2. Changing from solid fuel to gas
When you have decided to change from solid fuel to gas it is important that you have your chimney swept before installing the gas appliance. This is so you can be sure that there is no obstruction in your chimney that would prevent the dangerous fumes that gas fires emit from escaping safely.
Gas appliances, if properly calibrated, should create no solid material as a by product of their combustion. However it is important to be aware that the fumes that they create will have some affect on your chimney. Unless you have one of the steel liners fitted the fumes from the appliance will come into direct contact with the inside of your chimney. These carbon monoxide fumes combine with oxygen and water vapour to form a mildly corrosive carbonic acid.
Over a period of time this acid attacks your chimney lining, generally causing it to drop down your chimney in the form of a fine powder. The acid will also bring down anything that was encrusted on your chimney lining that the brush was unable to remove when your chimney was swept. (This is why it is important to have your chimney swep t regularly to avoid soot forming a solid crust).
Ultimately, once your chimney lining has been completely eroded, the acid will attack the bricks and mortar of the chimney itself, potentially creating gaps through which carbon monoxide fumes can leak.
Don't panic !
The above information is just to make you aware of what will be happening in your chimney very gradually over a long period of time.
For long-term safety I would recommend that you had a steel liner fitted inside your chimney. For the shorter term I would recommend that you had your chimney swept and smoke tested every 3 years and buy a carbon monoxide detector.