Every flue should be subjected to the following smoke test before the
heater is connected with it and preferably before the chimney has been
furred and plastered or otherwise enclosed. Build a paper, straw, wood,
or tar-paper fire at the base of the flue. When the smoke is rising in a
dense column, tightly block the outlet at the top of the chimney with
a wet blanket. Smoke that escapes through the masonry indicates the
location of leaks. Frequently this test reveals bad leaks into adjoining
flues or directly through the walls or between the linings and the wall.
Remedy defects before the chimney is accepted for use. Such defects are
usually difficult to correct; hence it is wise to watch the construction
closely as it progresses.