieved will vary with the type of building and application of the system, we have seen validated fuel savings of between 10% and 25%, with payback periods under two years. This is what today’s building operators need to maximise their energy efficiency objectives.

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