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HEATING, VENTILATION & SERVICES
Requirements of a wood burning stove Wood burning stoves require a flue/ chimney. The purpose of the chimney or flue is to discharge the products of combustion safely to the atmosphere. To work effectively the stove and the flue/ chimney should be considered as a complete system. The routing of the chimney/flue should be planned at the design stage. Most wood burning stoves and chimneys operate under negative pressure, relying on natural draft to transport the flue gases to the atmosphere. This requires a clear path to the top of the flue. A straight chimney/ flue is always the best solution but where this is not possible, to suit the design of the dwelling, the number of bends should be kept to a minimum and not exceed more than four. The angle of the bends should not be greater than 45° from the vertical, with the exception that 90° factory made steel bends or tees may be treated as being equal to two 45° bends. Horizontal lengths should be avoided.
The draw in the chimney/flue is even more important with the advent of Ecodesign stoves. One of the ways in which stove manufacturers have reduced emissions is to retain the products of combustion in the fire chamber for longer before releasing them into the flue. This needs to be kept in mind when designing a chimney/flue, as holding the flue gases longer in the fire chamber can reduce the velocity and temperature of the flue gases as they enter the flue. It is always advisable to check the stove manufacturer’s recommendations and that of the flue/chimney company. BFCMA members can advise on chimney design and installation. This includes using industry standard software to check that the flue design will create the required draw in the flue/chimney.
It is also important that the temperature of the gases are maintained above the dew point as they travel up the flue/ chimney. Good flue/chimney design requires consistent insulation along the complete length of the flue; clay, concrete and pumice flue liners require insulation to be prepared and added on site. Stainless steel system chimneys are supplied with effective insulation, and double wall pumice chimney systems have an air gap between the walls of the inner and outer blocks – the air gap combined with the natural insulating properties of pumice provide effective insulation.
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Although not physically connected, heat pumps and stoves can be viewed as an effective hybrid system
Environmental concerns alleviated The lower emission limits for wood burning in Defra’s Environment Act could make it difficult for new open fires to meet the lower emission limits. It has been common practice to include 200 mm flues and a standard fire opening in detached houses, leaving the choice of appliance to the homeowner. Wood burning stoves require a smaller flue diameter, typically 150 mm. Installing a smaller diameter flue also has the benefit of substantially reducing the air loss in SAP. SAP does not include secondary heating in the ‘notional building.’ However, this does not mean that a stove with a flue/ chimney cannot be added to the house design. Using the smaller flue diameter will significantly reduce the air loss calculation. There have been concerns about particulate emissions (PM) from wood burning. The Environment Act sets significantly lower emission limits for wood burning stoves than in the past. Many stoves already produce lower particulate levels than the Defra limits. The independently verified scheme, Clear Skies, identifies stoves that not only meet the emission limits required by Defra but also stoves that produce fewer emissions. The sustainable nature of wood burning comes from the fact logs are generally the off cuts that come from pruning trees. Woodlands need to be managed to stay healthy and part of that management is pruning. Interestingly, burning wood in a modern stove will produce less carbon than if it was left to decay on the woodland floor. Including a wood burning stove with a heat pump adds value to a house. If you consider doing so, remember the flue/ chimney requirements and incorporate them into the house design. The BFCMA is the UK’s only trade association representing the chimney and flue industry and works closely with Government, public bodies and other organisations to further the interest of the chimney and flue industry. It represents the majority of manufacturers and sole distributors in the industry.
Dennis Milligan is president of the BFCMA ADF SEPTEMBER 2023
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