Every flue should be subjected to a smoke test before the heater is
connected with it. This may be done as follows: Build a paper, straw,
wood, or tar-paper fire at the base of the flue, and when the smoke is
passing in a dense column tightly block the outlet at the top by laying a
wet blanket over it. If leakage exists at any point, it will immediately
become apparent by the appearance of smoke at the opening. Flues so
tested frequently reveal very bad leaks into adjoining flues or directly
through the walls or between the linings and the wall. When the smoke
test indicates leakage, the defect should be remedied before the chimney
is accepted for use. Remedying such defects is usually difficult, hence
it is wise to watch the construction closely as it progresses. Many brick
masons say that all flues leak. This is not true; every flue should be
tight.