Why aint my chimney drawing?

This is an all too common problem. Sometimes there is a very obvious problem and soloution and other times it can baffle the experts. Some flues work that shouldn't and some flues don't work that should. Is it an art or a science? I'm not qualified to answer but I can give some obvious pointers especially if your in the design/build stage. How flues work is pretty basic the gases from the fire are hotter than the surounding air and therefore rise. As they rise the fire pulls in more air to complete combustion thats why every appliance that burns should have a good air supply through a vent or similar. Not having enough fresh air will effect the draught of the chimney and could lead to carbon monoxide and other gases being fed back into the room and slowly poisioning you. Some people will say how a chimney works when a door or window is left ajar etc and this is that principle in action. Some flues give problems at the start as they are cold and the gases inside the flue cool down too quick and stop rising. Some chimneys, often external chimney breasts are like this but are ok once the fire gets going.

If your using solid fuel you will see and smell this pretty quickly as your room fills with smoke. If it's a gas burning appliance you won't see or smell it but it will be there so do what Duncan Stauart says and check flues and vents regularly and get one of these guys a CO detector.Another common reason for faulty flues is the termination point of the flue it simply may not be high enough or be in a hollow etc. Another getting more common problem is an opening thats too big. Some people try to emulate the big open fires as seen in Period houses and castles like this.


The opening thats standard in Ireland in a finished fire placeis usually 16" or 18" across and 22" high. Before the fireplace is fitted you are looking at a hole in the wall of about 20" across. The flue liner used is often an 8" internal diameter but is often referred to as a nine inch flue and once it's a few metres high the chimney will probably draw with no problems assuming good air supply. the problems we see more and more is that the client now wants an opening to be 2 or 3 foot wide and the same high. This can be done but may need a 10" or 12" flue liner again depending on the overal height of the flue. ideally speak to your builer, architect. or engineer as your building. If your already past that stage you have three choices starting with the hardest.

1. Redo your chimney withe the correct size flue liner. ( big messy and expensive)
2. Attache a high powered fan to the top. ( Expensive and some noise issues)
3. Reduce the size of your opening or install a stove which would in essence be reducing the size of the fire.

Stoves are double edged when having flue trouble. If the issue is too large an ope it may solve it but a stove is 3 times as efficient and therefore has lower flue gas temperatures so may cause a problem but unlikely.

A stove may help but will not solve an inherent problem and will not stop down draught. The vents on the front of the stove that allow air in for combustion will also allow smoke out if the draught of the flue is reversed.
Get an expert to assess the flue before you purchase an appliance.