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Chimneys, flues and wood burners

Renovating a property sometimes involves re-instating an existing fireplace and perhaps installing a new appliance such as a wood burning stove. In most cases planning permission isn’t required, unless you want to construct a new chimney stack. However, in Building Regulations terms, new chimneys are basically regarded as small extensions, so adding a new one will require consent. But the potential for life-threatening dangers from fire and toxic fumes means that even if you are installing a stove in an existing fireplace or lining a flue the work must comply with the Building Regulations (Part J deals with combustion appliances). Note that, by law, the installation of heat producing gas appliances (e.g. gas fires) must only be carried out by a Gas Safe registered engineer, who must notify Building Control.

Chimneys

Because chimney masonry is highly exposed to extreme heat and potentially corrosive flue gases it tends to require more frequent maintenance than the main walls lower down the property. Work should be carried out using suitable access equipment. There are several areas where periodic attention to chimneys may be required:

Flaunching

The flaunching is the mass of mortar at the base of the chimney pots that helps secure them in place. But after many years of exposure, flaunching can eventually crack and disintegrate.

Masonry repairs

Modern stacks incorporate a Damp Proof Course (DPC) in the lower courses to prevent any risk of water soaking down through the masonry below the roof level. But old stacks were built of relatively soft brick without a DPC. So damp can sometimes penetrate masonry just below the roofline which can suffer frost erosion and need localised repairs.

Flashings

Many leaks at roof level occur at junctions to stacks. Traditional lead flashings are the most effective method to prevent this. If your property has mortar fillets, they are best replaced with leadwork as the mortar is very prone to cracking.

Redundant stacks

Disused chimneys should be capped to restrict the ingress of rain, and also properly vented to prevent damp from condensation forming within old flues.

Structural support

Old stacks are rarely perfectly vertical, so a small amount of leaning is not unusual. However where significant movement has occurred, the best advice is to consult a structural engineer. ‘Stay bars’ are the traditional way to secure tall or exposed chimneys, which today take the form of stainless steel tie rods and straps. Where severe distortion has occurred to a stack, the only option may be to rebuild at least the upper courses of brickwork.

Chimney pots

Where a pot is missing, or has cracked or badly spalled it will need to be replaced. To protect flues from ingress of rain, a simple stainless steel ‘rain hat’ with an integral bird guard can be fitted.

Flues

Flues are basically vertical exhaust pipes enclosed by the chimney breast and stack masonry. They are designed to safely transport smoke and combustion gases out to the external environment, based on the principle that warm air generated by fires naturally rises. Often multiple flues are accommodated within a single chimney with thin internal partitions. Smoke containing combustion gases from fires is potentially dangerous to human health – both from breathing poisonous fumes and from the risk of fire. Leaks can sometimes go undetected, particularly within lofts or to adjoining houses. And if a flue becomes blocked – e.g. with a nest – a build-up of poisonous combustion gases will blow back, re-entering the room. It is even more dangerous where unlined flues are used for gas or oil-fired appliances that produce deadly, odourless carbon monoxide. So it’s essential to check the condition of flues before lighting fires. The airtightness of a flue can be tested using smoke pellets.

Flues fail for 2 main reasons:

  1. The acrid chemicals released in combustion gases eventually eat away at mortar joints causing cracks or holes that can allow poisonous gases to escape into rooms, lofts or adjacent flues. To prevent such dangers, active flues must be lined with the correct type of flue liner.
  2. Over time, soot and tar can build up inside flues, eventually igniting causing chimney fires. The solution is to have chimneys swept regularly.

Damp staining

Damp from rain pouring down un-capped chimney pots, or from condensing gases, can soak into loose internal masonry and bleed through chimney breast plasterwork causing ugly stains. Regular sweeping can help prevent this.

Disused flues

In most old properties today, some of the original fireplaces will be disused and boarded up. But unless chimney pots have been capped off, rain can come trickling down redundant flues causing damp patches (also caused by moist air condensing inside disused flues). Redundant pots should therefore be capped off with a vented hood fitted to the pots, with a vent also fitted to the boarded up fireplace below to encourage a healthy through-flow of air.

Structural alterations

In some properties one or more chimney breasts have been removed to make more space. Where the work has been done without Building Regulations consent, the remaining masonry above (leading up to the main stack) is often potentially dangerous as it is not fully supported. So unless a completion certificate was obtained for the work, it’s advisable to contact a structural engineer to verify whether it is safe.

Air supply

The provision of a sufficient amount of oxygen both for occupants and the efficient combustion of fires and appliances is a key part of compliance with Building Regulations. In houses where draughts have been totally sealed up this may require additional air vents to be fitted to the main walls for open fires and gas appliances - for example, in the form of 230 x 230mm airbricks inserted in the wall.

Smoky fires

There are several reasons why smoke can fail to safely disperse from a fireplace. Some chimneys are inherently too cold to draw well, particularly those on outside walls. Smoke will disperse more efficiently where internal flue walls are smooth with a flue liner. Smoky fires can also be down to blocked or damaged flues (hence the need for regular sweeping), a lack of indoor air supply, or stacks that are too short or overshadowed by surrounding buildings.

Lining flues

It’s essential that flues are lined, especially if you plan to install any gas, oil or solid fuel appliance in an existing fireplace because masonry flues are inadequate to cope with these more aggressive exhaust gases. Even where a flue is already lined, old steel liners eventually suffer from corrosion. So it is a requirement of the Building Regulations that when an appliance is changed the installers must check the condition of the flue to confirm that it’s clear of obstructions and is suitable for the appliance you plan to install. Flues should always be swept before fitting a stove or lining a chimney, and if necessary a smoke test carried out to check for gas tightness. In addition, gas and oil appliances need special chimney-top terminals to prevent blockage from birds or debris, and to help disperse gases. It’s essential that the flue liner is of the right type for the fire or appliance served. Flexible stainless steel liners are most widely used type as they are comparatively easy to install. Building Regulations recommend a minimum diameter of 150mm. Flexible single skinned liners are used mainly for gas fires and for oil or gas-fired boilers, whereas hardier double skinned liners are required for burning solid fuels and for wood burning and multi fuel stoves. The downside with flexible liners is they need renewal every 10 to 15 years, depending on how regularly the fire is used.

Wood burning stoves

Wood burning or multifuel stoves are far more effective at heating rooms than open fires, and also boast impressive green credentials. Note however that if you live in a smoke control area, you’re not allowed to burn fuels such as wood or coal that emit smoke, either in stoves or fireplaces (although smokeless fuels such as coke briquettes are permitted). Fortunately, some ‘cleanburning’ stoves approved by DEFRA are exempt. These produce low emissions when wood burning, and some are also approved for use with coal. Stoves can generate enormous heat within the flues and must never be used with flues that aren’t lined. Flue gas temperatures in modern high efficiency stoves can exceed 300 degrees and unlined stack brickwork can allow as much as 85 percent of this to pass through it. This intense heat can cause adjoining thatch or roof timbers to ignite. This is why it’s now a condition of installing new appliances that a suitably sturdy flue liner is fitted together with a register plate. Also, a carbon monoxide (CO) alarm must be provided in the room where the appliance is located (recommended positioning can be checked with Building Control). As an additional precaution in thatched properties temperature sensors linked to an alarm should be fitted. To maintain sufficient air supply to the room, additional air vents may be required to the main walls. Prior to commissioning the new stove, the installers should test the flue.