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ANSI Z21.91-2017: Standard for Vented Decorative Gas Appliances — Technical Overview
Scope, Performance Requirements, and Compliance for Gas-Fired Decorative Fireplace Equipment
Scope and Application
ANSI Z21.91-2017, also published as CSA Z21.91, establishes uniform safety and performance requirements for vented decorative gas appliances intended for indoor installation. This standard covers factory-built gas log sets, gas-fired fireplace inserts, and other decorative room heaters that are designed to be installed in a solid-fuel-burning fireplace or a listed vented enclosure. The standard applies to appliances that operate on natural gas, propane, or manufactured gas and are intended for use with a continuous pilot or intermittent ignition device.
The scope includes appliances rated at inputs up to 400,000 Btu/h (approximately 117 kW) and addresses all essential aspects: construction, combustion characteristics, venting requirements, safety controls, markings, and instructions. Appliances covered under Z21.91 must be permanently vented to the outdoors using a listed chimney or gas vent system; they are not permitted for unvented operation in living spaces.
Tip: Manufacturers should verify early in the design phase that the appliance’s maximum input rating does not exceed the limits defined in the standard relative to the venting system’s capacity. Overfiring can lead to incomplete combustion and carbon monoxide emission.
Technical Requirements and Performance Criteria
2.1 Construction and Material Requirements
The standard mandates that all components exposed to flue gases or burner flames must be constructed of corrosion-resistant materials. Burners must be rigid, securely mounted, and designed to prevent flame impingement on surrounding surfaces. Primary air shutters must be adjustable only with tools to avoid unauthorized tampering. Gas manifolds must be leak-tested at 1.5 times the maximum operating pressure, and all pressure-containing joints must withstand three times the service pressure without rupture.
2.2 Combustion and Venting Performance
Each appliance must achieve steady-state combustion with carbon monoxide (CO) concentration not exceeding 200 ppm in the flue gas (air-free). The standard requires a minimum of 50% of the appliance’s input rating to be delivered as radiant heat, ensuring efficient heat transfer while maintaining safe flue gas temperatures. Vent connectors must be sized according to Table 3 of the standard. The following table summarizes key input and performance parameters:
| Parameter | Requirement | Test Method |
|---|---|---|
| Maximum CO (air-free) | ≤ 200 ppm | ANSI Z21.91 Section 5.2 |
| Minimum radiant efficiency | ≥ 50% of rated input | Section 5.5.1 |
| Flue gas temperature rise | ≤ 350°F above ambient | Section 5.9 |
| Burner stability (wind test) | No flame lifting at 5 mph | Section 5.11 |
| Gas shutoff – pilot outage | Close within 90 seconds | Section 6.7 |
2.3 Safety Controls and Interlocks
All appliances must be equipped with a thermoelectric flame-sensing device, a standing pilot or intermittent ignition system (IID), and a gas shutoff valve that closes automatically upon loss of flame. The gas control system must include a manual shutoff valve accessible without removing any component. For appliances that include a blower or fan, a thermal interlock must prevent operation when the appliance temperature exceeds safe limits. The standard also details requirements for remote control and smart-device integration.
Warning: Failure to properly purge the gas line before lighting the pilot can damage the control valve. Always follow the manufacturer’s start-up procedures. Z21.91 requires explicit instructions for first light and annual maintenance.
Implementation Highlights for Manufacturers
Designing an appliance to meet Z21.91-2017 requires careful attention to burner geometry, air/fuel mixing, and heat exchanger materials. Four critical areas are:
- Venting system compatibility — The appliance must be listed only for use with specific vent configurations (B-vent, direct vent, or chimney liner). The manufacturer must state minimum and maximum vent lengths.
- Flame appearance — The standard requires that decorative flames simulate a natural wood fire. Burner ports must be arranged to produce realistic, yellow-tipped flames without sooting.
- Labeling and instructions — Permanent nameplates must include model number, gas type, input rating, and clearance to combustibles. Installation manuals must contain full wiring diagrams and troubleshooting guides.
