Is the whole country crazy, or are they onto something?

The belief that a supply of combustion air directly from outdoors is necessary for a wood stove to operate safely is, for many people, just common sense. Back in the 1980s we all preached the need for outdoor air to be ducted into every stove to make sure it gets the air it needs to operate normally.

Then in the late 1980s the research division of Canada Mortgage and Housing Corporation (CMHC) sponsored a big research project to find out how outdoor air supplies worked. The results were a shock to everyone. The testing clearly showed that outdoor air supplies don’t isolate the appliance, don’t prevent smoke spillage into the room and that wind around the house can cause reverse flow in outdoor air ducts.

Once we had all digested this news that turned our assumptions on their head, one simple fact stood out: air flows to zones of lower pressure through any available opening. Clearly, air does not flow where we want it to, no matter how much we wish it would.

It took several years and several more research projects to confirm these findings and convince key people that this was no fluke and that outdoor air was really not the answer to all our problems. Finally in 2001 a major change to the Canadian solid fuel installation code removed mandatory outdoor air supplies and instead substituted two options in its place. The code, CSA B365, calls for either a powered make up air supply to compensate for powerful exhausts like kitchen range hoods, or a carbon monoxide detector at ceiling level in the hearth room. It is a little more complicated than that, but that is the essential rule.

Following is the text of Section 5 Air for combustion and ventilation from the 2010 edition of CSA B365 with explanatory comments.

Start of quoted text and comments:

5 Air for combustion and ventilation

5.1 Protection against depressurization within a building

5.1.1 Heating appliances

For heating appliances burning solid fuel and operating on natural draft, protection against depressurization of the space where the appliance is installed shall be provided in accordance with Clause 5.1.3 or 5.1.4.

Clause 5.1.3. calls for a pressure test to be done on the house by closing doors and windows and turning on the largest exhaust systems to see how depressurized the house could become. The alternative is Clause 5.1.4. which requires that a carbon monoxide detector be installed. Building codes require that the CO detector be mounted at or near ceiling level in the room containing the wood heating appliance. See below.

Note: For the purposes of this Clause, solid-fuel-burning heating appliances have an air-to-fuel ratio less than or equal to 35:1 when tested in accordance with CSA B415.1 or 40 CFR, Part 60, Subpart AAA, Methods 5 and 28

This air-to-fuel ratio is simply a way to determine how leaky an appliance is. Any appliance with an air-to-fuel ratio of more than 35:1 is considered to be too leaky to be a heating appliance. The notes below clause 5.1.2 explain the characteristics of both heating and decorative appliances so the actual air-to-fuel ratio doesn't need to be tested.

 5.1.2 Decorative appliances

For decorative appliances burning solid fuel, protection against depressurization shall be provided in accordance with Clause 5.1.3. In a dwelling unit in which all chimney-vented appliances are solid-fuelburning appliances, protection against depressurization shall be in accordance with Clause 5.1.3 or 5.1.4.

Much of the concern about open fireplaces is that they can depressurize a dwelling, which can cause other fuel-burning appliances to spill exhaust into the room. However, if the only fuel burning equipment in the house is wood fired, a CO detector in the hearth room will suffice as protection against depressurization. In this case the technician has the option of installing a CO detector or running the pressure test and, if necessary, installing a make-up air system.


(1) For the purposes of this Clause, decorative solid-fuel-burning appliances have an air-to-fuel ratio greater than 35:1 when tested in accordance with CSA B415.1 or 40 CFR, Part 60, Subpart AAA, Methods 5 and 28.

(2) In general, heating appliances meeting the requirements of Clause 5.1.1 have doors with gaskets, have a means to control the rate of combustion, and do not incorporate a dilution device in the appliance or venting system. In contrast, decorative appliances usually have no doors, or doors without gaskets, and have no means to control the rate of combustion.

5.1.3 Protection against depressurization

The pressure decrease of the space containing a solid-fuel-burning appliance shall be determined using a test procedure such as that outlined in CAN/CGSB 51.71 or an equivalent. The results of the test shall be documented and provided to the user. Where the results of the test exceed the limits prescribed for solid-fuel-burning appliances in CAN/CGSB 51.71, makeup air shall be supplied to reduce depressurization. The makeup air system shall conform to provincial or territorial regulations or bylaws or, in the absence thereof, to the National Building Code of Canada (NBCC). Following the installation of the makeup air system, the test procedure outlined in CAN/CGSB 51.71 (or the equivalent) shall be repeated.

This is a fairly elaborate proceedure that would not often be carried out. However, if a wealthy client wanted a very big open wood burning fireplace AND a very large kitchen range exhaust, the pressure test and make up air system would be required to meet this code and could be justified.

5.1.4 Spillage detection and alarm systems

The testing and makeup air requirements of Clause 5.1.3 need not be met if a spillage detection and alarm system is installed in accordance with provincial or territorial regulations or bylaws, or in the absence thereof, with the NBCC.

