BOILER EFFICIENCY | OVERSIZING OVERSIZING MATTERS With energy prices soaring and carbon reduction a priority, ensuring heating systems are specified to operate as efficiently as possible is essential. In this article, based on their CIBSE Carter Bronze Medal-winning paper, Dr George Bennett and Dr Cliff Elwell explain the impact on energy use and comfort of oversizing boilers and how correct sizing can boost efficiency C hances are that, if you live in the UK, your home is heated by a gas boiler. Gas boilers are the dominant way to heat British homes, with more than 20 million in operation and more than 1.2 million still being installed every year. Electrification of heat through the deployment of heat pumps will play a pivotal role in the decarbonisation of heat, as set out in the governments target to install 600,000 such units per year by 2028. But with such a large install base of boilers and continued rate of replacement even in 2028 it is likely that ~600,000+ boilers will be installed improvements to boiler efficiency can, and should, contribute to decarbonisation, and are likely to carry across to hydrogen boilers. The last major improvement in boiler technology was the move to condensing gas boilers, which resulted in tangible savings of carbon and cost. But the technology hasnt stood still, and continues to be developed. Are there further cost and carbon savings to be gained from the humble boiler? The research we carried out for our CIBSE Carter Bronze Medal-winning paper Effect of boiler oversizing on efficiency: a dynamic simulation study aimed to answer these questions. Output DR GEORGE BENNETT is lead technical energy adviser at the Department for Business, Energy and Industrial Strategy (BEIS) DR CLIFF ELWELL is an associate professor at UCL Energy Institute The most common type of boiler in UK homes is the combination (combi) boiler, which provides space heat and on-demand hot water. These boilers have proven popular with homebuilders, consumers and installers because of their low cost, simple installation and space-saving form factor. However, such boilers must be able to meet the different requirements of a nearinstantaneous demand for hot water and space heating. The heating power demand to provide hot water to multiple outlets, such as showers and taps, can be many times the peak space-heating demand of homes, and even more on milder days. This difference in sizing requirement for space and water heating presents a challenge to the efficient operation of combi boilers. Domestic boilers do not typically operate at fixed output and, instead, adjust (modulate) their output to fixed levels, which are selected according to the demand. Unfortunately, the modulation ranges of modern boilers are typically around 5:1 peak to minimum heat delivered (for example, 25kW boiler minimum output would be 5kW). The conditions of operation for a boiler to achieve its design efficiency is steady state operation with low system water temperature flowing to and from the heat emitters in the building. This will ensure maximum capture of heat and latent heat from combustion products, and avoid the losses associated with starting and stopping. Should the minimum heat output of a boiler be too high to match the heat demand, the boiler cycles on and off, disrupting steady state low temperature operation and reducing efficiency. Measuring efficiency Previous monitoring of boilers in use has shown that efficiency falls short of the 28 May 2022 www.cibsejournal.com CIBSE May 22 pp28-29 Boiler efficiency.indd 28 22/04/2022 15:16