Northwest Chimney Services

Wood Stoves vs Open Fires

When looking at wood stoves for the first time, some people use their experience of open fires as a starting point, but there are many differences between them. There is also a significant difference between the burning of solid mineral fuels (e.g. coal) and the burning of wood. As fuels, both coal and smokeless fuel require a considerable air supply from beneath and above the grate to burn the volatile gases given off from the firebed. Wood, however, requires very little air from beneath the grate once combustion is established but needs sufficient air in the gas zone above the fire or in a secondary chamber to ensure efficient combustion. An open fire is at best 20% efficient (37% with smokeless fuel) as large volumes of warm air from the room are lost up the chimney along with the smoke from the fire. Modern ‘clean burn’ 1 wood stoves, by comparison, may be around 80% efficient. These stoves can be used to heat single rooms or small houses and are available with outputs from 3.5 kW to 20 kW.

Wood stove technology has been developed to a point where clean burning and efficient stoves are now commonly available. Operating in an almost sealed enclosure, and with a well-controlled and distributed air supply, a number of these stoves are now approved for use in smoke control areas. Several types of wood stoves are available: those designed to heat a single room (with or without a back boiler) or, when an internal boiler is incorporated, those that provide all of the heating and hot water requirements of a house. The use of modern designs results in higher combustion temperatures, and produces better fuel economy and fewer solid deposits than earlier designs. This technology provides the almost complete combustion of the tars and creosotes produced, resulting in a self-cleaning viewing window on the stove door and fewer deposits in the flue ways. For best results wood burning should take place on a bed of embers which trap fragments of charred wood and help to ensure the complete combustion of the fuel. Without an ember bed or a flat fire brick base, partially burned fuel would fall through the grate, resulting in reduced fuel efficiency.

Simple wood stoves

The basic wood stove produces heat in two ways: by radiation from its hot surfaces and by convection from air drawn in around the stove casing which is discharged from slots on top of the casing. Such stoves can generally heat only one room but in a house designed to be open plan, convection currents can move heat to the different parts of the house. Some inset convector stoves have air distribution outlets from the convection chamber, allowing convected heat to be distributed to other rooms. Most of the heat emitted from this type of wood stove is by radiation. However, ‘cool to touch’ convection stoves, which heat primarily by convection and not by radiation, are becoming increasingly available.

Multi-fuel stoves

Some manufacturers market stoves suitable for burning both coal and wood, but such stoves are usually a compromise design between the different requirements for burning these two fuels. A multifuel stove is unlikely to be optimised for either fuel, while stoves optimised for burning one fuel will usually burn other fuels inefficiently. However, some multi-fuel stoves have an additional insert grate for the fossil fuel option to allow efficient operation with both fuels. Problems may arise if both coal and wood are burned together. During combustion coal produces volatile gases containing sulphur while the combustion of even well-seasoned wood produces water vapour. Together, the sulphur and water vapour produce sulphurous acid and sulphuric acid which may corrode cold metal surfaces and shorten the life of the stove. This occurs when water in the flue gases condenses to form a liquid.

Flue liners

While brick chimneys are adequate for an open wood fire they should not be used for a wood stove. Closed wood stoves produce a more concentrated smoke and a higher moisture content in the flue gases. Condensation can occur when flue gases come into contact with a cold chimney surface, resulting in any residual tars and creosotes condensing in the chimney. These can soak into the brickwork, damaging the chimney, and also run back down to the stove causing a fire hazard. For this reason any existing chimney must be lined. Several methods of lining a chimney are available, for example inserting a flexible stainless steel flue liner manufactured specifically for solid fuel use, using a pumped refractory concrete lining or adding a clay or concrete liner to the brickwork. The chimney must be thoroughly sealed against gas escape along its whole length, and against rain ingress between flue liner and chimney at the top. Flue pipes should have the same diameter or equivalent cross-sectional area as that of the appliance flue outlet. Flue liners must always be installed within a brickwork chimney. Where a chimney isn't available a convenient method of constructing one is to use prefabricated, insulated, twin wall chimney components. We also supply and fit gas liners. Any work on a chimney is notifiable to the local authority Building Control Department unless it is carried out by a registered competent engineer in combination with a stove installation.

Even with a lined and insulated chimney it is important to burn only quality dry timber. The use of damp or unseasoned wood must be avoided and the fire must not be left to slumber for long periods (e.g. overnight). In this situation the stove never reaches its design operating temperature, resulting in incomplete combustion and the production of tars, creosotes and high levels of carbon monoxide (CO) in the flue gases. The tars and creosotes then condense in the liner because there is insufficient heat in the flue gases to maintain a temperature above their condensation point. This is to be avoided because an insufficient draw from the flue can lead to CO spilling back into the dwelling with serious or even fatal health consequences.

Flue height

The height of a flue for an appliance rated at less than 50 kW is regulated by the Building Regulations and, as such, will be determined by the local authority Building Control Department. Building Control Consent or a Building Warrant will be required for all wood stoves in order to ensure compliance with the regulations, and that the flue is correctly matched to the stove. Alternatively, a registered competent engineer who has been approved under a relevant scheme, e.g. HETAS, would be able to self-certify an installation. A flue which is too short will not produce sufficient negative pressure in the stove to draw the flue gases from the stove, and could allow these gases (including CO) to escape if sufficient combustion air is not available. Building Regulations require and specify minimum flue height which should be sufficient. A flue which is too tall will result in excess air flow through the stove which can cause combustion to occur too quickly; this, in turn, can cause the stove to overheat. Excess air flow in the flue will reduce the efficiency of the stove by carrying more heat than necessary up the flue. If this situation arises expert advice should be sought as draught stabilisation maybe required.