Open Fires, Chimneys and Flues

Chimneys were not widely used in domestic buildings until brick became available in the late medieval period. Before this, most dwellings used open hearths, with one large fire typically located in the centre of the building for both heating and cooking. The smoke would billow around the inside of the building before escaping through gaps or louvres in the roof.

In timber framed buildings a shorter bay was sealed off from the rest of the building with wattle and daub infill panels to allow for smoke to escape, this was known as a ‘smoke bay ’. Some early chimneys in solid wall buildings were constructed of timber framing with a wattle and daub infill known as a ‘smoke hood’. If they are found in-situ or even in use, they are important and rare survivals of historic fabric which should be conserved. The earliest brick chimneys were often built over open hearths.

As comfort standards improved and buildings became more subdivided, fireplaces were required in more areas of the building. As fireplaces became smaller and more efficient, so did chimneys. In the 18th and 19th centuries they became more complex, often combining several flues, each serving a separate fire.

The divider between flues within one chimneystack is called a withe. Withes were often slender, sometimes a single thickness of brick laid on edge, sometimes thin cut stones or occasionally a single layer of slate. This makes them vulnerable to damage when a chimney is swept, particularly at bends and other complex flue configurations.

Most brick flues are lined to prevent gases escaping through joints and cracks. The traditional method of lining chimney flues was to apply a mix of lime putty and fresh cow dung, known as ‘parge’ as the chimney was built. Parging continued to be common until the 1965 Building Regulations introduced a requirement for all new flues to be built with a liner during construction. During the 1960s the most commonly used flue liner was clay pipes, though with further tightening of the regulations, concrete and metallic flue liners took over.

Chimney pots did not come into common use until the 17th century and early 18th century. Most pots seen today usually date from the 19th century or later.

Care should be taken not to lose historic fabric during the opening up of old flues, or during the process of making alterations to flues in historic buildings. The fabric of chimneys in historic buildings and debris that accumulates in disused chimneys can be of archaeological importance, but soot and byproducts of combustion can also pose health hazards (via inhalation or skin contact). Precautions should be taken when working in these confine spaces, using appropriate personal protective equipment (PPE). Listed Building Consent may also be required.

Before returning an old chimney to use, its condition should be checked thoroughly including that of any withes. If a visual examination is not possible, the use of a borescope or CCTV should be considered. For larger buildings where there might be several flues and chimneys, wall penetrating radar could be used to identify voids. Even if the flue is to be lined, ensure that any necessary repairs are made before the liner is inserted.

If there are any concerns about the structural integrity or watertightness of a chimney, a structural engineer, surveyor or architect should investigate. The disturbance caused by the insertion of a flue liner and the stresses caused by the heating and cooling cycles can exacerbate any existing problems. A survey is particularly important if the chimney is to be reused for an open fire without a lining; the stack will be subject to greater heating and cooling cycles and to chemical attack from compounds in the smoke.

A 'room heater', an enclosed appliance in which fuels are burnt, can be a suitable alternative to an open fire in traditional buildings. A wide range of designs are available. They burn fuel much more efficiently than open fires, cause fewer draughts and can often help to reduce carbon dioxide and particulate emissions, when compared to open fires. Gas fired versions are also available.

Some room heaters are recessed or semi-recessed into a wall, often in an existing fireplace; others are freestanding away from the wall. Some recessed models have back boilers which can supply hot water or central heating. Freestanding stoves are types of room heater. Certain room heaters are certified as being able to burn wood and coal without creating smoke, allowing wood to be burned in smoke control areas.

Room heaters or stoves must be installed by a 'competent person'; for example, an engineer registered with HETAS for solid fuel, OFTEC for oil or Gas Safe for gas. The flue gas temperatures from a room heater can be significantly more than from an open fire so care is needed to reduce the risk of fire or heat damage. A flue liner will always be required, and if a flexible metal liner is used it should be double skinned. Some chimneys may not be capable of accommodating an adequate liner and so will be unsuitable for a room heater.

