Mind the gap

Boiler replacement, Chris Meir, Remeha
Chris Meir

Boiler replacement is widely acknowledged as a quick win to improved heating reliability and efficiency. So why do we hear of some boilers failing to deliver the projected performance and savings? It all comes down to ensuring good system design, says Chris Meir.

We all know how critical space heating is in a building, enabling its continuous operation and generating a comfortable environment for its occupants. But this essential service also typically accounts for a hefty amount of the building’s total energy consumption, so it’s important that the heating system operates as efficiently as possible.

A high proportion of UK non-domestic buildings still rely on inefficient or ageing commercial boiler stock for their heating, needlessly wasting energy and creating unnecessarily high bills. For this reason, a boiler upgrade is an acknowledged quick win to significant energy savings in the region of 20-30% (Source: Carbon Trust).

Of all the boiler types, condensing boilers offer the highest efficiencies. This is due to their ability to recover both the sensible and latent heat in flue gases – otherwise lost in non-condensing boilers – with the result that they are 10-12% more efficient.

Yet while this advanced technology is capable of achieving outstanding gross efficiencies of up to 97%, the unfortunate reality is that some condensing boilers still fail to achieve their maximum combustion efficiencies or deliver the anticipated energy savings.

So how to avoid this gap between potential and actual energy efficiency? It all comes down to good system design.

Operating multiple boilers in cascade configuration proved a rapid, high-quality solution to achieving greater heating reliability and efficiency at Royal Birkdale Golf Club.

Design correctly

To maximise the efficiency of any heating technology, it’s important to consider how it will operate within the system. In the case of condensing boilers, for full condensing to occur, the return temperature needs to be below dew point, which is normally around 54°C.

This can be a challenge in older buildings where radiator circuits are typically sized on 82°C flow and 71°C return temperatures. However, given the changes to the fabric and use of commercial buildings over the years, many radiators in existing buildings are often oversized, making it possible to rebalance them. Lowering the flow and return temperatures to 70/50°C or even 60/40°C will allow the condensing boilers to fully condense and achieve their maximum efficiencies without affecting comfort.

Size intelligently

This leads on to the importance of resizing. As the load may have changed since the last assessment, a ‘like-for-like replacement’ could result in over-sized boilers that perform less efficiently and fail to achieve the required building environment. For this reason, it is important to recalculate both the maximum and the minimum heat demand and match the current load accurately.

Install effectively

Condensing boilers perform more efficiently at part load, so sharing the load across multiple units rather than selecting one large output boiler will maximise boiler performance. Installing multiple boilers also increases the turndown ratio of the fully modulating condensing boilers so that they can match more closely the fluctuating heat load demand of the building.

The inbuilt redundancy of this arrangement brings further advantages, improving heating reliability, simplifying maintenance and extending the lifecycle of the boilers for improved whole life costs. At the same time, the availability of preconfigured cascade systems reduces on-site hot works for time and labour savings. And with forward-thinking manufacturers now offering bespoke-designed rig systems, there is the added assurance that the project requirements and specification will be met.


The next design element is controls, which should be addressed at the outset. Good control is vital to achieve higher efficiencies and enable optimum boiler performance to be maintained. We recommend including time, temperature, weather compensation and sequencing controls which must be fully integrated into the Building Management System to maximise energy savings.


Commissioning is a crucial part of the design process but it can often be rushed due to budget or time constraints. Good hydraulic, combustion and controls commissioning will help ensure that the heating system performs as designed.

Royal Birkdale Golf Club

Maintain the system

Boilers are not a ‘fit and forget’ technology, although they may sometimes be treated that way. We recommend implementing a proactive maintenance and servicing programme as this will flag any potential issues early on, reducing the possibility of costly plant failure. Control settings should also be checked routinely for optimum operation.

Research by BRE indicates that around 60% of UK non-domestic buildings (and 80% of homes) that will be in operation in 2050 are already built. So if we are to meet our stringent emissions reduction target of 80% by the same year, we need to make our commercial buildings as energy efficient as possible. And that means addressing the heating.

Condensing boilers have a key role to play as more stringent energy efficiency and low NOx targets are effectively driving a move towards them and away from non-condensing boilers.

In multi-component, low carbon heating systems too, condensing boilers typically play a part, working in conjunction with technologies like combined heat and power (CHP) units or heat pumps to deliver reliable, energy-efficient heating.

High efficiencies and ultra-low NOx emissions aside, the compact dimensions and flexible design options of condensing boilers provide space-saving solutions to access and space restrictions that are often found in refurbishment projects.

But to benefit fully from the huge energy savings potential of this advanced, high efficiency heating technology, it pays to ensure good system design.

Chris Meir is sales director at Remeha

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