History of Chimneys
Early home dwellers usually lived in structures with central fireplaces but without chimneys, allowing the smoke to find its own way out, just as nomadic Tibetans, living in tents called yurts, continue to do to this day. The main difference is that while the smoke from early dwellers fires drifted through incidental holes in the structure, the smoke from a yurt rises because it is heated. It passes to the top of the tent from where it is withdrawn partly by convection and partly by the Venturi effect. In that sense, the yurt acts as its own chimney, which means that these well-smoked people effectively spend their indoor lives living in a flue!
Earlier chimneys, found in single storey wooden-built structures, were centrally sited pretty much as a way of extending the hole in the roof down to fire-level. But when double-storey wooden buildings came to prominence, fireplaces and thus chimneys began to be situated on an outside wall.
In central Europe, tubular chimneys built of masonry were used as early as the 11th century. The technology spread like, well, wildfire and as rapidly as 100 years later, chimneys were commonly found in most secular buildings.
How Chimneys Work
Jonathon Greenall, the man behind Clearview Stoves, says, “You can run a bad stove on a good chimney but not a good stove on a bad chimney. That’s how important it is that a chimney works properly. First, you need to know what makes a chimney do its job. In other words, why does the smoke go up the chimney rather than drift out into the room?”
The upward movement of air and smoke in a chimney is known as the draft and we talk about how well a chimney draws.
Draft takes place because of the physical fact that hot gases are less dense than cold gases. Heated gases in a chimney are lighter than the air in the atmosphere and are, therefore, drawn up into it.
All other things being equal, a taller chimney produces more draft because of the simple fact that there tends to be more of this hotter, lighter gas in it, which creates more updraft.
All other things are rarely equal, though. The increased draft inherent to a taller chimney tends to be offset by the greater friction found in the chimney and by the tendency of the gases to lose their heat as they rise. A very dirty, roughly-lined, tall chimney, subject to poor insulation and possible air leaks could well be less efficient than a well designed shorter chimney.
Another fact of physics has an effect – albeit a lesser effect – on chimney performance. As air is blown over the end of a tube, a drop in pressure is created at the end of the tube. This principle was first discovered by an Italian, GB Venturi, almost 200 years ago and is the principle by which most carburettors inject petrol into the air stream. Wind blowing over a chimney has a similar effect, adding to draft. Wind eddying down the chimney has the opposite effect, of course.
It’s not often you’ll see this recommendation in print but it is a good idea for you to sit down and have a good read of a government document with the not-so-catchy title of The Building Regulations Approved Document J and the UK Implementation of European Standards for Chimneys and Flues.
It’s a surprisingly clear and interesting read and is about 70 pages in length (so there’s obviously not room for it here), with lots of drawings and useful ideas, as well as sensible and practical safety-related legislation. It’s essential reading if you are thinking of building or installing a chimney, fireplace or stove.
It is available for downloading from the Planning Portal (planningportal.gov.uk). Alternatively, hard copies of the document can be obtained from The Stationary Office, London 123 Kingsway, London WC2B 6PQ. Tel: 0870 600 5522. Or you can order online from The Stationary Office at tso.co.uk.
Chimney Types and Materials
Brick has often been used, even in stone buildings because, having already been fired, it is much better able to withstand intense heat.
The standard domestic chimney for a coal fire became 9″ x 9″, or one brick square, now rationalised so that the chimney in a new house has to be at least 225mm in diameter or the same, good old 9″!
An unlined brick flue is a shoddy piece of work. A lined but uninsulated flue is not all that much better. A smooth, insulated flue made from corrosion-resistant materials is essential if a chimney is to work efficiently and to last.
An efficient flue will cause less smoke blow-back with an open fire but can’t transform an inherently inefficient device. A stove far more efficient will produce much better results from an efficient flue.
Someone wanting to add a chimney where one was not built into the original structure could have a traditional structure built on the inside (this could be complex) or outside of a suitable wall or could add a stainless steel, insulated, purpose-made external flue.