- Testing for stability under draft — A 5 mph draft test (as per Section 5.11) ensures flames remain stable and do not lift off the burner when a door or window is opened.
Success Tip: Pre-compliance testing at an accredited laboratory (e.g., CSA, UL) can reduce certification time by up to 40%. Use the standard’s Table of Tests (Section 2) to build a test matrix early in the development cycle.
Compliance and Certification Notes
Certification to ANSI Z21.91-2017 is required for appliances sold in the United States and Canada. The standard is harmonized with related standards: ANSI Z21.88 (vented gas fireplace heaters) and CSA P.4.1 (testing method for gas fireplace efficiency). Key compliance steps include:
- Third-party listing — A Nationally Recognized Testing Laboratory (NRTL) such as CSA, UL, or Intertek must evaluate the product.
- Production inspection — The standard requires an initial type test followed by periodic factory inspections and witness testing.
- Markings — The certification mark and listing number must be permanently affixed to the appliance. Input ratings must be expressed in Btu/h for both natural gas and propane.
- Instructions — Installation and operating manuals must include specific warnings against using unvented operation and guide the installer in selecting approved vent components.
Danger: Installing an unlisted decorative gas appliance in a residence is a fire and life safety hazard. Uncertified appliances can produce carbon monoxide in excess of 1,000 ppm, leading to asphyxiation. Always verify the listing mark before purchase or installation.
After the standard’s effective date, manufacturers were given a transition period to update existing models. ANZI Z21.91 is revised periodically; the 2017 edition includes new requirements for electronic ignition systems, remote communication security (for smart appliances), and more stringent CO limits than the 2014 version.
Frequently Asked Questions
Q: What types of appliances fall under ANSI Z21.91-2017?
A: The standard covers vented decorative gas appliances such as gas log sets, fire baskets, and gas-fired inserts that are designed to be installed in a solid-fuel-burning fireplace or a factory-built vented enclosure. It does not cover unvented room heaters (ANSI Z21.11.2) or vented gas fireplace heaters (ANSI Z21.88).
A: The standard covers vented decorative gas appliances such as gas log sets, fire baskets, and gas-fired inserts that are designed to be installed in a solid-fuel-burning fireplace or a factory-built vented enclosure. It does not cover unvented room heaters (ANSI Z21.11.2) or vented gas fireplace heaters (ANSI Z21.88).
Q: How do I know if my gas log set complies with Z21.91?
A: Look for a CSA, UL, or other NRTL certification mark on the appliance and its packaging. The mark will include the standard number (Z21.91-2017). Additionally, the installation manual will contain a listing of approved vent components and a statement confirming compliance.
A: Look for a CSA, UL, or other NRTL certification mark on the appliance and its packaging. The mark will include the standard number (Z21.91-2017). Additionally, the installation manual will contain a listing of approved vent components and a statement confirming compliance.
Q: Can a Z21.91-rated appliance be used in a mobile home?
A: Yes, provided the appliance is specifically listed for mobile home use and complies with additional requirements in Section 8 of the standard, which addresses combustion air supply and structural attachment. Check for “Mobile Home Approved” on the rating plate.
A: Yes, provided the appliance is specifically listed for mobile home use and complies with additional requirements in Section 8 of the standard, which addresses combustion air supply and structural attachment. Check for “Mobile Home Approved” on the rating plate.
Q: What are the consequences of non-compliance?
A: Non-compliant appliances cannot be legally sold or installed in most jurisdictions. They will not pass building inspection, insurance claims may be denied in case of a fire, and the manufacturer may be subject to product recalls and liability lawsuits.
A: Non-compliant appliances cannot be legally sold or installed in most jurisdictions. They will not pass building inspection, insurance claims may be denied in case of a fire, and the manufacturer may be subject to product recalls and liability lawsuits.
Article published for informational purposes. Always refer to the latest edition of ANSI Z21.91-2017 (or its successor) for complete regulatory language. © 2026