A Tank in Time Saves Lines

For a leak-free plumbing system, be equipped for thermal expansion

House Doctor | Gary Dymski

March 6, 2003

Q. A few months back, I noticed my gas water heater pressure-relief valve was leaking. I replaced the valve, but the drip remained. A plumbing supply person told me the valve was defective, so I installed another valve. This valve also leaked. A few days later, I noticed that when any water faucet was first turned on, the water would gush out for a second or two, then would return to normal.

The valve is still leaking. I do not have any type of expansion tank on the water heater, just one on the oil-fired hot water furnace.

The leak is small, maybe a gallon each week. But why this happened all of a sudden is troubling. I've been in the house for 17 years, and I'm pretty handy. I'm sure I can fix this problem if I can figure out what's causing it. -Michael E. Wojcicki, West Babylon

A. You likely need a thermal expansion tank for the water heater. The water pressure in your plumbing system seems to have increased, perhaps because of new construction in your area or new water lines. Regardless, you should have had an expansion tank installed when the water heater was installed.

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When water is heated, it expands and volume is increased. Hot water plumbing, then, must make provisions for this expansion. An expansion tank gives water a place to go when thermal expansion occurs. When a water heating cycle ends, or when any faucet within the system is turned on, pressure is reduced, and water drains from the expansion tank back into the system.

To diagnose thermal expansion, first install a water-pressure gauge on the drain valve (it looks like an outdoor faucet) of the water heater. Once the gauge is in place - it screws on the valve like a hose - turn on the faucet. The gauge can now read the system pressure. Turn the thermostat down to its coolest reading.

Open a hot-water faucet inside and allow it to drain about 20 percent of the hot water in the tank. (Figure it out: If your tank is 50 gallons in capacity, 20 percent is 10 gallons.)

Once the hot water has been drained, turn off all faucets and fixtures, hot and cold, in the plumbing system and turn off the drain valve, too.

Check the reading on the water-pressure gauge. Return the thermostat to normal, so the water heater will cycle on. If the system is closed, the pressure gauge should start to climb. The pressure should increase, then hold briefly, then increase again. When it increases the second time, the pressure relief valve should open and release water. (You might want to put a bucket beneath it to catch the water.) When water pressure is reduced to the setting on the valve, the valve will close.

If your water pressure is above 60 pounds per square inch, it may be necessary to install a pressure-reducing valve, as well as an expansion tank. This valve reduces water supply pressure and can prolong the life of fixtures and valves. The pressure-reducing valve and the expansion tank are installed on the cold-water supply line, with the tank being placed on the line between the water heater and the valve. (Since you say you're pretty handy, you probably can install these devices. If you have any doubt whatsoever, contact a licensed plumber to do the work.)

Don't depend on the pressure-relief valve on the water heater to handle thermal expansion. This valve is designed as an emergency device, to release pressure. In you don't install an expansion tank, thermal expansion could cause damage in the plumbing system, including reducing the life span of faucets and fixtures.