Note: A carbon monoxide alarm may serve this purpose.

Most building codes require that the CO detector be installed at or near ceiling level in the hearth room because spilled exhaust will normally rise to ceiling level because it is hot relative to room temperature.

5.1.5 Documentation

Where a makeup air system is installed to control the building pressure in accordance with Clause 5.1.3, its performance characteristics and maintenance requirements shall be documented and provided to the user.

5.2 Outdoor combustion air ducts

A duct supplying combustion air from the outdoors shall be connected directly to the air inlet of an appliance that is certified for such a connection. The installation and the components of the combustion air system shall conform to the manufacturer’s installation instructions. See Annex D for information on outdoor combustion air.

This clause puts limits on the installation of outdoor air supplies connected directly to appliance combustion chambers. Because of the documented risk of reverse flow due to wind effect-induced negative pressure at the outdoor weatherhood, it is thought that combustibles near the duct should be protected from heating in case of reverse flow. This clause puts the onus on the appliance manufacturer to provide instructions on how to prevent overheating of combustion air ducts.

End of quoted text and comments.

Today in Canada outdoor combustion air ducts are rarely installed except where required by code for fireplaces and even when they are installed, the outlet is sometimes closed or disabled because they tend to cause other problems. So, do wood stoves in Canada starve for combustion air or fill houses with smoke and carbon monoxide? Are Canadians dropping like flies? Not at all. In fact, when the new provisions were added to the B365 installation code, the outdoor combustion air debate went silent and hardly anyone noticed.

To help users of the CSA B365 code understand why the changes were made, the following non-mandatory appendix was added to the document.


Annex D (informative)

Outdoor combustion air

Note: This Annex is not a mandatory part of this Code.


Requirements and recommendations for the provision of combustion air from outside the dwelling have been changed significantly over the various editions of this Code. This Annex provides the rationale for the changes.


B365 10 coverbIn the 1980s, it was thought that smoke spillage from wood-burning systems could be reduced or eliminated by providing a supply of outdoor air to the appliance. This outdoor air was supplied either directly to the combustion chamber of the appliance or indirectly in the form of a makeup air supply linked to, or in close proximity to, the appliance. Underlying the concept of outdoor air supplies was the belief that most combustion spillage was the result of wood-burning appliances not receiving sufficient combustion air from inside the dwelling. However, research conducted in the late 1980s by Canada Mortgage and Housing Corporation revealed that flawed wood appliance venting system design and not inadequate combustion air was the major factor in combustion spillage. Other research showed that outdoor air supplies were not a simple or effective cure for spillage and that some designs could create a fire hazard. Clauses D.3 to D.5 provide a summary of the research findings.


Indirect, makeup-type air supplies respond mainly to the house pressure relative to atmospheric pressure outside, not just to the air requirements of the combustion appliance. An indirect air supply functions simply as a hole in the house envelope, and its principal effect is to change the leakage pattern of the building. Therefore, an indirect outdoor air supply is not an effective strategy for resolving combustion venting problems because it cannot reliably prevent or reduce combustion spillage nor provide protection from depressurization.


Air supplies routed from outside directly to the appliance combustion chamber do not isolate the appliance from room pressure as was originally thought. Direct air supplies can reverse flow direction if the outdoor weatherhood is exposed to wind-induced negative pressure in excess of chimney draft. The flow reversal of hot exhaust gas through such a duct can create a fire hazard. Also, if the house is depressurized to a level greater than chimney draft, spillage will occur from any available opening in the appliance and venting system, so no protection from depressurization is provided by direct outdoor combustion air supplies. In other words, the negative house pressure at which an outdoor aired appliance will spill smoke is about the same as the level at which an appliance that takes its combustion air from the room. It is now generally agreed, based on research findings, that protection against depressurization can best be provided by evaluating and, if necessary, adjusting the pressure in the house by supplying makeup air from outdoors so that the pressure remains within the limits tolerated by the appliance, as is set out in Clause 4.1 and its referenced documents.


Good wood appliance venting system design, component selection, and proper operation have been shown to be more reliable in ensuring successful venting and in preventing spillage problems than supplying combustion air from outdoors. Although the 1990 edition of the NBCC included mandatory requirements for outdoor air supplies for fireplaces, those provisions were removed from the 1995 edition of the NBCC and replaced with a caution regarding the fire hazard posed by outdoor air supplies connected directly to combustion chambers. However, provincial or territorial building codes might still require the installation of outdoor air supplies for all fireplaces. If provincial or territorial regulations require the installation of outdoor air supplies, the technician should be aware that the possibility of reverse flow exists in air ducts routed directly to the combustion chamber, and that it is advisable to leave some clearance from the air duct to combustible material in case a reversal does occur.