Any fossil fuel-fired fire or stove, new or existing, needs ventilation to feed combustion. Check carefully as draught-proofing measures and other works may have reduced the air supply available for the fire. With insufficient ventilation, the chimney may not 'draw' properly and smoke and/or invisible odourless carbon monoxide may spill out into the room.

Carbon monoxide detectors must be installed to warn against the build-up of this toxic gas, but care is needed to position and maintain them – consult the manufacturer's instructions, Approved Document J or seek expert advice.

Not all chimneys can accept a new flue liner. Flues in traditional buildings are sometimes narrow, contorted or distorted by structural movement over the years.

Attempting to force a flexible metal flue liner down such chimneys will not always be successful and risks damaging historic fabric. Flexible metal liners also do not have a long life and will need to be inspected and replaced when necessary.

The space between the original flue and the new flue liner should be ventilated top and bottom to prevent condensation. Flue liners that require backfilling between the liner and the chimney should be avoided in traditional buildings.

Large fireplaces often have chimneys which are too large for modern needs. If so, a register plate can be installed to reduce the size of opening at the base of the chimney flue. This is normally a horizontal fireproof (metallic) plate with a central opening to allow the fire to be used and to which a flue liner can be connected above. Sometimes a hood is attached to the underside of the register plate to guide smoke into the flue from a large hearth with open fire. Register plates above open fires (other than gas effect fires) should be provided with dampers (see below). Disused fireplaces may be fitted with a register plate with no opening, often with small holes to provide necessary ventilation.

A chimney for an open fire will cause significant draughts when it is not in use. A good remedy for this is to fit a damper, which can be installed either in the throat of the chimney, the flue or at the top of the chimney. The best types are easily opened and closed using a handle that also indicates the position of the damper flap. Dampers should not block all the air entering the chimney as a small permanent flow is necessary to ventilate the flue when it is not in use. If a gas appliance is to be used any damper must be fixed in the permanently open position.

Chimneys in use should be swept regularly. This will help reduce the risk of poisonous gases spilling into occupied rooms, help prevent fires within chimneys and enhance the operation of the flue. The National Association of Chimney Sweeps recommends the following:

• Smokeless fuels - At least once a year
• Bituminous coal - At least twice a year
• Wood - Quarterly (when in use)
• Oil - Once a year
• Gas - Once a year

All gas appliances, including the flues, should be checked for safety at least every twelve months by a Gas Safe-registered installer. It is a requirement that landlords letting property should have all gas appliances and flues checked annually.

Whether the chimney is being used for a fire or not, a ventilated cap at the top of a chimney will prevent water, insect and bird ingress without preventing ventilation. Where a pot is not in place, there must be a protocol for dealing with birds and insects that might have inhabited the chimney.

The material of the cap and its installation should be assessed against its exposure to wind forces and ensure it complements the existing pot.

Approved Document J covers the installation or reinstatement of combustion appliances and fuel storage systems. More information on how this regulation affects historic buildings can be found here.

Building control consent is required unless the installation is undertaken by someone under the competent person scheme.

All works should be undertaken in line with Gas Safety (Installations and Use) Regulations 1998. Listed buildings are expected to comply with this regulation, even where they do not have to comply with Approved Document J.

The Clean Air Act of 1956 gave local authorities the ability to declare smoke control areas. In those areas it is an offence to emit any smoke from a domestic chimney, except for the very small amounts from burning authorised 'smokeless' fuels.

There is an exemption for appliances approved as being able to burn bituminous coal and/or wood without creating smoke. For further information about:

• smoke control legislation
• smoke control areas
• authorised fuels, and
• exempted appliances

or to check whether a particular building is within a smoke control area contact the local authority, visit Smoke control areas: the rules, or use the Smoke Control Area Interactive Map.

