Outdoor combustion air in the Canadian national building code

Made mandatory in 1990, removed entirely in 1995

The 1990 National Building Code (NBC) of Canada had mandatory requirements for outdoor air supplies for fireplaces, but, when the findings of Canada Mortgage and Housing Corporation (CMHC) research on outdoor air supplies were tabled during the revision cycle leading to the 1995 edition, the requirements were removed. 

Here is some background on the evolution of outdoor air supplies in the NBC.

The 1990 NBC contained the following Article (clause): "9.22.1.4 Combustion Air.  Fireplaces, including factory-built fireplaces, shall have a supply of combustion air. (See Appendix A)"

Appendix A-9.22.1.4. read in part: "The intent of this Article is to allow the fireplace to be operated without affecting, or being affected by, other appliances or exhaust equipment.  For this to occur, the fireplace must be provided with a supply of combustion air dedicated to the fireplace only; an opening to the exterior should be provided at or near the fireplace opening."

The Article went on to require outdoor air for factory-built fireplaces in accordance with the manufacturer's instructions and gave a series of prescriptive requirements for outdoor air supplies for site-built masonry fireplaces.

In the proposed revisions sent out for public comment in August 1993, it was proposed to delete Appendix note A-9.22.1.4. with the following reason given: "Combustion air supplies as currently prescribed are generally ineffective.  The requirement to provide combustion air is being deleted from CAN/CSA A-405, Design and Construction of Masonry Chimneys and Fireplaces and from the Code."

The '95 NBC contains the following: "9.22.1.4. Combustion Air.   Where a supply of combustion air is provided directly to the fire chamber of a fireplace, including a factory-built fireplace, the installation shall comply with the "Outdoor Air Supply" requirements provided by CAN/CSA A-405, Design and Construction of Masonry Chimneys and Fireplaces."  This is the only reference to combustion air for fireplaces.

The supply of outdoor air was made non-mandatory and this wording was included because the CMHC research that showed outdoor air supplies to be ineffective, also showed that direct-to-combustion chamber supplies could be hazardous because of the potential for wind-induced reverse flow of combustion gases through the supply duct.  The A-405 requirements proposed ways to provide outdoor air safely if you choose to supply it.

Like most building codes in North America, the NBC included outdoor combustion air requirements for combustion equipment on the assumption that it was a good strategy to reduce spillage susceptibility.  Unfortunately the assumption was acted upon before any research had been done to explore how outdoor air supplies actually behave.

The research reports that influenced the Standing Committee of Part 9 of the NBC are:

1) Fireplace Air Requirements, ORTECH for Canada Mortgage and Housing Corporation, 1989

2) The Effects of Glass Doors on Masonry Fireplace Spillage and Surface Temperatures, Virginia Polytechnic Institute for Canada Mortgage and Housing Corporation, 1994

Copies of these reports are available from the CMHC information centre at (613) 748-2367.  

Although the two studies were conducted by two labs with different set-ups, different protocols and different appliance types (1. factory-built, 2. masonry), they arrived at the same conclusion: The susceptibility to combustion spillage due to room depressurization is not affected in a predictable way by the presence or absence of air supplied from outdoors, whether supplied to the combustion chamber or indirectly through a supply duct terminating near the fireplace.

In both studies the reference room depressurization at which spillage was induced was 10 Pa.  In 'Fireplace Air Requirements', none of the five tested fireplaces spilled at 5 Pa depressurization despite the fact that all were very different in their configurations and features, although all did have glass doors.  The tests at the two depressurization levels were done with and without outdoor combustion air supplies.

Once the research findings were in and analyzed, the underlying physical process became clear:  That is, air flows to a zone of lower pressure through any available opening, regardless of our wishful thinking.  In retrospect, this principle appears rather obvious, although for most of us it was not until revealed in the lab.

As a result of the findings of these two studies, and against the backdrop of dozens of other CMHC studies of combustion venting and building aerodynamics, it was recognized that managing the indoor pressure environment was the only viable option for preventing health- and life-threatening combustion spillage from chimney-vented atmospheric appliances.  This is particularly the case with automatic oil and gas equipment of this type because they have dilution devices downstream of the combustion chamber: barometric draft controls in the case of oil appliances and draft hoods in the case of gas appliances.  Dilution air cools the exhaust, weakens draft and offers a ready path for combustion spillage, roughly equal to the spillage susceptibility of open fireplaces.  Automatic operation of gas and oil systems takes place independently of householder knowledge and participation and may continue for long periods undetected and that is why this type of spillage is considered potentially life-threatening.

But it was also recognized that hand-fed controlled combustion woodburning equipment does not use dilution devices and has high spillage resistance during most operational periods except for the tail out of the coalbed phase of the fire as system temperatures cool.  This type of spillage is not considered hazardous because it cannot continue for long without householder intervention (reloading), and this implies awareness of any malfunction.

Therefore, the '95 NBC at 9.32.3.8. Protection Against Depressurization requires make up air for exhausts exceeding 150 cfm where chimney vented wood, oil and gas systems are installed in the building.  Where the only spillage-susceptible equipment present is woodburning, there is an exception requiring only the installation of a carbon monoxide detector to provide warning of spillage should it occur.  A performance alternative to the prescriptive approach of 9.32.3.8. is offered in the form of a reference to the CSA ventilation code F326.