Cutting back on boiler waste

Industry comment

Published: 17 March

Author: Alan O'Brien

Type: Industry comment

Year: 2009

Dry cycling of boilers can waste a great deal of energy, even when a BMS is in use. Tony Willis, Technical Sales Director with Sabien Technology Ltd, explains how this waste can be minimised

It has been recognised for a long time that many boilers waste energy through ‘dry cycling’ when they are not actively contributing to the building heating and hot water demand. And this is despite sophisticated modern controls such as building management systems (BMS) and building energy management systems (BEMS) being in place.

To address the problem of dry cycling effectively, therefore, it is necessary to optimise the system through intelligent analysis of the boiler’s flow and return water temperatures. This can be achieved by retrofitting an ‘intelligent boiler load optimisation’ system which ensures the boiler’s designed set point temperature is always maintained, so that comfort levels aren’t compromised. Such a system also needs to work in harmony with existing BMS/BEMS and other temperatures controls, when present, so it enhances the overall performance of the system and achieves maximum energy savings and CO2 reductions.

Furthermore, the system’s intelligence needs to be fully adaptive to changing load demands such as those created by existing weather compensation or, and building optimisation controls.

Our work in this area shows that this ‘intelligent boiler optimisation control’ will deliver energy and carbon savings of typically between 10% and 25% (depending on site conditions and applications) to give a payback of between six months and two years. For example, at AVIVA/Norwich Union, Sabien Technology’s M2G delivered savings of up to 17%, while for O2 the same technology saved up to 27% on boiler energy consumption.

Dry cycling

When a boiler is standing idle it acts like a giant radiator, losing heat to its surroundings – even a modern, well insulated boiler will typically lose 1-2% of its heat through radiated/standing losses. In addition, draught within the flue system may draw cooler air through the boiler, potentially adding a further 2-3% to standing losses. This is further exacerbated with the need to purge the combustion chamber each time a pressure jet burner fires (cooling down something you are trying to heat up!) which creates further boiler heat losses.

Inevitably, these standing losses will cause the boiler to fire unnecessarily to overcome the temperature losses while the boiler is idle. However this firing, or dry cycling, does not contribute to the actual building’s heating demand; it is simply compensating for these ‘standing losses’ and thus wasting energy.

This situation is caused by the fact that over 80% of boilers in the UK are oversized to ‘play safe’ or to meet extreme weather conditions that are only rarely encountered. This increases ‘dry cycling’ therefore increasing the number of times the boiler will fire unnecessary.

In the past simple timed delay devices, (which we do not advocate or recommend),have been used to try and overcome ‘dry cycling’. These work by delaying firing for a pre-set time after the system calls for heat. Very often, though, that call for heat will reflect a genuine requirement for heat. Consequently the delay will simply cause the boiler’s designed set point to be depressed, thus causing the boiler to fire for extended periods - resulting in increased energy consumption and compromising the comfort levels in the building.

’Intelligent boiler load optimisation’ has the ability to recognise and identify ‘dry cycling’ by constantly monitoring the boiler’s thermal response to changing loads every 10 seconds. Two digital temperature probes measure and monitor the boiler flow and return temperatures. The onboard software calculates the temperature gradient over time and determines when the boiler should fire for true building/heating demand and when it should remove/inhibit unnecessary boiler firing and energy consumption.

Crucially any control system that inhibits the boiler from firing must be fail safe and always ensure the boiler’s designed set point is met, therefore avoiding any additional servicing and maintenance requirements.

For all of the above reasons, there are strong economic and environmental incentives for taking a closer look at current boiler operation, and installing ‘intelligent boiler load control’.

As featured in Water, Energy and Environment - February / March 2009