Getting to grips with
boiler dry cycling
(written for BSEE)
Optimising boiler plant performance requires attention to detail at
every level and it’s unrealistic to rely on the BMS to do everything.
Tony Willis, Technical Sales Director with Sabien Technology, explains
how boiler dry cycling is a case in point, and how newly developed
technologies can address this problem
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 – and 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
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. And 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
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
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 program 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 2 years. This is what today’s building operators
need to maximise their energy efficiency objectives.
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