Developments in Canadian houses during the past several decades have increased the possibility of back-draughting (also known as reverse flow) in natural-draught chimneys. These developments include: a) the use of high-power-range exhaust fans capable of creating a negative pressure sufficient to back-draught chimneys. b) the use of more tightly sealed exterior envelopes of houses, which make it easier for exhaust equipment to create a strong negative pressure within houses. In this report a theory will be presented that illustrates the conditions under which backdraughting can occur in a house. In addition, measured data from an experiment performed on a very well-sealed house incorporating a natural-draught chimney are presented and the results are compared with the theory.

KEYWORDS: chimneys, draughts, Canada, housing, flues, testing, boilers, exhaust fans, natural draughts, insulated housing, downdraughts.