When a fireplace smokes, it should be examined to make certain that the
essential requirements of construction as outlined in this bulletin
have been fulfilled. If the chimney is not stopped up with fallen brick
and the mortar joints are not loose, note whether nearby trees or tall
structures cause eddies down the flue. To determine whether the fireplace
opening is in correct proportion to the flue area, hold a piece of sheet
metal across the top of the fireplace opening and then gradually lower
it, making the opening smaller until smoke does not come into the room.
Mark at the lower edge of the metal on the sides of the fireplace. The
opening may then be reduced by building in a metal shield or hood across
the top so that its lower edge is at the marks made during the test; or
the trouble can generally be remedied by increasing the height of the
flue.