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Building Products March 2018

COMMERCIAL Definitions and design considerations MARCH 2018 | BUILDING PRODUCTS 63 performance of the chimney. • The heat output of the appliance and the total kW/hr rating of the boiler room need to considered, along with appliance efficiency. • The type of fuel to be used will have an impact on emissions. • DEFRA smoke control areas require the installation of an exempted appliance. However, the correct chimney height has still to be calculated. • Building regulations requirements must always be referenced. • The requirements of BSEN15287-2:2008 should be followed for appliances with a rating above 50kW. • When considering the requirements of the Clean to determine the chimney height take into account the type of fuel burned and the maximum rated output of the appliance.” The velocity at which flue emissions are discharged is also critical to determining how readily emissions are dispersed – the higher the discharge speed, the better the dispersion rate. The BFCMA again: “When designing the chimney/flue system it is important to design the chimney/flue so that the required discharge velocity can be achieved. With boiler systems it is often necessary to reduce the internal diameter of the chimney top by using a tapered cone. Care should, however, be taken to calculate the correct size of tapered cone. “The maintenance of a constant discharge velocity can be difficult when more than one appliance is serviced by one chimney. In this situation, an exhaust fan may be required.” Adjacent buildings can also have an impact on the flue height and must be taken into consideration when calculating it. As the BFCMA points out: “The objective is to achieve an effective and safe chimney discharge height rather than a minimum discharge height.” It believes the designer should start with matching the appliance and the flue by paying attention to the appliance manufacturer’s recommendations. Then, it recommends looking at the chimney height and emissions in the light of the following checklist: • Determine the necessary height to create the optimal draught to meet appliance operating requirements (Approved Document J provides guidance). • Factor in site-specific environmental impacts on the chimney draught. The local terrain (proximity of trees and higher buildings, for example) will have an impact on the The flue is the working part of the chimney, conveying the products of combustion safely to the atmosphere (whereas the chimney includes the shaft within which the flue is housed). Design considerations for flues include: • Appliance type – Flues have traditionally worked under negative pressure, drawing the products of combustion from the appliance. In this operational form, a successful chimney ‘draw’ depends on the difference in air pressure between the appliance and the top of the chimney (created by the height), and the difference in temperature between the appliance’s exhaust gas and the outside temperature. However, many appliances – especially those incorporating fans – now operate with positive pressure where the air or gas pressure is greater than that of the atmosphere. • Design drivers – The temperature and pressure rating of the appliance will drive the flue choice. The maximum and minimum output of modulating appliances should also be considered. Where possible, bends in the flue should be avoided, as a straight vertical chimney performs better. • The route taken by the flue – This can have a significant impact on performance; generally, the straighter and more vertical the chimney the better. • Testing – The proposed flue should be designed in plans and tested using a flue test programme before installation. The test calculation will show if the flue route will facilitate the required draught to allow the flue gases to escape freely to atmosphere. • Fans – A fan may be required and an appropriate fan controller should also be specified at the design stage. The flue must be capable of removing the flue gases if the power or fan fails. • System chimneys – Where system chimneys are used, always use the standard offset components available from the chimney manufacturer. Flue types and their applications Stainless steel flues are split into different systems according to the application they are designed for: Twin wall system flues – High quality stainless steel system chimneys comprise two concentric stainless steel metal walls with preinstalled insulation filling the annular space between them. The systems are available in different models for use with: • Condensing and non-condensing multi-fuel appliances. • Diesel and gas fuel generators and CHP systems. • Appliances operating under high positive pressure at low and high temperatures. • Can also be used as a connecting flue pipe. Single wall flues – These are designed for use on multi-fuel applications as a chimney liner or connecting flue pipe on condensing and noncondensing applications. Flexible liner – Stainless steel flexible liners can be used to reline an existing chimney. There are two types: single skin liners for use with gas appliances and twin skin liners, manufactured from overlapping strips of high grade stainless steel to give a smooth sealed flue-way for use with wood and multi-fuel appliances as well as condensing gas and oil appliances. Air Act for wood burning appliances with a burn rate above 45.4 kg/hr, the Biomass Assessment Tool (TG09) should be used (http://bit.ly/2BGgt3j). • A location is designated a local air quality management (LAQM) area if the air quality breaches the required limits. In a LAQM area, it is the local authority’s responsibility to measure and approve the installation of the appliance. • Finally, dispersion modelling (the mathematical simulation of how air pollutants disperse in the atmosphere) may be required for larger appliances. No one point will, on its own, determine the final required height. Usually more than one will have an actual impact and affect the final determined termination height of the chimney. Regulations that govern the flue design include the Construction Products Regulations Mandate 105 ‘Chimney, flues and specific products’ which stipulates that the chimney and flue should be considered as a part of the building. The Construction Products Regulations also require that all chimney and flue products are CE marked. A CE mark shows that the manufacturer has checked that the products meet EU safety, health or environmental requirements. It is also an indicator of a product’s compliance with EU legislation and currently allows the free movement of products within the European market. The CE mark classification should be used to select the correct flue for the chosen application. Flue sizing, to establish the optimum flue height and diameter, should be carried out in accordance with BSEN 13384-1:2015 for single appliances and with BSEN 13384-2:2003+A1:2009 for more than one appliance. The internal diameter of the flue must be matched with the outlet on the appliance. It should never be less than the outlet diameter of the appliance. The appliance manufacturer’s chimney sizing recommendations should always be followed. specflue.com © Schiedel


Building Products March 2018
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