The article on heat exchangers that was published in HP128 included language from Section 405.1 of the Uniform Solar Energy Code (USEC). The article stated that double-wall exchangers were required on solar energy systems heating potable water, with no exceptions. Some readers took issue with this statement.

To clarify: In the context of codes, an "exception" is a special blanket alternative to a code provision. It will be published below the provision and start with the word "exception" in bold print. Exceptions usually have one or more conditions that must be adhered to for the exception to be an accepted alternative to the requirement. There are no exceptions to 405.1 published in the 2006 USEC.

The USEC also includes an Appendix D that gives guidance for "Engineered Systems," including this definition: "Engineered Solar System: A system designed for a specific building project with drawings and specifications indicating materials to be installed, all as prepared by a person registered or licensed to perform solar design work." The guidance in the appendix gives conditions that must be adhered to in order to conform to the intent of the code when a single-wall heat exchanger is specified in an engineered system:

Appendix D 3.2 Single-wall heat exchangers shall be permitted if they satisfy all of the following requirements:
1) The heat-transfer medium is either potable water or contains essentially nontoxic transfer fluids having a toxicity rating or class of 1 (see Section 206).
2) The pressure of the heat-transfer medium is maintained at less than the normal minimum operating pressure of the potable water system (UPC:L3.2). Exception: Steam complying with Section L3.2 (1).
(3) The equipment is permanently labeled to indicate that only additives recognized as safe by the FDA shall be used in the heat-transfer medium.

Appendix D 3.2 pertains to engineered systems, the approval of which is up to the local authority having jurisdiction (AHJ), a.k.a. the local building or plumbing department.

At first glance, Section 405.1 and Appendix D 3.2 may seem to be at odds. This has led many AHJs to make new, specific rulings related to the use of single-wall systems that are gaining popularity nationwide, while ensuring their safety. This past November, for example, Oregon's Building Codes Division proactively approved the use of single-wall systems statewide as long as several conditions are met.

Chuck Marken • Solar Thermal Editor