Making chimney calculations simpler

Making chimney calculations simpler

 

By Chris Laughton, Managing Director of The Solar Design Comp

In October 2010 a guide to the latest England & Wales Building Regulations for combustion appliances was released in the form of Approved Document J. This contained guidance on methods for calculating chimneys and flues, referencing the approved calculation procedure within BS EN13384. The guide gives this method as a suitable basis ‘for deciding whether a chimney design will provide sufficient draught’ (ADJ pg. 30). This standard allows deviance from the usual rules-of-thumb providing a calculation is under-taken.

The standard contains a number of complex calculations with multiple variables. This is not surprising as chimney draught varies due to a wide range of factors. Temperature is one of these factors, and so it must be considered whether the chimney is located inside or outside the building as well as chimney insulation levels. The speed of flue gases, diameter, height and number of bends all play their part.

Producing successful calculations necessitates juggling complex hydraulic equations. These are all interconnected, as when one variable changes such as neighbouring building heights or humidity, then all others are also affected. This is an iterative process which is very difficult to optimise manually, and such calculations are normally done using dedicated computer software.

For smaller sub-50kW systems it is often sufficient to follow the guide rules-of-thumb, but for systems producing over 50kW or with multiple appliances, a calculation becomes the preferred choice for accuracy. For this reason, installers and engineering consultants are increasingly turning to computer software to run different chimney possibilities and prove that a chimney system will work in accordance with BS EN 13384.

CASE STUDY: a 25kW wood pellet boiler

A 25kW wood pellet boiler was planned to be installed in an unheated outbuilding adjacent to a dwelling of a similar height. A fragile roof covering meant that a vertical route was not preferred. The chimney design was required to meet BS EN13384-1, to indicate if a sufficient draught was available and if using three bends would mean a condensation risk at the top of the chimney.

The details of the installation were input into kesa-aladin software, which calculates to this standard and incorporates the relevant manufacturers’ data. As well as the appliance type, length, height, diameter and number of bends, the height above sea level and neighbouring obstacles had to be described.

A 7m-high, 200mm-diameter, twin-wall stainless steel flue-way was first considered. However, this initially passed the draught test but failed the condensation test and so was at risk of excessive corrosion and freezing.

The diameter was reduced to 150mm, but this still failed the condensation test. However, a 5m-high flue at 150mm met all the requirements of condensation and minimum draught, and so could be shown to meet BS EN13384-1 at a considerably lower cost than originally envisaged.

Good, clean combustion requires a good chimney. To achieve improved air pollution and meet the Government’s Cleaner Air agenda, chimney design needs to be optimised. Using the EN13384 standard offers improved defence against negligence claims. Using software to calculate to this standard is a time and risk-saver in the long run. Installers and engineering consultants increasingly prefer the freedom to make their own calculations between different chimney brands, with the convenience of manufacturer’s details available in a digital database alongside the calculation.

Chimney calculation software transforms the initially onerous EN 13384 calculations into a more visual method and allows customers to appreciate the process in more detail.