Oversizing matters: Effect of boiler oversizing on efficiency

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

The difference in sizing requirement for space and water heating is a challenge to the efficient operation of a combi boiler

Chances 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 government’s 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 hasn’t 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.

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 near-instantaneous 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 advertised values and expectation, giving the potential for real-world improvements. Research has also shown (from the same authors) that on/off cycling is widespread in combi boilers in homes across the UK, where the minimum power output is typically above those required to match winter space-heating load.

The research in our paper aimed to investigate whether the limited modulation range of combi boilers, together with the mismatch of space and water heating demands, is a significant contributor to the reduction of in situ efficiency, and whether this can be mitigated.

Our study used dynamic simulation modelling to illustrate the impact of boiler oversizing on cycling, quantify the impact on efficiency, and explore measures to mitigate negative effects. The models mapped a range of operation and oversizing characteristics, coupling an established building model (TRNSYS) with physically accurate representations of the heating system in a co-simulation environment in Matlab.

Current legislation and labelling overlook PSR as a determinant of system efficiency, failing to incentivise appropriate sizing

In a simulation environment, the efficiency of the whole heating system could be tracked across multiple heating system configurations within the same virtual home, thereby eliminating the uncertainty of field measurement comparison across buildings, with all the uncertainty of construction, occupants, weather, and so on.

The level of oversizing present in most homes is considerable, and it is quantified as the ratio of boiler maximum output to the design heat load of a building, known as plant size ratio (PSR).

By simulating a common combi boiler size of 28kW (with a modulation range of 5:1) in a typical EPC grade C home with a design heat load of 3.3kW, we found that the level of boiler cycling was similar to that found in real boilers. Further simulations showed that the typical oversizing (PSR >3) and associated cycling behaviour brings an efficiency penalty of 6-9%.


The building fabric model enabled the impact of PSR on internal temperatures to be investigated. When the heating schedule was unchanged from that typically employed (07:00-09:00 and 16:00-23:00), then the increased efficiency at lower PSR was accompanied by lower internal temperatures, impacting thermal comfort.

However, extending the heating periods enabled the more modestly sized boiler to maintain the internal temperature at the required level without sacrificing efficiency. In the dwelling and heating system simulated, the efficiency gain from lower PSR outweighed the increased gas demand from longer operating times, resulting in energy savings.

There is a clear link between raising PSR, increased cycling and an associated decreased efficiency; however, in the UK, boilers are regularly oversized with respect to space heating, especially combination boilers that must cover peak hot-water demand.

Current legislation and labelling – including Energy-related Products and Standard Assessment Procedure – overlook PSR as a determinant of system efficiency, failing to incentivise appropriate sizing.

Reducing boiler oversizing by addressing installation practices and certification has the potential to improve efficiency significantly at low cost, decreasing associated carbon emissions and saving customers money.

Smaller, more suitable boilers can achieve their design efficiency in situ, but must be operated for longer periods to achieve the required internal temperatures – but this can be done without sacrificing efficiency or energy demand.

The correct sizing of boilers, coupled with more continuous heating schedules, decreases carbon emissions and can be implemented in the short term, before the stock is transferred to lower-carbon technologies such as heat pumps. 

About the author
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