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The Design Of Chimneys and Flues

Chimney and Flue Design

How do we get the smoke to go up a chimney? A properly built chimney will naturally "draw" the rising hot air roaring up the chimney which stops smoke coming into the room and as a result the fire burns better by sucking air and oxygen through the fire.

To make the chimney draw it has to be designed properly with a constriction, a narrowing of the flue. A chimney without a constriction will not draw but can easily be modified to get it working correctly. The scientific principle is known as the venturi effect.


Getting a flue or chimney to "draw"

I met a man who had a fireplace built in his new house and was told by his builder that he would need an electric fan in the chimney to draw the smoke out. A fan? That doesn't sound right does it, chimneys have been in use for thousands of years and they didn't have electric fans back in mediaeval times now did they?

There is in fact an ancient art to getting a flue to draw. There used to be skilled artisans who would travel around the countryside setting people's flues so that fires would burn properly, the smoke would go up the chimney and not come out into the room.

These days of course the science is well understood.

The Venturi Effect

The secret to getting a flue to draw is to have a constriction in the chimney. This work on the venturi principle and is named after an Italian physicist Giovanni Batista Venturi (1746 - 1822). Air moving up the chimney has to compress to get through the gap of the constriction, once through the chimney opens out and the pressure in the rising air falls causing a partial vaccum. This area of lower pressure sucks air into it through the gap. The air passing up through the constriction and on up the chimney speeds up and the flue starts to draw, pulling air up the chimney.

This venturi concept is used in Carburettors to suck air and petrol into a car engine and drawing air in to mix with the gas in the burners of your gas stove. Have you ever noticed that power station cooling towers are narrower in the middle? The Venturi principle is also the principle that makes yacht sails and air craft wings work, the curve in the upper surface of an aircraft wing makes air travel over the wing faster than under the wing causing lower pressure on the upper surface which sucks the plane upward. The ability to lift the weight of an aircraft demonstrates the power that the venturi principle can generate.

So above a fireplace we put a chimney with a constriction above the fire. Getting the constriction just right used to be a real artform. These days it is a science and we see the venturi principle used in many ways such as pulling petrol into your car engine using a venturi in the design of a carburretor and drawing air in to mix with the gas in the burners of your gas stove. Have you ever noticed that power station cooling towers are narrower in the middle?

Now I am sure you are reading this and thinking what on earth is the relevance of all this. Well I have come across people that want a fire place however they find it almost impossible to find a person with the knowledge to build a flue that works correctly. People don't understand and hence the builder who thought that you need to install a fan.

Setting a flue

If you have a chimney that doesn't draw properly or you wish to build one you need to build the flue with a narrow part above the fireplace at the bottom of the chimney. It is a good idea to make it in such a way that you can easily modify it later if you need to just in case you have not got the correct amount of constriction.

Start off with a constriction that is half or two thirds the size of the rest of the chimney. Light the fire and try it and adjust it as necessary.

You need to know that:

  1. If the constriction is not narrow enough the chimney will not draw well.
  2. If the constriction is too narrow the chimney will draw too well and the wood or coal you are burning will burn away far too quickly.
  3. If the constriction is even narrower it will slow down the amount of hot air that can get up the chimney and will slow the fire down.

You need to find the sweet spot with a good compromise or you could install a "Damper."

Dampers are used to control fires

In the days when most houses had fireplaces and chimneys the chimney would usually have a "damper" installed to change the size of the constriction in the flue and in this way control the fire. A damper was either a piece of cast iron (cast iron withstands the heat better than steel) that slid across the flue or turned like a butterfly valve in the flue to restrict the flow. As you increased the constriction you could increase the amount of draw and get the fire roaring - very useful when first lighting the fire or quickly heating the room up. But as you increase the constriction further you would reach a point where further constriction would "dampen" the fire slowing down the rate of flow up the flue and this would save fuel, it would make the fire burn slower and use less coal or wood. This is how they managed to keep a fire going all night.

Venturi flues can keep your house cool.

More importantly this same venturi principle can also be used to ventilate roof spaces and keep houses cooler.

In Queensland, Australia the old house builders used to install a small vertical chimney with a decorative cowl on the top in the centre of the highest ridge of a roof. The vents were about 300mms in diameter and perhaps 800 mms long with a smaller diameter of pipe in the middle to form a constriction.

The roof space under a corrugated iron roof can get very hot and has very few gaps to allow ventilation so the old house builders came up with the idea of these vertical roof vents and found they were very effective. The vent would draw hot air out of the roof space drawing cooler air in under the eaves along the bottom edge of the roof. The constant air flow through the roof space would take away the heat and keep the house cool.

These days many houses have tiled roofs and roof space ventilation is not an issue as there are usually lots of gaps between the tiles which allow the air to circulate freely. However if your roof is fairly well sealed and has a lining under the tiles or if it is corrugated iron, colourbond, asbestos or, as I saw recently, copper sheet, an effective roof vent may be a sensible option.

The thing about using a venturi is that this natural phenomenon can be used to create air movement even on the stillest of days. It is very simple, very low cost and with no moving parts to go wrong.

It is early days yet but as initiatives to combat global warming combined with the high cost of energy start to really take off we may well see some very clever developments using the venturi effect to help keep our houses cool.

Copyright © Phil Wilson August 2008
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