The problem with outdoor air supplies

It seemed like such a good idea until it was tested.

Most building codes in North America require that woodburning equipment have a supply of combustion air provided from outside the dwelling.  These rules were put in place on the assumption that most smoke spillage from fireplaces (and stoves, furnaces and so on) is caused by their inability to "get enough air" and that the outdoor air supply would provide enough air and therefore reduce spillage. 

When, in 1989, Canada Mortgage and Housing Corporation sponsored some research it became clear that both assumptions were wrong.  It turns out that most smoke spillage from woodburning systems is caused by bad design and that outdoor air supplies don't do much of anything to reduce spillage.

Here is further information on the subject of outdoor combustion air supplies.

Excerpts from Fireplace Air Requirements, the CMHC study that shed light on the outdoor combustion air question.

A more detailed discussion of the behavior of outdoor air supplies.

How the CMHC research affected the treatment of outdoor air supplies in the National Building Code of Canada.