Sustainability
Tackling boiler dry cycling is key to energy saving
Building services maintenance engineers are often expected to help their clients or company to reduce energy consumption. To do so, they need to ensure that all control aspects have been addressed, says Geoff Newman
operators constant- ly strive to improve their sustain- ability credentials – as well as reduce energy costs – they are increasingly turning to their specialist suppliers for assistance. In this respect, building services engineers have a key role to play and many are now becoming pro- actively involved in the energy management aspects of the plant they maintain. Some are even including energy savings in their contract targets, with financial penalties if they fail to deliver. For all of these reasons it is
It is likely the inherent problem of boiler dry cycling is occurring and is going unnoticed
important that a building’s energy use is optimised, addressing the operation of not just whole systems but also individual items of plant. However, it’s important to note that there will always be vary- ing levels of optimisation. Even in buildings where extensive optimi- sation has been delivered there are often further opportunities to reduce energy consumption. This point can be clearly illustrated by considering the situ- ation with boiler plant. It’s not uncommon for commercial boiler plant to have a number of control elements in place to optimise the boilers’ and heating systems’ performance. These may include boiler sequencing, weather compensation, demand control, modulating burners and even ensuring the correct air to fuel ratio is being achieved.
Many of these are delivered through routine maintenance and commissioning, standalone con- trols or via a building management
36 | March 2014 | HVR
system (BMS). Each will be delivering significant cost savings and reduced carbon emissions. However, it is likely the inherent problem of boiler dry cycling is occurring and going unnoticed, resulting in unnecessary energy costs – even with these sophisti- cated controls in place.
Tightening control
Boiler dry cycling is a symptom of standing losses (heat lost via the boiler casing and flue). Once the heating system is satisfied the boil- er(s) will turn off. Naturally the boiler cools down; as the tempera- ture drops below the setting on the boiler’s thermostat, the boiler will fire to recover its standing losses, yet the building requires no heat. This is boiler dry cycling and it will be a continuous occurrence throughout the period the boiler is operating, potentially wasting significant amounts of energy. Importantly, even the latest boilers with modulating burners,
sequencing and BMS control are likely to be suffering from boiler dry cycling. Typically the existing controls, including BMS, are con- figured to control the heating sys- tem as a whole from the common header (the blended temperature of all boilers), rather than monitor- ing and controlling each individ- ual boiler. It is impossible to iden- tify which boiler is dry cycling from the common header so dry cycling often goes unnoticed, resulting in unnecessary energy consumption.
The issue of boiler dry cycling can be an expensive one if not addressed. Analysis illustrates that preventing boiler dry cycling will deliver average cost savings of 12 to 15 per cent across an estate, with a typical payback under 18 months. This example clearly demon- strates the potential benefit of introducing additional control when an area of energy wastage, such as boiler dry cycling, has not been addressed. However, when
considering retrofit controls it is important to verify that these will not interfere with existing control strategies or compromise comfort levels in the building(s).
Again, this can be illustrated by considering dry cycling. Boiler dry cycling is not a new phenomenon, engineers have been aware of it for many years. Early attempts to control it tended to delay boiler firing based on historic firing patterns and not real-time data or artificially lower each boiler’s set point temperatures – thereby com- promising comfort levels and potentially conflicting with the existing controls e.g. BMS. Clearly neither of these options is acceptable. The purpose of intro- ducing additional control is to add to the savings that are already being made by the existing con- trols, not interfere with the opera- tion of those existing controls. An alternative approach is to use a modern boiler load optimisation control strategy that constantly m
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