Maximising efficiencies with CHP and condensing boilers

As CHP becomes more widely specified as a low carbon solution to generating both heat and power, Mike Hefford, Head of Low Carbon Technologies at Remeha Commercial, looks at how combining this technology with gas condensing boilers can maximise the savings potential.

Buildings are big users of energy, accounting for around half the UK’s total primary energy demand. As organisations look to improve their energy efficiency, reduce their carbon emissions and cut their energy costs, Combined Heat and Power (CHP) technology is increasingly specified as the sustainable solution to efficient heat and power within buildings that require significant, consistent demands for space heating and electricity generation.

How does CHP work?

CHP technology converts a single fuel into both electricity and heat in a single process at the point of use. CHP units operate by burning a fuel, usually natural gas, in an engine to drive the electrical generator and provide power for the building. The hot exhaust gas produced by the engine enters a heat recovery exchanger to feed the boilers and provide LTHW for space heating and HWS generation. Unlike conventional power stations, where approximately 50-60% of the heat created as a by-product of electricity generation is emitted into the atmosphere, a CHP plant captures a large part of this otherwise wasted heat, converting it for heating purposes.

Primary energy savings

It is due to this ability to generate both heat and power on site, at the point of use, that CHP units can operate at higher efficiency levels, offering a total fuel efficiency of 85-90% compared with an efficiency figure of around 40-45% for traditional generation. In other words, most CHP units can bring a primary energy saving of 30% and a 20% reduction in emissions. The figure is even higher with condensing CHP units like Remeha’s R-Gen 20/44 and 50/100 NG models which can achieve outstanding total fuel efficiencies of between 103 and 104%, reducing greenhouse gas emissions by up to 60% and primary energy consumption by up to 40%.

Achieving maximum benefits

The potential savings are considerable, but as with all technologies, there are a number of factors to consider in order to achieve the maximum benefits from CHP.

Feasibility – The first step is to undertake a feasibility study and site visit to ensure that CHP is the appropriate solution. While CHP is suitable for both new and existing buildings, it best suits organisations with year-round heat demands, usually running a minimum of 5,000 hours annually. This makes it the perfect solution for residential developments, hospitals, care homes, hotels and leisure centres. The longer CHP is running, the more efficient it is. So while CHP plants will still work effectively in other buildings, the return on investment will be longer as the financial payback period is dependent in part on the number of operating hours.

Sizing – Accurate sizing of the thermal and the electrical base loads and the heat-to-power ratio is essential as incorrectly specified and installed CHP will simply not deliver the maximum savings. It is important to remember that the thermal output from CHP is a by-product of the electrical output. A well-sized CHP system will use all the electricity and heat in the building. For this reason, our advice is to size the CHP unit to the lowest average heat demand with condensing boilers specified in conjunction to meet the supplementary heat requirements.

CHP and gas condensing boilers

The key to a well-designed system is to consider the system as a whole, a process which requires a thorough understanding of all the components and how they operate. Rather than considering the CHP unit and condensing boilers in isolation, we should engineer to integrate the two technologies effectively to achieve the maximum efficiencies from both components.

With a high proportion of our existing buildings still dependent on gas for heating, CHP is extremely well-suited to retrofit projects, helping raise the efficiency of the building to meet low carbon targets. However, whether fitting a CHP unit externally onto an existing system as a ‘bolt-on’, or installing a CHP unit with gas condensing boilers in a fully-contained internal package or energy centre, the same principles apply for optimum system efficiency: good design, good integration and good installation.

Keeping it simple

The CHP unit operates as the lead boiler with the condensing boilers providing additional heat during peak periods or when required. Avoid over-designing the systems – we often see overly complicated systems incorporating more 3-port valves and pumps than are actually required, which serve only to bump up the installation costs.

Instead, focus on maximising the efficiency of both technologies. When designing a CHP/condensing boiler system, it is important to bear in mind that condensing boilers work most effectively at lower water temperatures where they recover both sensible and latent heat from the flue gases. Condensing occurs at around 54°C when the flue gases are at or below their dew point. So in order to achieve the higher efficiencies, the boiler return water must be at this temperature or lower to allow continuous condensing to take place.

For this reason, the hydraulic integration of the two technologies has a major impact on how the system will perform. A potential pitfall is to connect the CHP in parallel onto the system heating return as this will preheat the return and will make the condensing boilers less efficient. A more appropriate solution would be to connect the CHP to the low loss heater of the condensing boiler as this helps to promote full condensing, thereby helping achieve the higher efficiency levels.

CHP systems offer huge potential for carbon and energy savings in both new buildings and old. However, accuracy and skill in configuration and integration are essential in order to achieve the maximum results. For this reason, it is beneficial to use a manufacturer or supplier with in-depth knowledge of both CHP and condensing technologies who can assist in smarter system design. We at Remeha Commercial look forward to sharing this insight with consultants and specifiers to support organisations and help the nation move to a more sustainable future.

For more information on the Remeha R-Gen range, email us at

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