Q. We just installed natural-gas heating. Originally we burned coal and than oil. All three fuels have gone up an old brick chimney. I've been told I should install a Class B metal flue inside the chimney. A black watery sediment has formed on the outside of the chimney. Your comments, please? A reader Winona, Ontario
A. "The black substance could be either smoke stains generated by the combustion of fuel oil, or creosote from earleir combusting coal," says C. W. Faulkenberry, a staff engineer with the Brick Institute of America, McLean, Va.
"In either case, have a chimney sweep clean the interior of the chimney. Clean the exterior brick using a mild household detergent and the 'bucket and brush' method."
Finally, he cautions: "Creosote is highly flammable and has the potential for causing a chimney fire. The cleaning process recommended above would remove most of the deposit regardless of its makeup; and since natural gas is clean-burning, the problem should not recur."
For more information you can get a copy of "Fireplaces and Chimneys," put but by the Department of Energy. Write to the Consumer Information Center, Pueblo, CO 81009. Ask for booklet No. 150H; the cost is $1.
Clearly, no extra flue is recommended.