Why do I need to have my chimney swept ?
The short answer is because your life and property are at risk if you don't.
2. WHEN AND HOW OFTEN SHOULD I HAVE MY APPLIANCE SWEPT?
The Health and safety executive bodies Recommends that chimney sweeping should take place at least:
Once a year for appliances burning Smokeless fuels
Once a year for appliances burning Gas
Once a year for appliances burning Oil
Twice a year for appliances burning Domestic Bituminous Household Coal
Quarterly for appliances burning wood
Smokeless fuels :-
Yes even Smokeless fuels produce products of combustion (fly ash) that leave deposits on the surface of the flue that need to be swept off at least once a year.
Domestic Bituminous Household Coal :-
Where as household coal produces soot in much larger quantities generally the cheaper fuels are of lower quality and produce the most soot, also as they tend to spark more, hence there is a greater chance of chimneys catching fire, if they are not swept often enough. Twice a year is the safest choice.
WOOD BURNING STOVES:
Wood stoves are notorious for producing vast quantities of creosotes along with water vapour. If you burn wet unseasoned logs, and if the appliance is fitted to a flue that is not insulated, tar will quickly build up in the cold flue, you then have a recipe for disaster. Wood stoves should only be fitted to well insulated flues, and must only burn well seasoned wood so that the moister content is down near 25%, (that's room dried wood) and then they must be swept every three months when in use. Unless this advice is followed the flue can become so tarry that no amount of sweeping will make any difference, (think of the tar on the road surface and the effect of pushing a brush over it) the only answer will be to have your chimney reamed out, and hope the chimney will stand up to such violent action.
Gas appliances are in the main clean burning. But are very susceptible to blockage where there is no Gas Approved Terminal protecting the top of the flue e.g. if leaves or other debris can enter the flue and come to rest on a cobweb that can be all that is required to cause a partial blockage. Also if the flue is aged and the old lining is crumbling sand and grit can fall and slowly block the outlet of the appliance. If the appliance was fitted before the chimney was swept clear of all soot then that too will fall and cause a blockage as will bird's nests. So always give your gas appliance a safety check once a year by a Corgi Registered Gas Fitter, and he will tell you if it needs to be swept.
Oil burning appliances :-
Oil burning appliances if maintained well serviced according to the manufactures instructions, will give no trouble, but will benefit from annual sweeping as they do produce soot. However if the regular servicing requirement is not met they it can soot up rapidly, the soot from oil burning appliances it seems to me is the most obnoxious of them all, as to whether it is more carcinogenic than the others I don't know but great care should be exercised when they are swept.