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Viessmann Optimising Pause Times in Lp Gas Boilers

Boiler manufacturers are pushing the development of condensing combi and wall mounted boilers for improved energy efficiency. Christian Engelke, technical director for Viessmann, examines developments in boiler technology and operation to help equip the industry for the future.

Advanced high efficiency lp gas condensing storage combi and wall mounted condensing boilers are among the most economical on the market today. In addition to their compact size, they offer a high level of comfort and excellent reliability.

One technology already helping to make conventional lpg-burning products highly efficient is Viessmann’s Lambda Pro intelligent combustion control. Yet, even more can be done to lower fuel usage and reduce carbon emissions, which is where dynamic pause time optimisation comes in.

Lambda Pro was launched in 2005 by Viessmann and enables the boiler to automatically compensate for fluctuations in combustion quality and, more importantly for lp gas, for fluctuations in air pressure. The combustion controller works alongside the burner to constantly monitor the air to gas ratio.

Instead of the lpg gas/air mix remaining fixed from commissioning, the mix is automatically adjusted as required to maintain the correct ratio – in a similar way to the fuel management system in a car engine. It works efficiently irrespective of the type or quality of the lp gas.

The control replaces the gas valve control and changing to lpg takes just a few seconds with no changing of any parts. The technology is now becoming more widely included on higher end boilers, such as Viessmann’s Vitodens 200-W combi and system boilers, Vitodens 222-F boiler with DHW store and the Vitodens 242-F integrated boiler and solar DHW storage.

LPG BOILER PAUSE TIMES

For a long time, we have been familiar with the use of defined pause times (or burner anti-cycling function), where a burner remains off for a chosen period after each operating cycle, only starting again once the pause time has expired and the actual boiler water temperature has fallen below the lower limit.

This rigid control is not that efficient, as it cannot be adapted to changing conditions. Selectable pause times has also proven less than successful as, typically, they have not been matched to local conditions during commissioning.

With dynamic pause time optimisation, an algorithm evaluates temperature gradients in the heat up curve of a switching interval, i.e. the rate in Kelvin per second at which the boiler water temperature increases. Pause times are then matched depending on the heat up rate according to a specified function based on complex simulations and practical experience.

They also stop early in the event of preset circumstances, for example, a manual heating adjustment or a quick temperature drop, as you would expect from a fully dynamic system. Viessmann’s own research into the effects of pause time found that methods for dynamic pause time optimisation can be implemented without detracting from reliability or any loss of comfort to the homeowner.

From a starting point of 77 cycles per day with a fixed pause time, the number of burner starts was reduced by approximately one third to around 50 cycles per day. Dynamic pause time optimisation had a particularly positive effect in April to October, when the cycling frequency was reduced from up to 170 burner starts per day to below 100, more than 40 per cent.

Average burner runtime per switching interval with both the integral and differential methods was extended by one third from 7.65 minutes to 11.45 minutes, while the overall annual runtime remains almost unchanged.

Dynamic pause time optimisation can improve efficiency rates beyond the accepted level of up to 98 per cent (in relation to gross calorific value). The improvement arises from a reduction in pre-purge phases in conjunction with a reduction in cycles, extended burner runtimes during spring/autumn (regulated modulating operation) and lower standby losses due to lower average boiler water temperatures.

Condensing boilers can be improved to provide an even better return on investment to the householder and, of course, improve heating comfort in the process. Dynamic pause time optimisation, such as on the Vitodens 200 range, together with other interventions, such as extended modulation ranges and certain control features, could improve lp gas consumption by 10 per cent per year compared to non-optimised systems.

Contact Viessmann on 01952 675000 www.viessmann.co.uk

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