How Chimneys Work
A chimney breast runs through a property from the very bottom to the very top. On each floor there is usually a fireplace and each fireplace has its own dedicated flue. The flue is a cavity that runs through the chimney breast from just above the fireplace to its own dedicated chimney pot. It is the flue that smoke and emissions from a fire pass through and the movement of these emissions is known as the draw or up-draught. A chimney breast can contain multiple flues running side by side each other as they move up the chimney breast, depending on how many floors the property has.
It is important to understand how your chimney works so you know that it is working properly and safely. The information in the tabs below should help with this. If you any questions, please call us on 01273 726 989 or 07742 829 848.
The draw works due to two simple principles;
1. hot air rises and
2. air moves from a high pressure environment to a low pressure environment and the air pressure at the top of the chimney is usually lower than at the bottom of the chimney
When you light a fire the air inside the chimney warms up and becomes less dense than the air outside and above the chimney. As the warm air rises it draws in cooler air from the room which fans the flames, warms up and again rises.
As heat from the fire is pushed into the room and cool air in the room continues to be pulled into the fireplace, the room warms up. This cycle will continue as long as there is sufficient fuel on the fire and air in the room.
If there is damage inside the flue, perhaps cracks in brickwork, bricks missing etc, you are unlikely to know about it as the flue is completely enclosed and inaccessible. It is usually the only part of a property that is not regularly maintained. It is possible for smoke and emissions from a fire to escape through cracks or holes in the flue wall and enter other rooms in the same property, other flues that are running alongside and from there into other rooms and even into rooms in attached properties. This is why smoke tests are performed, to make sure smoke only escapes where you want it to.
That said a smoke test is no guarantee that a leak wont develop at some point in the future. The only way to ensure smoke cannot leak out of a flue is to have it lined. There are a number of ways of doing this and the most simple and cost effective is to insert a flexible steel lining into the existing flue. For more information on this, please see out Lining Your Flue page.
Why Do They Go Wrong
There are a variety of reasons why your chimney might not be working properly and we have listed the main ones below.
Fireplaces and the flues that serve them are designed to work together, the fireplace size is proportionate to the flue size allowing for a good 'draw' - as warm air rises up the chimney it draws in cooler air from the room which fans the flames, warms up and again rises. If the fireplace and flue have not been designed correctly or changes have been made to the fireplace that mean it is not longer in proportion to the flue, the flue may no longer be able to handle the amount of emissions the fireplace produces. Smoke wont be able to move up the flue quick enough, it will back up and eventually come back into the room.
Vice versa if the flue is wide and the fireplace has been changed to make it much smaller, smoke may move up the flue very slowly and could easily be pushed back down if strong wind can blow down the chimney. Making sure your fireplace size suits your flue size is important as you don't want smoke coming into the room.
A chimney needs to be the right height to function properly and its height is determined by the building its attached to, its position on the building and other buildings or trees around it. If your chimney is too short and your fireplace smokes, you will notice this gets worse when its windy. Again your chimney sweep should be able to offer appropriate advise and help you resolve any problems.
Installing the right type of fireplace or stove, lining the flue and installing an appropriate pot and cowl can resolve these and other issues. Your chimney sweep should be able to offer advice and support with these type of problems. For more information see our FAQ - Why does smoke come into the room?.
Blockages at the top of the chimney or in the flue are a common problem. This could be due to a number of reasons, perhaps the brickwork of the chimney stack or in the flue is old and crumbling. Maybe there is some debris causing the blockage such as a broken pot or cowl, birds nest debris or maybe even a bird.
Blockages can also occur due to thick layers of soot, tar and other residue which reduce the size of the flue to a point where there is no longer enough free area to vent the fireplace properly. Approx. 1/2" buildup can restrict the air flow by around 17% for a typical masonry fireplace chimney. A 1/2" buildup can cause as much as a 30% reduction when using a lined flue.
Having your chimney swept will remove blockages. Your chimney sweep should be able to spot issues that could cause problems and should be able to offer advice and support to help you resolve them. Fitting an appropriate Chimney Cowl may be a simple solution to nesting birds.
Soot and Creosote
Soot and other residue like creosote, a byproduct of burning wood, not only cause blockages as detailed on the Blockages tab, they can mix together and form a tarry substance common called chimney tar. This is flammable and if ignited can burn at a very high temperature. Chimney fires can not only cause very series damage to the chimney, they can spread to the rest of the house and out of the top of the chimney.
Having your chimney swept regularly will remove much of the soot and prevent the build of tar. Tar can’t be removed by sweeping alone and may require a chemical intervention to clean properly.
The type of fuel you use can help reduce soot and other reside. Burning 'green' wood (recently cut and not thoroughly dried out) produces a lot of creosote and water vapour which condense in the flue to form creosote and tar which dries to form a crust that can be flammable and can block the chimney.
You should only burn properly dried or 'seasoned' wood (dried for around a year under shelter from rain but with air circulating). This produces far less creosote and water. If you burn coal you should choose a smokeless or eco coal rather than ordinary house coal. Unlike ordinary house coal, soot from smokeless coal is not flammable and in general smokeless coal produces less emissions but it generally produces less heat and is more expensive. You can find out more about different woods and how good they are to burn on our Fuel Performance page.
The ventilation in your home can affect the performance of your fireplace or stove. As warm air rises up through the chimney it draws colder air from the room into the fireplace. This fans the flames and in turn warms up and rises up the chimney. As heat is given back into the room, it warms up so there is less and less cool air available to be drawn into the fireplace.
Energy efficiency concerns encourage us to make our home as air tight as possible so no heat can escape. This can be a problem if your home is too airtight. If there is no appropriate ventilation at all, cooler air wont be able to get to the fireplace causing the fire to burn sluggishly and 'smoke', in other words smoke from the fire can come back into the room.
If your home is too ventilated, i.e. the air in the house is escaping through many other places, again it is not drawn to the fire. in fact a lot of ventilation elsewhere can create its own draft overpowering the draft in the fireplace chimney. This can cause the emissions from the fire to come back into the room.
As emissions from fires are toxic, they contain smoke, soot and carbon monoxide, you must ensure the room with the fireplace has adequate ventilation, to ensure it burns properly and all emissions can escape safely. Your chimney sweep should be able to advise you how to ensure your room is properly ventilated.