Rob Berridge, Director at Rob Berridge Heating Design Consultants, says that bigger is not necessarily better when it comes to boiler outputs.

Recently, HVP published an article written by Martyn Bridges who is not only the Director of Technical Communication and Product Management for Worcester Bosch, but is also the chair of Boiler Technical Committee for the Heating & Hot Water Industry Council (HHIC).

For those who don’t know, the HHIC is one of the largest trade bodies in the UK, which, on its website (, specifically states that the organisation is, 'committed to effectively driving, supporting and influencing the sustained growth of the UK domestic heating and hot water industry'.

Those who know me know that this article was clearly directed at what I have tried to highlight, so the kind guys at HVP have allowed me to pen a response in my own, independent way.

I have almost single handedly challenged the industry status quo over plant sizing and system design but have also proven time and time again, (through working systems that I have designed) that recognised engineering methods can and do work far more efficiently on an absolute fraction of the ‘rule of thumb’ kW specification we have all unfortunately come to think of as accurate and common practice! The boiler market has completely run away with itself and, regardless of modulation ratios, they are massively too big for what the average house or commercial building will ever require.

It is a known, physical and scientific fact that watts and flow rate are intrinsically linked and cannot be separated. Resistance in pipework also plays a huge part. Take combi boilers for instance: it makes absolutely no sense whatsoever to have such large flow rates on combi boilers that exceed velocities of 1.5m/s (metres per second). All of them only have a 15mm inlet and outlet, and this equates to a finite and safe amount of flow rate. Couple this with any component flow restriction within the heating up process of hot water and you potentially worsen the velocity and component erosion/noise issues of exceeding 1.5m/s. Indeed, 1.5m/s on 15mm copper (table X) pipe equates to a maximum flow rate of 13l/m (litres per minute).

There is also the huge problem of installers not matching the manufacturers' stated, factory flow rate/requirements of these types of boilers to the incoming flow rate of the supply. This will have vast effects on real world efficiency, as most combi boilers cannot be range rated on the hot water side. Mains water flow rates fluctuate vastly anyway throughout a typical day, so it seems logical to set them lower for a more guaranteed consistency and ‘factory efficiency'?

Water companies only guarantee 0.7bar to an estate boundary at ground level if an engineer is to install the boiler to the mains supply (they do not need a mains supply), but these pressures and flow rates vary wildly around the country. Plus, legally, if your incoming water main has a low flow rate, you are only allowed to boost it to 12l/m. A far more consistent and efficient supply would be a traditional tank system (buffer) and booster pump with a set flow rate of 13l/m or one of the growing options of mains boosting solutions. 

My point is, these high velocities should not be dumped on heating or water systems and I would strongly argue that the higher kW outputs of these types of boilers will never be fully used and so cannot be as efficient as traditional boiler and tank systems. Sure, there is a huge market for combi boilers as we have an awful lot of small, one shower room properties but, frankly, if you have a deep bath and want it to fill quickly, the most efficient way to do so is to heat your huge amount of water over a long period of time and not instantaneously.

All manufacturers produce combi boilers and most that know me, know that I do not favour them even though I have one of my own. Lack of space has driven the market of these units and they are only really suited to single bathroom properties, so I would always argue that units should not produce more than 13l/m at a 35 to 45oc temperature rise. Most outlets are actually producing 55 to 60oc temperatures which is unusable anyway, and we need to then blend the water down to a more useable 38oc. This is madness in my opinion and we could easily add a legionella protection facility to all those who would question the lower temperatures.

My points generally are not focused on domestic hot water flow rates, I think combis are wrong and nowhere near as efficient as traditional boiler and tank systems and that is what I want to focus on.

What is the point of having huge output boilers of any type (combi, system or heat only) that frankly, can never be fully used? I’d rather have a slightly bigger wallet! The average heat loss for a UK property is around 6 to 8kW, yet we are constantly told that we can range rate down these unnecessarily high output boilers to match system requirements.

Why? Surely a correctly specified boiler with the right output heat exchanger (like heat pumps) will be far more efficient, smaller, cheaper etc? Indeed, smaller output units are commonly sold on the continent, so why do we not have them here? This whole debate could be easily settled if manufacturers simply offered these smaller output units on the UK market.

It is my guess that manufacturers (coupled with trade bodies) are selling a, ‘ruse’ so that they can sell boiler plant that they profess, will be as equally efficient in a 1 bed flat or a FIFTY room mansion or castle? Let's face it, that is exactly what most of the available boilers could heat, and I can absolutely prove the calculations. This ‘one size fits all’ approach saves manufacturers a lot on production costs, marketing etc but I argue that we have an energy wastage crisis that needs addressing! 

Martyn's point that, ‘also, with the rise in popularity of home extensions, this variable output is useful to help futureproof the heating system, should the homeowner install a new extension’ is simply not valid. If a whole house is an average of 6kW heat requirement, what on earth size of extension would that future proofing cover when the standard 30kW unit is installed? Yes, my thoughts exactly. 

Every modern boiler is a great piece of kit these days. Unfortunately, the systems they are being installed onto are more often than not very poorly designed, maintained, and initially installed. We see horror shows on a regular basis. All I have ever tried to do is raise awareness of the fact that over 90% of our boilers are massively oversized. Indeed, on a recent commercial project, I saved the clients over £60,000 on material costs alone just by doing a correct calculation.

Correct sizing of plant and system design ensures a cheaper, more efficient, smooth, balanced, and extremely long lasting system for clients and who wouldn’t want that, eh? The manufacture and supply of more appropriately sized boiler plant for all of our buildings would go a long way in our fight against energy wastage, both in manufacturing material costs, running costs and longevity. A boiler is not a car, and bigger is absolutely not better.

It is absolutely wonderful that Martyn has entered the conversation as this is a huge and very relevant subject at the moment. I only want the very best for this industry of ours with fair and frank conversation. Accordingly, I would like to invite him, all manufacturers and the wider trade bodies to have a public debate on these issues and help to promote best practice and efficiency as God knows, we are going to be up against it over the next two decades.