It’s easy to overlook the importance of chimney and flue design, says Dennis Milligan of the British Flue and Chimney Manufacturers Association, explaining some of the basic rules to bear in mind
When constructing a new dwelling, the route of the chimney needs to be thought about – at the design stage. Most residential chimneys/flues rely on natural draught – the pressure difference between cold and hot air – to draw the flue gases up the chimney to the atmosphere. A potential reduction in velocity and temperature could reduce the draw of the chimney or flue. A straight chimney is always the best solution, but where this is not possible, due to the construction 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 be no greater than 45° from the vertical.
One of the keys to a well performing chimney or flue is consistent insulation along the complete length of the flue. The last thing you want is for the temperature of the gases to fall below the dew point. Clay, concrete and pumice liners require insulation to be prepared and added on site. Stainless steel and clay system chimneys are supplied with effective insulation. 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 provides effective insulation along the length of the chimney. Maintaining the flue gas temperature will enable the flue gases to freely escape to the atmosphere.
The height of the chimney/flue is also important. In accordance with Document J, the minimum chimney height recommended for the minimum performance of wood burning and multi fuel appliances is 4.5 metres from the top of the appliance to the top of the chimney. It should be stressed that this is the minimum height – some Ecodesign stoves may require more height. It is always advisable to check with the stove manufacturer’s recommendations. The installer should carry out a draw test when the stove has been installed to make sure it is working correctly.
It is worth noting that there are different types of flue for different flues. For example, the flue required for a gas fire is different to that for a wood burning stove. Condensing appliances require a flue that has been designed to work with condensing appliances.
Installing a flue or an appliance is not a DIY job. In England & Wales you can engage a ‘competent person’ with sufficient knowledge and skill to carry out the job, and the authority to undertake it. The Ministry of Housing, Communities & Local Government (MHCLG) has approved a number of competent persons schemes like HETAS, Certsure and OFTEC, with HETAS the largest solid fuel scheme. A competent person is able to self-certify that their work complies with the Building Regulations.
If you have been looking for a new stove you cannot have failed to hear about the Government’s Ecodesign scheme; the Defra requirement to reduce particulate emissions. Chimney and flues are not part of Ecodesign, however, the flues being sold now are ready for use with Ecodesign appliances.
The purpose of a chimney or flue is to discharge the products of combustion safely into the atmosphere. When re-opening an existing chimney, it is important to check that the flue is sound and has no cracks or gaps. The most common way to create a sealed flue in a defective chimney is to reline the chimney with a flexible flue liner – stainless steel and plastic for gas appliances and stainless steel for solid fuel.
All mineral burning appliances need air to work efficiently and safely. It is essential that the appropriate air supply, as required by the appliance manufacturer and Building Regulations, is provided into the room where the appliance or open fire is situated. An inadequate supply of combustion air can create problems. Situations assumed to be downdraught (and spillage of smoke and fumes back into the room, which can be unpleasant and dangerous), are more frequently caused by insufficient provision of combustion air. Never block air vents installed to provide combustion air. If the appliance does not get all the air it needs to burn the fuel efficiently, incomplete combustion will occur resulting in the production of carbon monoxide and, if badly deprived of air, copious volumes of soot.
All wood burning and multi-fuel appliances should have a carbon monoxide alarm fitted within the same room as the appliance. The carbon monoxide alarm should comply with BS EN 50291-1:2010, and must be installed to the manufacturers’ installation instructions, and current Building Regulations. An alarm is required because carbon monoxide is a dangerous but odourless and invisible gas.
Chimney and flues require little maintenance. The one essential job is to sweep the chimney/flue on a regular basis. The frequency of sweeping depends on the usage of the appliance. However, as a minimum, the chimney should be swept at the start of the ‘heating season.’
It is also good practice to check at least every year or two the exposed parts of a chimney, flashings and terminals for signs of damage. Just like the outside of a house, chimneys can suffer from the wear and tear of extreme weathering.
Dennis Milligan is president of the BFCMA