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...

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