Efficient boilers meet efficient heating systems

Published:  06 September, 2012

boiler, space heating, Hamworthy Heating
Maximising efficient operation — Bob Walsh.

As modern condensing boilers rise in popularity, Hamworthy Heating’s Technical Director, Bob Walsh, looks at how to maximise the benefits of already highly-efficient boiler models through careful system design.

The switch from cast-iron boilers to condensing models has been one of the most spectacular shifts within the heating industry to date. In 2002, cast-iron heat exchangers represented 70% of all new commercial boiler sales. Today, the proportion is just 18%. In a mirror image to cast iron’s prospects, condensing boilers are now 80 % of commercial sales, compared to 20% in 2002.

The move to condensing boilers has primarily been driven by the need to reduce carbon production by national and international agreements and laws: the Kyoto Protocol; the Climate Change Act and, subsequently, the Carbon Reduction Commitment; the Energy Act; and tighter UK Building Regulations.

Indeed, the DECC’s progress report released in June this year shows that the UK is on the right track to superseding carbon reduction targets, with reported emissions (including emissions trading) in 2011 being 26.4% below the baseline. Despite this, the target is viewed by some as no longer challenging, since the reduction in emissions could be attributed to the decrease in industrial activity owing to the recession.

Yet, the target set by the UK’s own Climate Change Act requires us to go even further, with a reduction of 80% by 2050. Meeting this target will demand even more adaptation and ingenuity, not in condensing boiler design but in careful system integration.

Gradually over the last 50 years, the controls on condensing boilers have improved, and we are now reaching a new benchmark.

Looking back from today’s standard, the early boiler controls left room for improvement. Boilers had simple on/off controls, so they were either on 100% or off. They were controlled to provide part-load output by bringing on (or switching off) each boiler in sequence to meet the system demand for heat. The old on/off boiler had its maximum efficiency when operating at full load.

Today’s modern condensing boilers have modulating burners and controls to suit, so they operate and respond to part-load conditions, typically modulating down to 20% of full output. Unlike old boilers, maximum boiler efficiency is achieved at part load, so even greater efficiency can be gained when operating the boilers in turn at part load, before modulating each up to full load in turn.

Given the high efficiency levels of modern boilers, it is important to design and control the heating system to fully exploit that efficiency.

One of the remaining challenges for condensing boilers, whole-system temperature control, is a topic that has needed to be addressed for some time. The challenge for traditional systems has been the dilution effect of the non-firing boilers when a unit is part loaded. For example, an installation of four boilers delivering 82°C flow temperature, might need a flow temperature from one boiler of 91°C to achieve a mixed-flow temperature of 82°C once the dilution effect of the three non-firing boilers is taken into account. That higher temperature requires an adequate head (pressure) at the boiler to comply with Health & Safety Executive Guidance Note PM5.

This challenge is overcome by Hamworthy’s Fleet range of condensing boilers by fitting a circulating pump on each boiler to isolate the non-firing modules. This isolation, in turn, maintains the temperature at the system design setting.

We have also introduced the Merley, the latest generation boiler sequence controller. Using proven, reliable Siemens controls platform technology, the Merley can be used with Hamworthy boilers to share the load between multiple boiler modules in the most efficient manner possible, providing cascade control for up to 16 modules based on time and temperature requirements.

For condensing boilers, this controller offers numerous control strategies, providing user programmability to suit building occupancy. Time and comfort control, time clock and optimiser, variable- or constant-flow temperature, night setback and frost protection are all included.

Maximising the benefits of condensing and high-efficiency boilers through careful system design and integration will ensure that boilers are the most sustainable and economic solution for many years to come, and they will have a significant role to play in carbon reduction and real energy savings.

Bob Walsh is technical director with Hamworthy Heating.

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