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ANSI Z21.90-2015 / CSA 6.24-2015: Technical Essentials for Gas-Fired Appliance Venting Systems
Ensuring Safe and Efficient Removal of Combustion Products in Residential and Commercial Installations
Gas-fired appliances rely on properly designed venting systems to safely expel combustion products. The bi-national standard ANSI Z21.90-2015 / CSA 6.24-2015 sets forth comprehensive requirements for the construction, performance, and certification of venting systems intended for use with such appliances. Developed jointly by the American National Standards Institute (ANSI) and the Canadian Standards Association (CSA), this standard is critical for manufacturers, installers, and building officials seeking to ensure safe and reliable venting in residential and commercial applications.
Scope and Applicability
ANSI Z21.90-2015 / CSA 6.24-2015 applies to factory-built venting systems designed for gas-fired appliances operating on natural gas, propane, or similar gaseous fuels. The standard covers the following vent types:
- Type B vent – double-wall non-metallic or metallic vents for draft-hood-equipped appliances (e.g., water heaters and furnaces).
- Type L vent – double-wall metallic vents for gas-fired appliances with limited-temperature exhaust (e.g., some commercial cooking equipment).
- Special gas vents – dedicated systems for condensing and Category IV appliances (often PVC, CPVC, or stainless steel).
- Direct-vent system components (if part of a listed vent system).
The standard does not cover masonry chimneys, single-wall metal pipe (unless specifically referenced), or vents for solid-fuel or oil-fired appliances. It serves as the basis for listing and certification of venting system components by recognized testing laboratories (e.g., UL, Intertek, CSA).
Important: Installers must verify that a venting system is listed and labeled for the specific appliance type and fuel used. Mixing components from different certified systems voids the listing unless explicitly allowed by the manufacturers.
Technical Requirements
Materials and Construction
Each vent type must be constructed from materials that resist corrosion, thermal degradation, and mechanical failure. The table below summarizes typical material requirements for common vent categories.
| Vent Type | Inner Pipe Material | Outer Pipe Material | Max. Continuous Operating Temperature |
|---|---|---|---|
| Type B | Galvanized steel (0.017 in. min.) | Galvanized steel (0.019 in. min.) | 480 °F (250 °C) |
| Type L | Stainless steel (304 or 316, 0.015 in. min.) | Galvanized steel (0.019 in. min.) | 530 °F (277 °C) |
| Special Gas Vent (Category IV) | PVC, CPVC, or polypropylene (per ASTM D1785, D2846, or F2618) | Same as inner (single-wall) or metal jacket | Appliance-specified (typically ≤ 140 °F continuous) |
Performance Testing
Venting systems must pass a series of rigorous tests to ensure safe operation under anticipated conditions:
- Temperature resistance test – components must withstand elevated flue gas temperatures without deformation or failure.
- Leakage test – under positive or negative pressure (depending on vent class) the system must not exceed allowable leakage rates.
- Rain and snow intrusion test – for outdoor sections, the vent must prevent entry of precipitation that could block or damage the system.
- Combustibility and clearance test – maintain safe distances to combustible materials as per manufacturer instructions (typical clearances for Type B: 1 in. from combustibles).
- Structural integrity test – resistance to wind loads, thermal expansion, and mechanical stresses.
Tip: When selecting a vent system, always check the certification marking for the maximum flue gas temperature of the appliance. Using an undersized or low-temperature-rated vent can cause rapid deterioration and unsafe operation.
Implementation and Compliance Considerations
Listing and Marking
Each component of a venting system must bear a permanent marking showing:
- Manufacturer’s name or trademark
- Vent type (e.g., Type B, Type L, Special Gas Vent)
- Clearance to combustibles (minimum)
- Maximum allowable operating temperature
- Standard reference (ANSI Z21.90/CSA 6.24)
- National standard of Canada or UL listing mark as appropriate
Installation instructions must be supplied with the first component of each system and must include explicit guidance on assembly, support, terminations, and prohibited uses.
Field Installation
While the standard is primarily a factory-product standard, it influences installation through the listing. Key compliance points for field installation include:
- Always use components from a single listed system; do not mix brands unless documentation confirms compatibility.
