A.Dave Yates, a plumbing contractor in York, Pa., responds: Your home's incoming municipal water line contains a backflow preventer (BFP) or a pressure-reducing valve (PRV) or both; either can lead to this problem. As cold water is heated, it expands and must have somewhere to "grow." Before BFPs or PRVs were required on incoming water service lines, this thermal expansion could be absorbed by the municipal water system. However, now that most homes are closed systems (water can enter, but can't return to the street), there's no place for this expansion to go. That's what killed your first water heater.

Your installer diagnosed your problem but may not have installed a large enough expansion tank to correct it. Residential expansion tanks start at 2.1-gallon capacity and go up from there. Regardless of the size, Federal Department of Transportation regulations limit the air charge in a thermal expansion tank to 40 psi for shipping. But if your...

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