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Q.We recently had to replace our relatively new gas-fired water heater when its internal flue tube collapsed, ruining the thermostat and causing hot combustion gases to spew out the front. The installer, who claimed that the water heater was ruined by thermal expansion, added a thermal-expansion tank to the system. But the temperature- and pressure-relief valve on the new heater still leaks occasionally, and even though we've had it replaced, the problem persists. What's going on?

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 home's water pressure is 80 psi, the installer needs to adjust the air charge to match final delivery pressure; if he doesn't, you will lose half the tank's capacity as it takes on the added 40 psi.

Even a 2.1-gallon expansion tank that's been properly charged to 80 psi, though, can be inadequate for a 40-gallon water heater. Here's why:

A 40-gallon water heater can be subject to as much as 1 gallon of thermal expansion (the exact amount depends on inlet and storage temperature range). But by applying Boyle's law (which states that volume at a constant temperature is inversely proportional to pressure, or P1 x V1 = P2 x V2), we find that a 2.1-gallon thermal-expansion tank is undersized even with the correct air charge of 80 psi. For example, when P1 = 80 and V1 = 2.1, if V2 = 1 then P2 will equal 168; this is why your 150-psi T&P relief valve is leaking periodically.

Substitute 1.1 for V1 (if the plumber fails to adjust the tank's air pressure upward) and .35 for V2 (the volume left in the pressure tank if you get another 3/4 gallon of thermal expansion), and you could see a potential expansion tank pressure of more than 250 psi. But with a 4.5-gallon expansion tank — the next, larger size of residential thermal-expansion tank — the water in your system will have plenty of room to expand without blowing your T&P relief valve or ruining your water heater.

By the way, water pressures in municipal water systems seldom remain constant; they tend to spike at night as system-wide usage drops off. Those spikes in pressure are then trapped within your home's plumbing system, which will increase the demands on the thermal-expansion tank for proper protection. By spending $15 or $20 more for the larger thermal-expansion tank, you have purchased a pretty cheap insurance policy for your hot-water heater.