The ideal state should be a deposit of fine soot, grey/black in colour. Less desirable are cornflake-like crispy deposits hanging off the walls of the flue. Anything in the way of a black shiny glaze or worse still, brown or black sticky or liquid deposits, indicate that the flue gases are falling below the dew point and condensing into tar.
In a healthy chimney, deposits should come off easily when a tight-fitting sweeping brush is used to rod the flue: if they don't, it is a clear warning signal that you need to ask for help to find out what you are doing wrong. You are probably either running the stove too slowly, using wood which is insufficiently seasoned, or both. You must then quickly change what you are doing so that such symptoms disappear.
Once you have established a safe pattern of use, you should be able to space out your chimney sweeping to perhaps just once a year. But twice is generally advisable, and in practice it must be swept as often as necessary to keep it clean. Remember the adage, 'A clean chimney never caught fire!'
Never be tempted to let the chimney go from one year to the next without being swept. Apart from the sooty deposits, fly-ash (fine ash carried up with the hot rising gases) may have settled out to a considerable depth at any bends in the flue. You may also have unknowingly acquired a bird's nest or two during the spring, and end up with a room full of smoke and/or a chimney fire come the autumn when you light up for the first time.
And chimneys do not like catching fire under any circumstances. If you have clay liners installed, they will be likely to crack during a fire as they are unable to stand the 'thermal shock', and once they have cracked, any condensation or rain water will be able to soak into the surrounding brickwork resulting in stains and smells. If a stainless steel liner is fitted, you may succeed in buckling or even smelting it -- but in any event it will be ruined as taking stainless steel above red heat changes its metallurgical properties, resulting in accelerated corrosion. And if no liner is fitted, then old brick chimneys, often only half a brick thick, may get hot enough to ignite adjacent timbers.
Points to remember:
- Inspect your chimney regularly -- and change the way you run your wood burning stove if necessary. Ask for professional help if you are in any doubt.
- Have the chimney swept as often as necessary to keep it clean.
- The use of a catalytic cleaner in your stove as a preventative measure will help to break down any deposits and makes subsequent sweeping easier and more effective.