Releasing the full potential of condensing boilers

Published:  20 January, 2007

Viessmann
The use of a burner with a large surface area in Viessmann’s Vitocrossal condensing boilers reduces the emissions of oxides of nitrogen and carbon monoxide.

Sales of condensing boilers to in the commercial market are approaching 50% — but STUART PURCHASE is concerned that they are not able to realise their full potential.

Whether driven by rising fuel prices or the urge to reduce carbon-dioxide emissions, the sales of commercial condensing boilers in the UK are rising steadily. Over 40% of the commercial boiler market is now condensing and, according to Stewart Purchase of Viessmann UK, rising.

Emissions

Commercial condensing boilers now achieve percentage efficiencies in the high 90s (gross). While their carbon-dioxide emissions are much lower than conventional boilers, there are other undesirable emissions, the mains ones being oxides of nitrogen and carbon monoxide.

Stewart Purchase is managing director of Viessmann and tells us that the company has worked to reduce those emissions by developing a family of burners exclusively for its own boilers.

Called MatriX, these burners combine the features of atmospheric and pressure-jet burners. The globular catalytic matrix allows the gas to burn as hundreds of small flames — giving a much larger surface area of flame than a pressure-jet burner. This larger flame is also cooler, resulting in lower emissions of oxides of nitrogen and carbon monoxide. Emissions of these gases from the MatriX burner are said to be well below any present or likely regulatory requirements. Indeed, the emissions are well below the stringent German Blue Angel requirement.

These burners are used in Vitocrossal boilers up to 314 kW and can modulate smoothly down to 25% of full output to maximise combustion efficiency at part load.

Frustrated

Given the potential of condensing boilers to reduce carbon emissions, Stewart Purchase is understandably frustrated that they are not always given the opportunity to deliver. He points out that for only about 30 days of a 200-day heating season is the boiler faced with design conditions of 0 or -1°C outdoors. ‘Most of the time,’ he says, demand is on average only half the design input — but most control regimes drive the boiler flat out all year round.

‘No wonder,’ he continues, there are questions whether condensing boilers are worth having. Against standard flow and return temperatures of 80 and 60°C they would hardly ever condense.’

However, even at these operating conditions, condensing boilers do deliver worthwhile savings, as they push up the operating efficiency even if they are not condensing all the time.

The key to maximising the potential of a condensing boiler is to reduce the flow temperature so that the return water is below about 58°C, enabling condensing operation to occur.

Stewart Purchase argues that there is no need to oversize radiators, as is often suggested. For most of the heating season, radiators are already well oversized. Existing buildings are already likely to have seen improvements such as double glazing, and offices now have heat gains from computers that did not figure in the original heating calculations.

‘The simple answer,’ he explains, ‘is to measure, minute by minute, the inside and outside temperatures and let the boiler choose its optimum and lowest flow temperature.’

Control

That approach to control is embodied in Viessmann’s Vitotronic control system, with input from outdoor and indoor sensors. The flow temperature is reduced to the lowest possible and switches to a higher temperature when there is a demand for domestic hot water.

Whereas condensing boilers typically reduce fuel consumption by 25% on average, Stewart Purchase tells us that results from all types of installation show that the Vitotronic control system achieve at least a further 10 percentage points by reducing flow temperature according to the weather.

Those two savings added together suggest that the energy used by boiler plant for space heating and hot water can be reduced by an average of 35% — which makes a very large contribution towards Government targets for reducing carbon emissions.



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