Some of today’s boilers offer high modulation ratios, but, considering average heat loads in the UK, is such a wide range ever required? Alisdair Cumming – Research and Development Manager at Baxi – explains why a modulation ratio of 5:1 will provide efficiency, comfort and reliability for the majority of UK homes.
In recent years, boiler manufacturers have been striving to produce ever wider modulation ratios, and 10:1 has been the ambition of many.
This is because, theoretically, increasing a boiler’s modulation ratio will improve fuel efficiency, and will increase user comfort and boiler longevity by preventing on/off cycling.
It’s important for a boiler to modulate and to match the output power of the boiler to the thermal load of the system.
This is especially true for combi boilers, where the customer will want fixed hot water temperatures irrespective of the volume of water required, and the output of the boiler is generally based on the amount of hot water – rather than the space heating – required.
However, it’s important to ask whether a 10:1 modulation ratio would ever be required for a typical UK property – e.g. would a 30kW boiler ever actually need to modulate down to 3kW?
Heating loads
To answer this question we need to consider average heating loads across the UK. Many dwellings in our country are old buildings with relatively poor insulation, and this means an output of around 8–10kW is typically required to keep a house warm in winter.
Therefore, a 30kW boiler would never need to modulate down to 3kW because this wouldn’t heat the property sufficiently.
The most popular boiler outputs in the UK are 24kW and 28kW, and – unless a property has low heating requirements because it has been built to ultra-modern standards and is exceptionally well insulated – 2.4kW or 2.8kW is not going to keep the occupants warm and comfortable.
So, for example, if a 24kW or 28kW boiler with a 10:1 modulation ratio is installed in a typical UK property, it’s likely that the lowest kW point will never actually be used and no efficiency gains will be made.
Aiming too high
As already discussed, it is important for a boiler to modulate, but a very high ratio might not actually be necessary for the vast majority of UK properties. Achieving higher ratios also involves a lot of additional technology and cost, and results in several downsides.
Firstly, with a high ratio, the flame in the boiler is very small, and therefore weak. If it’s disturbed (e.g. by additional air movement), the flame might become unstable and unreliable, or even go out. Resulting in poor flame detection and/or nuisance tripping, the boiler will need to restart, which is what modulation is designed to avoid.
Secondly, higher modulation ratios don’t necessarily mean lower energy usage. The primary efficiency consideration of the boiler is the return water temperature – the lower the better. The gains made in energy efficiency by moving a modulation ratio from 5:1 to 10: 1 with the same return water temperature are negligible.
Thirdly, the components used in boilers with wider modulation ratios are more delicate, and this can lead to reduced reliability.
Striking the balance
During the design and development stages of its latest boiler, Baxi wanted to find the right balance – a modulation ‘sweet spot’. The aim was to develop a boiler with an optimum modulation range that would provide efficiency, comfort and reliability for the majority of UK homes.
Taking all the issues surrounding modulation ratios into consideration and looking at average heat loads across the UK’s housing stock led to the conclusion that the ideal modulation ratio is 5:1.
For 99% of property types in the UK, a modulation ratio of 5:1 seems ample, and it’s perfectly acceptable for the other 1% too.
A 5:1 modulation ratio will keep UK homes warm enough, and, where combis are concerned, will provide enough hot water (even for the most demanding of showers as long as the boiler is sized correctly), and will deliver stable hot water temperatures.
Theoretically, a boiler modulation ratio as high as 10:1 could be beneficial for highly insulated, ultra-modern homes, as well as some commercial properties, but for the vast majority of UK dwellings, we believe that the optimum ratio is 5:1.