Under the Air Quality (Domestic Solid Fuels Standards) Regulations 2020 and the Clean Air Act 1993 certain solid fuels are certified for use in England.

It is best practice to ensure that a smoke test in accordance with BS 6461 is undertaken to ensure the appliance and flue have been installed correctly.

Chimneys can be key features that contribute to the significance and character of historic buildings. They may also fulfil structural functions. In listed buildings consent is needed to remove chimney pots and dismantle chimney stacks. In conservation areas, chimneys are also frequently subject to planning controls.

Removing internal chimney breasts is best avoided if possible. Such work would need structural engineering advice, Building Control approval and, if the building is listed, Listed Building Consent.

All disused flues require regular maintenance. The greatest risk is from bird nests or other debris entering the chimney from above, or from building debris (old parging or broken withes) from the flues themselves. Either form of debris can block flues in unexpected places, leading to damp, smells, staining and other problems. The simplest way to check for and clear such debris is to have the disused flues swept.

Venting externally

When a disused chimney is sealed this should ideally be done with a hard capping and vented externally both at its lowest point and highest level.

Internal to external ventilation

If open to the building's interior, a flue can use a vented cap at the top and be partially blocked at its base to restrict airflow but provide adequate ventilation to prevent moisture build-up. This can be achieved via a balloon, closing an existing damper or by installing a register plate with ventilation holes. Balloons are less appropriate for occasionally used flues. Removing and reinstalling them can be awkward and dirty and they will melt if left in place accidentally. A better solution is a damper, although these are costlier and more disruptive.

Some warm air will continue to be lost up the flue. Draughts and wind noise will not be eliminated. There may be a risk of condensation inside the flue as warm moist air from inside the building flows through it, particularly when it is cold outside. Internal to external ventilation is best suited to internal chimneys rather than those on the outside walls of buildings.

Venting internally

Where the chimney stack above roof level has been dismantled, internal ventilation might be provided at high-level into the upper floor room or into the roof void. Through-ventilation is then provided by warm internal air. This can lead to problems if the chimney is on an outside wall, as condensation can readily form on the cold outer skin from moisture-laden internal air. If the flue is sooty ventilating it internally can lead to unpleasant smells. Internal ventilation is not suitable where the upper portion of any chimney stack is still in place as the upper portion, which is then lacking any ventilation, can become damp.

A disused flue, that is no longer needed for ventilation purposes, might provide a good place to run services, mechanical ventilation ducts or the flue from a new energy efficient boiler. This would need a competent person to assess if the chimney and flue are compatible with the intended use, that no damage would be done as a result, that maintenance and access is still possible, and that adequate ventilation of the flue itself can still be maintained. Gas Safety Regulations requires that the flue is accessible throughout where there are joints for routine annual inspection. The same risks of damaging historic fabric are relevant here as for sweeping and inserting flue liners.

If a disused flue is vented from inside to outside it can provide useful 'trickle' ventilation, for example in rooms with well-fitting windows that allow only limited air infiltration. If a suitable flue runs past, extra vents can even be provided for rooms that did not originally have fireplaces. When used to provide ventilation it is important to ensure that any chimney is clean of combustion debris.

In a domestic building, disused flues could house ducts for extractor fans from kitchens and bathrooms running to a discreet airbrick on the back of a disused flue. This can often provide a less intrusive solution than a cowl projecting through a roof line or a vent through a façade.

Disused flues can also be used to house ventilation fans, for 'fan assisted' extract ventilation. Fans can be housed within the opening to the flue and connected directly, run the full distance of the chimney and sealed around the point of discharge. Thought needs to be given to if a fire damper would be required and access to the fan for maintenance is required.

When venting extractors into a chimney, make sure that an air duct runs the full distance along the flue and is sealed around the point of discharge, for example the air brick. Otherwise, moist air venting into the flue could condense, causing damp and staining problems.

In any of the above cases, the same risks of damaging historic fabric are relevant here as for sweeping, inserting flue liners or using them for service runs.