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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
As building operators constantly strive to improve their sustainability 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 proactively 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 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 varying levels of optimisation. Even in buildings where extensive optimisation has been delivered there are often further opportunities to reduce energy consumption.

This point can be clearly illustrated by considering the situation 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 controls or via a building management 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 sophisticated 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 boiler(s) will turn off. Naturally the boiler cools down; as the temperature 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 configured to control the heating system as a whole from the common header (the blended temperature of all boilers), rather than monitoring and controlling each individual boiler. It is impossible to identify 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 demonstrates 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 compromising comfort levels and potentially conflicting with the existing controls e.g. BMS.

Clearly neither of these options is acceptable. The purpose of introducing additional control is to add to the savings that are already being made by the existing controls, not interfere with the operation of those existing controls.

An alternative approach is to use a modern boiler load optimisation control strategy that constantly measures and analyses the temperature profile of each boiler in real time. This enables the controller to identify and prevent the boiler from dry cycling and, more importantly, allow the boiler to fire immediately if there is a genuine demand for heat. If a BMS is in place, the controller integrates with it, taking its signal from the BMS.

Just as importantly, it recalculates the values every time the boiler reaches its required set point temperature. This means it adapts to BMS/optimiser variable set-points and does nothing to conflict with other existing controls such as weather compensation, demand control or sequencing. The boilers' designed set points and the system temperature (common header temperatures) are never altered.

Crucially, such units require no maintenance or seasonal calibration and can be easily moved to new boilers if existing plant is replaced.

The exact savings that can be achieved by tackling boiler dry cycling vary from one project to another but, as an example, installation of 111 M2G intelligent boiler load optimisers at HMS Sultan was shown to deliver a 12 per cent energy saving (based on CUSUM analysis), with a return on investment within 1.4 years. Similarly, Lincolnshire County Council reduced its gas consumption by 15 per cent across 23 properties by retrofitting M2G and is now installing M2G to over 1,200 boilers within its school estate.

Validating savings
Whatever the nature of the energy-saving project, it is important that any savings can be verified using a validation methodology that is clearly understood by all parties. A useful and internationally recognised approach to such validation is the International Performance Measurement and Verification Protocol (IPMVP). The IPMVP defines a best practice framework for quantifying the results and benefits of energy efficiency investments. It also ensures there is total clarity and transparency for all parties in relation to the scope of work from initial planning, the measurement methodology, through to the validation of results.

The underlying point of this article is that when you think all bases are covered, they may not be. So rather than assuming all is under control, take a closer look to ensure you have a full understand of what is happening.

// The author is the business development director with Sabien Technology //
11 March 2014

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