Reducing energy-wasting dry cycling in boilers
It is now more important than ever to fine-tune every aspect of building-services performance. Tony Willis explains why boiler dry cycling is often overlooked — and why most control strategies fail to address this area of waste
It is probably true to say that up to a few years ago the key criterion for a heating system was that it met the requirements for space heating and domestic hot water. Nowadays, with increasing pressure from a raft of environmental legislation, the efficiency with which it does so is also a priority — and the way the boilers are controlled is as important as the inherent combustion and thermal efficiencies of the system.
In a well designed, properly commissioned and regularly maintained system most of the performance parameters are controlled efficiently through the building-management system (BMS). Unfortunately, one area of control that is not addressed, and often goes unnoticed, is boiler dry cycling.
Given that boiler dry cycling can wastefully account for as much as 25% of fuel consumption (based on figures from over 90 projects in the last five years) and that this heat is not utilised by the building system load, this is clearly an area that requires attention. Indeed, there have been many attempts to do just that in the past, but to date all have failed. It is only now, by using more recently developed technologies, that this vital aspect of boiler performance can be optimised.
To understand this, it is important to be clear about what dry cycling is. A boiler in standby or off at set point acts as a large radiator and loses heat to its surroundings (a typical temperature differential, or ∆T, between a boiler and the surrounding air could be as high as 60 K). Even a modern well insulated boiler will have standing losses of 1 to 2% — with additional losses from the flue and, in some cases, from purging the combustion chamber before re-firing.
Heat is lost from the boiler as standing losses causes its controls to fire it to compensate, even when there is no requirement for space heating or hot water. This problem is at its worst during Spring and Autumn, when boilers tend to switch on and off or modulate — particularly when boilers are oversized (which accounts for around 80% of the UK’s boiler plant). Contrary to the beliefs of some, dry cycling also occurs with modulating boilers.
Although it would seem logical to assume that the BMS should handle the problem of dry cycling, there are several reasons why it doesn’t and why additional measures are required to work in harmony with the BMS.
First, it is important to recognise that a BMS is typically designed to optimise a building and not individual items of plant. Thus the BMS will monitor and respond to common flow and return blended temperature from all boilers but will not monitor the performance of individual boiler temperature profiles under variable-load conditions.
For example, in an installation with three boilers that are sequenced by the BMS (Fig. 1), one boiler may be running at full capacity, the second may be firing intermittently to top up the first, and the third boiler may be on standby. The overall picture (i.e. the blended temperature of the whole system) that the BMS ‘sees’ may be fine, but the second and third boilers will be experiencing some standing losses and dry cycling. This will only be picked up if boilers are monitored individually, which is the main reason why dry cycling often goes unnoticed.
Second, a BMS is made up of a number of control strategies to control various aspects of a building’s performance. Because dry cycling is so rarely recognised, most building-management systems do not include dry-cycling control as standard.
Clearly, it should be possible to re-configure the existing BMS, and, indeed, a number of our clients have approached their BMS providers to explore this option. However, this is not as simple as it seems; estimates of re- development time vary from 18 to 24 months, extra sensors and inputs/outputs are required, and the boiler-room outstation will probably need to be upgraded as well. So, yes, dry cycling can be controlled through the BMS — but only at a prohibitive cost.
This is where new and affordable technology mentioned above comes into play.
Early attempts to control dry cycling failed because the technology then available could not differentiate between a genuine ‘call for heat’ to maintain comfort and a ‘call for heat’ resulting from just the standing losses.
In contrast, intelligent boiler load optimisation can differentiate, so that set point and comfort are not compromised. This is achieved 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. Consequently, it is fully adaptive to changing boiler load/heating demand.
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 help building operators meet their environmental obligations.
There’s no doubt that the 21st century will bring many environmental challenges, so it makes sense to use 21st century technologies to meet them.
Tony Willis is technical sales director with Sabien Technology Ltd.