Types of Flues
The type of flue you install (or already have) will dictate the type of fire you can fit, so it is important for self-builders to choose their fire first and match the flue to this. Alternatively, simply install a Class 1 flue which will be able to handle almost any type of fire.
- Class 1 Flue: A brick, clay, precast pumice, or occasionally metal flue of a minimum internal diameter of 178mm (7″). This type of flue will usually accommodate any type of open solid fuel fire or an open-flued gas fire.
- Class 2 Flue: A clay, precast concrete, pumice or metal flue designed for certain types of gas fire or solid fuel stove. Due to the narrower depth and smaller cross-sectional area of the flue, not all fires can be installed in this type of flue.
- Balanced Flues: A balanced flue exits from the back of a gas fire and obtains its air for burning from the outside and in turn expels the products of combustion through the same flue.
- Fanned flues look similar in appearance to balanced flues but take their air for burning from the room in which they are installed. Electrically powered, they can be located away from an external wall. Both options are a good choice where there is no useable chimney.
Installing a New Flue
The best way to install a new flue in, say, a chapel conversion is to build a central fireplace and a flue right through the centre of the house, bearing in mind the Building Regs. with regard to surrounding flammable materials. The flue stays warmer, heat is more efficiently used throughout house and the top of the flue is nearer to the ridge.
Keep a stand-alone flue as straight as possible. Every bend adds friction and sharp bends are worst of all.
Even an insulated stand-alone flue may smoke horribly in the middle of winter if on the outside of a building.
If you’re building a new house with an open fireplace, don’t fit a concrete lintel too far back in the flue opening because this makes it impossible to fit an insulated flue liner later. Also, beware a builder who wants to fit a square liner. It will touch chimney bricks all round and cannot be properly insulated.
Slow-moving gas, like water in a meandering stream, will deposit solids on the insides of the chimney. If you see smoke rolling picturesquely out of the top of your chimney, it’s too slow! A warmed flue liner improves momentum.
The chimneys in Clearview Stoves showroom are cleaned about every 10 years but some peoples chimneys have to be swept every couple of years. Poor combustion can involve 50 times more smoke going up the chimney and unburned creosote and soot will stick to the sides of the chimney.
The Ratio Problem
For an open fireplace to function properly and for the chimney to draw, the ratio between the chimney and fireplace needs to be exactly right. Flues above 6m tall should generally be not less than 1/7th to 1/8th of the area of the fireplace opening, e.g. a 225 mm (9″) diameter flue will support a fireplace opening up to about 550 x 550 mm (22″ x 22″) (see Building Regulations J 2.2.) For bungalows, the ratio should be reduced to 1/6th. A major advantage of precast fire chest and chimney flue systems is that the ratios have been carefully worked out and so are guaranteed to function properly.
If you are building a new chimney, take note at what works for the surrounding properties. They will already have been built or adapted for the local environment.
The main cause of smoke coming into the room from an existing chimney that once worked is a blocked flue. The solution is to get the chimney swept. Insufficient air coming into the room can also cause blow-back. Bear in mind that, traditionally, houses had lots of draughts while today they are often too airtight for an open fire. If you dont give the fire sufficient oxygen, it will find its own, pulling great gulps of air down the chimney and belching smoke out into the room. If necessary, add a vent to an outside wall adjacent to your stove or fireplace.
When fitting a liner, joints should have their male spigots facing down. The idea is not to make sure all the smoke goes up it will if joints are properly sealed but that water running down gets right to the bottom where it will be boiled away again.
Did you know?
Chimneys were traditionally lined with parging, a render mix used to prevent gases escaping through mortar joints and cracks. Parging was usually roughly carried out and is often simply the same mix as the brick mortar. However, as recently as 50 years ago, a recommended mix consisted of lime, sand and ox hair or cow dung!
Thanks to Jonathon Greenall and Helen Davis at Clearview Stoves (01588 650401) for their practical advice regarding flues.