ANSI Z21.10/CSA 4 (Water Heaters)

Standards within the ANSI Z21.10 (or the CSA 4) series specify guidelines for newly produced gas water heaters. These water heaters provide heating to a massive portion of the population, and, in the United States alone, around 5 million are shipped annually.

What Are the ANSI Z21.10 Standards for Water Heaters?

There are two ANSI Z21.10 standards: ANSI Z21.10.1-2026/CSA 4.1-2026: Gas water heaters, volume I, storage water heaters with input ratings of 75,000 Btu per hour or less and ANSI Z21.10.3-2019(R2024)/CSA 4.3-2019(R2024): Gas-fired water heaters, volume III, storage water heaters with input ratings above 75,000 Btu per hour, circulating and instantaneous.

ANSI Z21.10.1/CSA 4.1

Their applications are clearly displayed by their titles, but to clarify, the primary difference between these two standards is the input ratings of the storage water heaters that they cover. ANSI Z21.10.1-2026/CSA 4.1-2026 applies to newly produced, automatic storage water heaters having input ratings of 75,000 Btu/hr or less, constructed entirely of new, unused parts and materials.

ANSI Z21.10.3/CSA 4.3

Alternatively, ANSI Z21.10.3-2019/CSA 4.3-2019 applies to newly produced, large automatic storage water heaters having input ratings above 75,000 Btu/hr, instantaneous water heaters, and circulating water heaters, including booster water heaters, constructed entirely of new, unused parts and materials.

Which Heaters Does ANSI Z21.10 Cover?

The water heaters or appliances covered by both standards are for use with natural gas, manufactured gas, mixed gas, liquefied petroleum gases, and LP gas-air mixtures, as well as recreational vehicle installation for use with liquefied petroleum gases only, mobile home and recreational vehicle installation convertible for use with natural and liquefied petroleum gases when provision is made for the simple conversion from one gas to the other, and combination potable water/space heating applications.

ANSI Z21.10.1-2026/CSA 4.1-2026 and ANSI Z21.10.3-2019/CSA 4.3-2019 help to assure reliability and efficiency within their covered water heaters through expansive guidance for construction and performance, and they also include manufacturing and production tests.

Development of ANSI Z21.10/CSA 4 Standards

Each of these standards possesses two designations, and this derives from actions carried out in response to the Free Trade Agreement between the United States and Canada on January 2, 1988. At this time, significant attention was given to the harmonization of safety standards addressing gas-fired equipment for residential, commercial, and industrial applications in the US and Canada. The elimination of differences between the standards of these two nations was viewed as a means to remove potential trade barriers, providing an atmosphere for North American manufacturers to market more freely and taking a step toward international standards.

In result, several subcommittees were established, and, while ANSI Z21.10.1-2026 is CSA 4.4-2026 and ANSI Z21.10.3-2019 is CSA 4.3-2019, the documents also include sections for items unique to Canada and the United States to be followed by their respective users.

Changes to ANSI Z21.10.1-26/CSA 4.1-2026

ANSI Z21.10.1-2026/CSA 4.1-2026 was recently revised. The ninth edition of the American National Standard for storage water heaters with input ratings of 75,000 Btu per hour or less updates the eighth edition of the same standard that was released in 2019. In its recent revision, it saw the following changes of note:

• The Scope was revised to clarify applicability with respect to fuel types, installation environments (including manufactured homes, park model trailers, recreational vehicles, and outdoor installations), and combination potable water/space heating applications.
• Clause 4, Construction, was revised and expanded to clarify requirements for materials, corrosion resistance, component accessibility, burner construction, venting interfaces, condensate management, and protection against combustion products.
• Clauses 4.1, General Construction and Assembly, 4.2, Materials, 4.24, Flue tube wall thickness and coating, 4.32, Vent and air-intake pipes of direct vent systems, and 4.33, Instructions, have been updated to strengthen provisions applicable to direct vent and Category II, III, and IV water heaters, including venting materials, condensate disposal, vent-air intake systems, and installation documentation.
• Clauses 4.1, General Construction and Assembly, 4.15, Manually operated gas valves, 4.16, Gas appliance pressure regulators, 4.30, Automatic flue damper devices, and 4.34, Marking, have been revised to improve alignment and clarity of requirements applicable to water heaters intended for installation in manufactured homes, park model trailers, and recreational vehicles.
• Clauses 4.10, Automatic gas ignition systems, and 4.21, Automatic gas shutoff systems, have been revised to update requirements for automatic gas ignition systems, automatic gas shutoff systems, and temperature-limiting devices to reflect current safety expectations and referenced component standards.
• Clause 4.31, Electrical equipment and wiring, has been revised to clarify requirements for electrical construction, wiring methods, protection, marking, and evaluation of low-voltage, line-voltage, and direct-current systems.
• Clauses 4.33, Instructions, and 4.34, Marking, have been revised to improve clarity, consistency, and usability of installation instructions, operating instructions, warning statements, rating plate information, and safety markings.
• Clause 5, Performance, has been revised to improve clarity of test conditions, installation configurations, and evaluation methods, including provisions related to venting systems, damper devices, and direct vent water heaters.

ANSI Z21.10.1-2026/CSA 4.1-2026: Gas water heaters, volume I, storage water heaters with input ratings of 75,000 Btu per hour or less and ANSI Z21.10.3-2019(R2024)/CSA 4.3-2019(R2024): Gas-fired water heaters, volume III, storage water heaters with input ratings above 75,000 Btu per hour, circulating and instantaneous are available on the ANSI Webstore.

Questions About ANSI Z21.10/CSA 4 Standards?

Both ANSI Z21.10.1-2019/CSA 4.1-2019 and ANSI Z21.10.3-2019/CSA 4.3-2019 were prepared by the Z21/CSA Joint Technical Advisory Group on Standards for Gas-Fired Water Heaters and were formally approved by ANSI.

Please direct any technical questions relating to this American National Standard to the developer. You can find the contact information for all ANSI-accredited standards developers here: List of ANSI-Accredited Standards Developers (ASDs).

For further information, please refer to: Who to Contact for Standards Related Questions.