Every water heater has a temperature and pressure relief valve near the top, with a pipe running down toward the floor. It is the heater's most important safety device. If it drips, it is opening, and it is opening because something is wrong with the pressure. Ignoring it is a mistake; so is simply replacing the valve without asking why.
What the valve does
If the water in the tank gets too hot, or the pressure gets too high, the valve opens and lets water out until things are back within limits. Without it, an overheating tank could fail violently.
Why it drips
Thermal expansion. Water expands as it heats. In an old house the expansion pushed back into the town main and nobody noticed. Modern houses have a check valve or a backflow preventer on the water meter, which stops water going back out. The heated water then has nowhere to go, and the pressure climbs each time the heater runs, until the relief valve opens. This is the most common cause today, and it has become common in Rockland County as water suppliers fit backflow devices at the meter.
High house pressure. If the pressure in the house is above about 80 psi, the relief valve is near its limit all the time. A pressure gauge on an outdoor faucet tells you. The cure is a pressure-reducing valve on the main supply, and, once one is fitted, an expansion tank, because the reducing valve also stops water going back out.
A failed valve. Valves that have opened many times, or that are old, do not always reseal. If the pressure is normal and the valve still drips, replace it.
Overheating. A thermostat that has failed can let the tank overheat until the valve opens. Very hot water at the taps, and steam from the relief pipe, mean turn the heater off and call a plumber.
The expansion tank
An expansion tank is a small steel tank, about the size of a football, fitted on the cold water pipe near the heater. Inside is a rubber diaphragm with air on one side. As the water heats and expands, it pushes into the tank against the air, and the pressure in the system stays steady.
- It must be sized for the water heater, and its air charge set to match the house pressure when it is fitted.
- The diaphragm eventually fails. Tap the tank: the top should sound hollow. If it sounds full of water throughout, or the relief valve starts dripping again after a few years, the expansion tank has failed and needs replacing.
- Many towns now require one when a water heater is replaced. Ask the plumber.
Testing the relief valve
Once a year, put a bucket under the pipe and lift the lever briefly. Water should rush out and stop when you let go. If nothing comes out, or it keeps dripping afterwards, the valve needs replacing. Some plumbers advise against testing old valves, because they may not reseal, so if the heater is old, leave the test to the annual service.
Never do this
Never cap or plug the relief valve's pipe, never fit a smaller valve, and never remove the valve because it drips. A capped relief valve turns a water heater into a pressure vessel with no safety.
When to call a plumber
- A relief valve that drips or discharges regularly
- House pressure above 80 psi
- To fit or replace an expansion tank or a pressure-reducing valve
- Any sign of overheating
Find a licensed plumber near your town.