A Study of Necessary Changes to Harmonize Standards Requirements Related to Protection Against Depressurization for Vented Combustion Equipment

Prepared by John Gulland, Gulland Associates Inc. For Canada Mortgage and Housing Corporation
September, 2002


The Technical Sub-Committee (TSC) for standard CSA F326-91, Residential Mechanical Ventilation Systems is working to update the standard. The complexities of the task are being managed through the formation of several Task Groups (TG) charged with addressing particular issues identified by the sub-committee. The work of two of these TGs are relevant to this project: the TG on Combustion/Depressurization and the TG on Harmonization of Standards.

The TG on Combustion/Depressurization is reviewing and refining the requirements in F326 related to protection against depressurization that can cause venting failure in combustion equipment. Although its work is not yet completed, there is a clear consensus that the test procedures set out in CGSB Standard 51.71, The Spillage Test, will, subject to detailed review and refinement, form the basis of the approach recommended to the F326 TSC.

The TG on Harmonization of Standards could not be active until clarity emerged on the general approach recommended for F326, particularly as it relates to the issue of protection against depressurization for vented combustion systems. The necessary clarity now exists. This report is intended to assist the members of the TG on Harmonization in comparing the relevant standards and in making recommendations, first to the F326 TSC, and then to the TSCs responsible for the standards under review, of changes that would be required to bring their requirements into technical agreement with those proposed for F326.


  1. Through a detailed review of the relevant standards, document the approach taken in each in addressing potential problems of combustion air supply and house depressurisation.
  2. Starting with the assumption that the approach proposed by the TG on Combustion/Depressurization in the current round of revisions to F326 is the most technically appropriate, identify the changes needed in the other standards to make their treatment of combustion venting and house depressurization compatible with that in F326.
  3. Develop specific proposals to be eventually submitted to the various committees responsible for the standards under review.
  4. Present these proposals to the Harmonization Task Group of the F326 Sub-Committee and the Coordinating Committee on Combustion Venting and make any changes identified by these groups.
  5. Identify any difficulties with the starting assumption in Point 2.

This report and its appendices is intended to meet objectives 1, 2, 3 and 5.

Standards reviewed for this analysis:

  • CSA F326, Residential Mechanical Ventilation Systems
  • CGSB 51.71, The Spillage Test
  • CSA B139, Installation Code for Oil-Burning Equipment
  • B149.1, Natural Gas and Propane Installation Code
  • CSA B365, Installation Code for Solid-Fuel Burning Appliances and Equipment

General concepts

Changes in the materials and techniques of house construction over the past 25 years have resulted in reduced leakage rates of building envelopes. Throughout this period there has been much discussion within the responsible codes and standards committees concerning the use of chimney-vented combustion devices in houses of relatively air tight construction. The issues that animated these discussions include:

  1. The building envelope may be so tight that natural leakage will not supply the combustion air (including excess and dilution air) requirements for chimney-vented equipment.
  2. Standard household exhaust devices may depressurize the building enough to induce spillage of combustion products from chimney-vented equipment into the living space.
  3. Air consumption of a chimney-vented appliance may depressurize the dwelling enough to affect the operation of other equipment. This issue has been raised mostly in relation to open wood burning fireplaces.

Underlying these concerns is the objective of acceptable performance of combustion equipment which can be defined as:

  1. successful ignition without spillage
  2. correct flame quality
  3. complete venting to outdoors of the products of combustion

A suitable supply of combustion air for chimney vented equipment can be defined as one that is available as required from a location which is at or close to atmospheric pressure. Efforts to ensure adequate combustion air for new installations, or to correct apparent problems related to the issues listed above, have until recently focussed on providing a supply of combustion air from outdoors either directly to the appliance combustion chamber, or more commonly, indirectly to the space in which the appliance is installed. Both laboratory research and field experience have shown that these efforts to ensure acceptable performance by supplying combustion air from outdoors have not generally been successful.

Of the five standards reviewed, three are combustion equipment installation codes (B139, B149 and B365), one is a ventilation system standard (F326) and one is a house pressure test standard (51.71). The five standards were reviewed for their treatment of the issues of combustion air supply and protection against depressurization.

Findings from the review of the standards

Two very different approaches are evident among these five standards:

  • B139 and B149 are combustion air-centred: Their requirements assume that tight house construction will not permit natural leakage to supply combustion air. Both standards have extensive requirements for the provision of combustion air from outdoors. The problem of exhaust device depressurization is not addressed in any detail within the standards. In fact, the recognition in B149 that its combustion air and venting tables are appropriate only when depressurization of the space does not exceed – 5 Pa (Appendix C) presents installers and inspectors with a problem because the standard includes no detail on how to proceed if the value is outside this limit.
  • F326, 51.71 and B365 are depressurization-centred: Their requirements assume that houses leak enough to provide combustion air, but that depressurization of a house by its exhaust devices may disrupt normal operation of chimney-vented combustion equipment. (Standards 51.71 and B365 have requirements recognizing that open or decorative fireplaces may consume enough air to affect the operation of other equipment.

Why the combustion air-centred approach needs review

It is assumed in standards B139 and B149 that oil and gas appliances may not receive the combustion air they require if the dwelling ". . . has windows and doors of either close-fitting or sealed construction, and the exterior walls are covered by a continuous, sealed vapour barrier and gypsum wallboard (drywall) or plywood or similar materials having sealed joints . . . " (B149) or is ". . . built in accordance with the National Building Code of Canada, 1985 (or subsequent edition)" (B139). This assumption is not supported by housing research literature. The natural leakage rate of new Canadian houses is generally sufficient to supply combustion air requirements for closed combustion chamber natural draft vented equipment without inducing any significant amount of house depressurization. In addition, the usual methods of providing combustion air from outdoors are generally ineffective and may induce venting failure under certain conditions.

