hoveling

from The Century Dictionary.

• noun A mode of preventing chimneys from smoking, by carrying up two sides higher than those which are less liable to receive strong currents of air, or leaving apertures on all the sides, so that while the wind blows over the top the smoke may escape below.
• noun A chimney so built.

from the GNU version of the Collaborative International Dictionary of English.

• noun A method of securing a good draught in chimneys by covering the top, leaving openings in the sides, or by carrying up two of the sides higher than the other two.