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Greater control is required

18 August 2011

Measures taken to reduce the energy consumption and carbon emissions of buildings can lead to energy wastage in other areas. Tony Willis, Technical Sales Director of Sabien Technology, explains why boiler dry cycling may be on the increase and going unnoticed.

Over the last few years we have seen a much stronger focus on measures to reduce the energy consumption and carbon emissions of existing buildings. And clearly this is a positive development, as tackling the performance of the UK’s building stock is the only way the government’s emissions targets will be met. However, when such improvements are made it’s important to evaluate their impact on the existing control of the building services plant, as failing to do so can lead to actually wasting further energy.

Boilers are a case in point. For example, one of the obvious ways to reduce a building’s energy consumption is to improve the insulation of the building, to minimise heat lost to the building fabric /outside. This has the net effect of reducing heating requirements -however the existing boiler plant will still have the capacity of the original design. Therefore the heat output capacity of the boilers may now be under-utilised.

There has also been a further drive to increase occupancy levels/density in many office buildings, as organisations seek to rationalise space usage and reduce overheads. The result is an increase in internal heat gains from additional IT equipment etc., again leading to a reduction of the base heating load demands.

In addition, many buildings are now incorporating low carbon heating technologies such as solar thermal or heat pumps to meet some of the space heating or domestic hot water load. The boilers are still retained as back-up and to meet peak demand but there will be extended periods where the output capacity of the boilers is not being operated to the best efficiency.
Clearly, all of these situations are very positive in terms of reducing the overall heating energy consumption but it’s also a formula that can increase a phenomenon known as boiler dry cycling – which results from standing losses.

The fact is that all boilers can suffer from standing losses. Even a well-insulated boiler loses heat to its surroundings so that the water temperature in the boiler falls below the current set-point of the boiler’s internal thermostat / load control. When this happens, the boiler may fire simply to recover the temperature of the wasted heat – even though this energy is not being used to meet demand for heat from the building. This is known as dry cycling.

The situations described above – increased internal heat gains, better insulation and use of alternative heat sources – all serve to reduce the time the boilers fire. Consequently, the standing losses and dry cycling are likely to increase.

Dry cycling

Unless measures are taken to control it, boiler dry cycling happens with the majority of boilers. Indeed, this can be further exacerbated by the common practice of installing over-capacity boilers to meet extreme weather conditions that may only occur for a few days a year (i.e. they are over-capacity for most of the year).

In fact, even during the very cold winters we experienced in 2009/10 and 2010/11, we have identified boilers that were dry cycling. For example, in the winter of 2009/2010, measures to prevent dry cycling at RAF Odiham reduced the boilers’ gas consumption by 15%.

This is why the measures described above, that reduce space heating loads, have the potential to increase boiler dry cycling – and why it is so important to prevent this. Crucially, such control must be established without changing set-point temperatures, compromising comfort or conflicting with existing controls and/or the Building Management Systems (BMS).

In this respect, there are some misconceptions about how dry cycling can be controlled. Many people naturally expect that their BMS will prevent boiler dry cycling, but that’s not what a BMS is normally designed or programmed for. A BMS strategy is typically designed to optimise a building rather than individual items of boiler plant. In relation to heating plant controlled by a BMS, this will typically monitor and respond to the common header flow and return blended temperature from all boilers; it won’t be monitoring individual boiler load profiles under variable load conditions.

Furthermore, a BMS is made up of a number of different strategies to control various aspects of a building’s performance. Because boiler dry cycling is commonly not recognised, the majority of BMS systems do not include boiler dry cycling control as standard. Technically, it is possible to re-program a BMS but the timescales are long, so the work is expensive. It will also be necessary to install extra sensors and inputs/outputs, often requiring an outstation upgrade and therefore may not be commercially viable.

Consequently, a retrofit solution that integrates with the BMS is a more cost-effective solution. Unfortunatel yearly attempts at retrofit solutions were an abject failure and, sad to say, some of these have reappeared recently in the current stampede to take advantage of the ‘green pound’. These ‘failures’ either delay the boilers’ firing, or artificially reduce the boiler set points, both of which allow temperatures to fall in the spaces being heated. In addition, they can cause direct conflicts with the BMS strategy by artificially changing the set point.

What they are not doing is differentiating between a genuine call for heat and compensation for standing heat losses in real time.

In contrast, intelligent boiler load optimisation using modern, patented technologies, analyses each boiler’s temperature profile in real time to ensure the boilers only fire in response to a genuine demand for heat from the building. This is achieved by constantly monitoring the boiler’s thermal response to changing loads, calculating the temperature decay 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.

Crucially, the set point has to be reached before these calculations can be made, so this technology cannot change the set point. This ensures that it works in harmony with the BMS and other controls, such as weather compensation, rather than causing conflicts.

Intelligent boiler load optimisation, patented in Sabien’s M2G unit, also monitors each boiler individually to provide very precise control of the boiler plant at individual boiler level. In this way, it is designed to enhance and augment the performance of the BMS or other controls, providing the fine-tuning that is required to further reduce energy consumption.

Intelligent boiler load optimisation has been tried and tested in a wide range of applications, usually integrated with existing BMS and weather compensation controls. Depending on the individual boilers and the systems they serve, energy savings range from 10% to 25%. Average savings across an estate are typically around 12% to 15%, with paybacks ranging from a few months to two years. Current users include BT, Lloyds Banking Group, Vodafone, Hampshire County Council, Defra, and the Environment Agency.

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