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Q.Is it acceptable to use plastic piping for the pressure-relief discharge on a domestic water heater? The BOCA code in my area requires that the discharge piping be rated at or above the temperature of the system, but it’s not clear whether that refers to the temperature of the water in the tank or the temperature setting of the relief valve.

A.Redwood Kardon responds: Without protection, a domestic water heater whose thermostat has failed would see a continuous rise in temperature and pressure. When the water pressure exceeded the capacity of the tank (typically 300 psi), the tank would burst with enough force to send an average-size car 125 feet in the air (see "Exploding Water Heater," Eight-Penny News , 9/93).

A temperature- and pressure-relief valve is designed to prevent such catastrophic failures. In my area, we follow the IAPMO (International Association of Plumbing and Mechanical Officials) Uniform Plumbing Code , which allows the use of galvanized steel, hard-drawn copper, chlorinated polyvinylchloride (CPVC), polybutylene (PB), or code-listed straight sections of relief-valve drain tube.

It’s true that the temperature ratings for CPVC and PB are well under the settings (210°F or less) for most relief valves. The consensus among inspectors I’ve talked to, however, is that the relief cycle for an excessively high-temperature or high-pressure condition is so intermittent and short-term that CPVC or PB are acceptable materials for this application.

Redwood Kardon is a building inspector for the City of Oakland, Calif., and author of Code Check : A Field Guide to Building a Safe Home . An online version of Code Check can be found on the Web at www.codecheck.com.