- Maintain required clearances and support spacing as per manufacturer’s instructions.
- Terminate the vent at least 3 ft above any forced-air inlet or roof line (local codes may vary).
- For Category IV (condensing) appliances, ensure the vent is sloped to allow condensate drainage and is made of approved plastic or stainless steel.
Compliance advantage: Using a properly listed and installed venting system reduces the risk of carbon monoxide backdrafting, structural fires, and flue gas condensation damage. Many local building codes now mandate adherence to ANSI Z21.90/CSA 6.24 for all gas-fired vent installations.
Danger: Improper termination of a gas vent (e.g., too close to windows or air intakes) can allow lethal carbon monoxide to re-enter the building. Always follow the vent manufacturer’s termination requirements and applicable codes (such as NFPA 54/ANSI Z223.1 and CSA B149.1 in Canada).
Compliance and Certification Notes
Manufacturers seeking compliance with ANSI Z21.90-2015 / CSA 6.24-2015 must submit samples to an accredited third-party testing agency. The testing program verifies all material, dimensional, and performance criteria. Successful products receive an official listing and are included in the agency’s published directories.
It is important to note that the 2015 edition supersedes earlier versions and introduced updated requirements for:
- More detailed instruction manuals and marking
- Improved rain intrusion testing
- Harmonization of clearance requirements between U.S. and Canadian markets
Building inspectors may request documentation proving listing for both the vent system and the connected appliance. Designers should also verify compatibility with appliance vent categories (Category I, II, III, or IV) as defined in ANSI Z21.1 and the appliance’s installation manual.
Regular review of the standard is recommended because updates may occur after 2015 (such as amendments or new editions) that could affect compliance obligations.
Q: What types of gas-fired appliances are covered under ANSI Z21.90-2015?
A: The standard covers venting systems for most residential and commercial gas-fired appliances, including furnaces, boilers, water heaters, dryers, and cooking equipment, as long as the vent system is factory-built and listed for the appliance category. However, it does not apply to venting for solid-fuel or oil-fired appliances, nor to masonry chimneys.
A: The standard covers venting systems for most residential and commercial gas-fired appliances, including furnaces, boilers, water heaters, dryers, and cooking equipment, as long as the vent system is factory-built and listed for the appliance category. However, it does not apply to venting for solid-fuel or oil-fired appliances, nor to masonry chimneys.
Q: Can plastic (PVC/CPVC) vent pipe be used with any gas appliance?
A: No. Plastic vent systems are only permitted for condensing (Category IV) appliances that produce low-temperature exhaust (typically below 140 °F). The vent material must be certified to the relevant ASTM standard (e.g., D1785 for PVC) and listed for use under ANSI Z21.90/CSA 6.24. Using plastic venting with non-condensing appliances will cause melting or fire.
A: No. Plastic vent systems are only permitted for condensing (Category IV) appliances that produce low-temperature exhaust (typically below 140 °F). The vent material must be certified to the relevant ASTM standard (e.g., D1785 for PVC) and listed for use under ANSI Z21.90/CSA 6.24. Using plastic venting with non-condensing appliances will cause melting or fire.
Q: If I already have a vent system installed before 2015, do I need to replace it to meet this standard?
A: Not necessarily. The standard applies primarily to new installations and to products manufactured after its effective date. Existing ventilation systems that were installed under previous editions of the standard or under other applicable codes are usually considered acceptable as long as they remain in safe operating condition. However, if you replace the vent or make significant modifications, the new components must comply with the current edition.
A: Not necessarily. The standard applies primarily to new installations and to products manufactured after its effective date. Existing ventilation systems that were installed under previous editions of the standard or under other applicable codes are usually considered acceptable as long as they remain in safe operating condition. However, if you replace the vent or make significant modifications, the new components must comply with the current edition.
This article provides a general overview of ANSI Z21.90-2015 / CSA 6.24-2015 for informational purposes. Always refer to the official published standard and consult a qualified professional for specific design and compliance decisions.
© 2026 – This content is intended for informational use only and does not constitute professional engineering or legal advice.