Combustion air from outdoors for an appliance that is not specifically engineered to function in this mode is normally provided indirectly through a duct that terminates near the appliance. A direct connection of the outdoor air duct to the appliance air inlet would be considered a superior approach except for concerns about unintended effects on combustion. B139 provides that "An outside combustion-air duct may be directly connected to the burner if the appliance is so certified." B149 does not contain such a limitation, but the language in the standard makes clear that indirect supplies are assumed through statements such as "An outdoor air supply . . . shall be provided to either an enclosure or a structure in which an appliance(s) is installed."

Field-installed indirect combustion air supplies function to passively increase the leakage rate of the building in proximity to the combustion equipment. As a result, these supplies are more properly referred to as make-up air. The design and sizing provisions in the standards assume that the building interior and the area around the outdoor termination of the air supply are at or close to atmospheric pressure. This approach has technical limitations that can result in the appliance not having access to a suitable supply of combustion air. These limitations include:

  • As currently prescribed, indirect combustion air supplies are not adequately sized to supply the total combustion air requirements of the appliances they are intended to serve at the maximum allowable driving pressure of – 5Pa.
  • Wind effects at the outdoor termination of the duct can cause significant fluctuations of the pressure in the duct which affects the flow direction and volume of air supplied to or withdrawn from the space.
  • Stack effect during cold weather normally produces negative pressure at lower levels of the building where most fuel-burning appliances are installed. This stack effect can induce flow in a passive make-up air supply duct, whether or not the appliance is firing. During extremely cold weather, this stack effect-induced flow can cool the space excessively and may interfere with the performance of the appliance when it fires. Stack effect may produce a pressure difference at the appliance that exceeds the –5 Pa default depressurization limit for appliances that are not otherwise certified, a problem that can impair appliance function if its venting system runs up the outside wall of the dwelling and/or penetrates the building envelope below its highest level.
  • The operation of exhaust devices may depressurize the dwelling in excess of the –5 Pa default value. The indirect make-up air supplies, as currently prescribed, cannot be expected to provide sufficient relief to prevent such depressurization.

For these reasons, passive indirect sources of make-up air are not considered a reliable means of supplying combustion air in tightly-sealed buildings. It should be noted that these limitations do not apply to relief openings in small enclosures such as furnace rooms that are designed to permit air for combustion and ventilation to flow from other parts of the building.

It is also worth noting that the test procedure described in Standard CGSB 51.71 could be used to detect situations in which the combustion air consumption of an oil- or gas-fired appliance is suspected of depressurizing the space in which it is located. This would simply involve establishing the prescribed test conditions and, instead of turning on selected exhaust devices, firing the combustion appliance and measuring the depressurization of the space.

The technical limitations of passive and even direct supplies of outdoor combustion air, combined with the knowledge that most new houses have sufficient natural leakage to supply combustion air without significant depressurization, suggest that the most effective means of ensuring acceptable performance is to determine the potential for excessive depressurization through testing and to take appropriate action based on the test findings. This general approach is reflected in standards CSA F326, CSA B365 and CGSB 51.71. The most recent set of proposed revisions to the National Building Code Section 9.32 Ventilation Systems is also generally in harmony with this approach.


CSA F326 and CGSB 51.71

The TG on Combustion/Depressurization will report to the F326 TSC on the mechanism by which agreement can be achieved between its requirements and those of CGSB 51.71, which will likely be referenced within F326. Through the sharing of information and considerable overlap in membership between the TSCs for CSA F326 and CGSB 51.71, it is expected that once both standards are reviewed and revised, agreement will be achieved.

CSA B365

This standard was published as a new edition in 2001. The extensive revisions made to the standard included a new Sub-section 4.1 Protection Against Depressurization aimed at bringing its provisions into close agreement with the approach taken in the most recent proposed revisions to the National Building Code of Canada, Subsection Protection Against Depressurization. In summary, this approach involves either the testing the potential for excessive house depressurization using the CGSB 51.71 procedure and taking appropriate remedial action, or installing spillage warning devices such as CO detectors. The option of providing spillage detection and alarm is unique to hand-fed wood burning equipment and is justified by the direct involvement of the user in the combustion process and the fact that spillage from these systems is readily detected by relatively inexpensive smoke and CO detectors.

Because B365 was revised recently and its requirements in this area were brought into general agreement with the NBC, which in turn is consistent with the general approach anticipated for F326, B365 can be said to agree conceptually with F326. Once the F326 TSC completes its work and recommends a final draft for publication, some slight changes to B365 might be warranted to bring it into close agreement. At this time, no such changes can be recommended.

B139 and B149

To bring Standards CSA B139 and B149 into agreement with F326 will require a shift from the combustion air-centred approach to ensuring acceptable appliance performance to the depressurization-centred approach. This shift will involve significant reworking of Subsection 4.4 Air for Combustion and Ventilation in B139 and Section 7. Venting Systems and Air Supply for Appliances in B149

Appendix A offers the TSC for B139 a draft set of requirements that would bring the standard into closer agreement with F326. Appendix B offers the TSC for B149 a draft set of requirements to assist it in shifting from the combustion air-centred to the depressurization-centred approach to ensuring acceptable appliance performance.  Note: Appendices A and B have not been included in this online version of the report.

The subject area specialist members of both TSCs will undoubtedly refine the respective drafts of proposed changes to account for technical issues that will emerge in the revision process. As the process evolves, the TSCs may require support from housing specialists in dealing with the complexities of house/combustion system interactions.