Stoves and chimneys working as one

Dennis Milligan of The British Flue & Chimney Manufacturers Association (BFCMA) explores the upcoming requirements for stoves, and the best design, installation and maintenance techniques to ensure their best performance.

Next year will see Defra introduce the requirement for Ecodesign stoves that have been designed to meet Defra’s new stringent particulate (PM) emission limits. From 1st January only new stoves that meet the Ecodesign emission limits can be put on the market for sale in the UK and Ireland. However, it will still be possible to buy a stove that was manufactured and offered for sale before 1st January, but it is advisable to future proof your purchase and help the environment by buying an Ecodesign stove.

Defra is also making it easier for stove owners to buy dry wood. It has been long known that dry wood burns better than wet, but it is now known that dry wood gives off significantly fewer PM emissions. The small bags of wood that you can buy from supermarkets and petrol station forecourts must have a moisture content below 20%. This has been found to be the moisture limit required to keep down particulate emissions. It also helps to keep the flue cleaner. Look for the Ready to burn logo on the bag.

Flues & chimneys
A stove requires a flue to conduct the products of combustion safely to the atmosphere. To work well the stove and flue must work together as a unified system. When a new stove or flue is installed, it is important that a draw test is carried out to prove that the stove/flue system is working properly. The installer should carry this out when he has installed the stove.

Most residential chimneys/flues rely on natural draft, 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 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.

The words chimney and flue are often used interchangeably here. A simplified way of looking at it is the inner tube that transports the flue gases is the flue. The chimney is the combined inner tube and the outer casing. A clear example is a brick chimney with a stainless steel liner. When constructing a new dwelling the route of the chimney should be thought about at the design stage.

Further reducing emissions
Stove manufactures have employed a number of design features to reduce PM emissions.

One of the ways in which they have reduced emissions is to retain the products of combustion in the fire chamber longer before releasing them into the flue. This needs to be factored into a chimney/flue design as this can reduce the velocity and temperature of the flue gases as they enter the flue. Some Ecodesign stoves may require more flue height.
It is always advisable to check with the stove manufacturer’s recommendations and the chimney/flue 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 chimney.

Insulation & seals
Consistent insulation along the complete length of the flue is important if the stove and flue are to work correctly. The last thing you want is for the temperature of the gases to fall below
the dew point.

There are a number of different materials that can be used for chimneys and flues. Clay, concrete and pumice liners require insulation to be prepared and added on site. Stainless steel 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 provide effective insulation along the length of the chimney. Maintaining the flue gas temperature will enable the flue gases to freely escape to the atmosphere.

When re-opening an existing chimney, it is also 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 is situated. An inadequate supply of combustion air can create problems. 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 can occur resulting in the production of carbon monoxide and, if badly deprived of air, 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 Buildings Regulations. An alarm is required because carbon monoxide is a dangerous odourless and invisible gas.

Chimney/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.

Dennis Milligan is president of the British Flue and Chimney Manufacturers Association