Quick answer
A water heater relief valve drips for one of three reasons. Most often, thermal expansion in a closed plumbing system pushes pressure toward the valve's 150 psi limit because the expansion tank has failed or is missing. Second, debris or wear stops the valve seating. Third, and rarest, the water is overheating toward 99 °C. Never cap or plug the discharge pipe.
Your water heater relief valve is dripping because it is either doing its job or no longer able to. The temperature and pressure (T&P) valve near the top of the tank opens at 150 psi or 99 °C, whichever comes first, and sends water down the discharge pipe toward the floor. When it drips matters more than how much, and the pattern sorts the three causes quickly.
Drips only while the tank is heating: thermal expansion in a closed system
Water expands as it heats. In an open system the extra volume pushes back into the street main and nobody notices. Add a backflow preventer, a check valve at the meter or a pressure-reducing valve, and the system is closed: the expanded water has nowhere to go, pressure climbs with every heating cycle, and the T&P valve weeps a few tablespoons to relieve it.
The cure is an expansion tank on the cold line, a small steel tank with an air cushion behind a rubber bladder. When it fails or was never fitted, the relief valve becomes the expansion device. These signs point that way.
- Dripping follows a shower, a laundry load or any heavy hot-water use, then stops once the tank is back to temperature.
- Tap pressure surges for a second when a faucet is first opened.
- The expansion tank sounds dull and solid all over when tapped, where a healthy one sounds hollow on the air side.
- Pressing the air valve on the expansion tank releases water, which means the bladder has ruptured.
Homes on a private well are usually spared, because the well pressure tank absorbs the expansion unless a check valve sits between it and the heater. Whether your house needs one is covered in do I need an expansion tank on my water heater.
Drips all the time: debris on the seat or a worn valve
A steady drip that ignores the heating cycle is the valve itself. Scale flakes lodge on the seat, often right after someone lifts the test lever on an old valve, and the disc can no longer close. Springs also weaken with age and start to open below their rating. A valve in this state gets replaced, not cleaned: it is a safety device, and one that leaks may also be one that sticks.
Hot bursts or steam: genuine over-temperature from a stuck thermostat
The rare cause is the dangerous one. If the taps are running far hotter than usual, the discharge comes in hot spurts and the tank rumbles, a thermostat has stuck closed or an electric element has grounded and is heating without control. The valve is now the only thing limiting the tank, so act before calling.
- Gas tank: turn the control knob to OFF.
- Electric tank: switch off the double-pole breaker.
- Leave the cold supply open so the tank is not sealed, keep clear of the discharge pipe, and call for service the same day.
What the valve is preventing is described in can a water heater explode.
Never cap or plug the discharge pipe
The discharge pipe has to run full-size from the valve down toward the floor or a drain and end open, with no cap, no plug, no shut-off valve and no threads on the end that would accept one. A capped relief valve turns a nuisance drip into a sealed pressure vessel. Put a bucket under it and book the repair.
What the repair involves: valve replacement and expansion-tank pre-charge
- Pressure test first: a gauge with a tell-tale needle on the drain valve or a laundry tap records the peak over a full heating cycle. Static pressure above about 80 psi calls for a pressure-reducing valve.
- T&P valve replacement: the tank is partly drained, the old valve unthreaded, and a new one of the same pressure, temperature and capacity rating fitted with the discharge pipe reconnected.
- Expansion tank: sized to the heater and the house pressure. The air side is pre-charged with a tire gauge and pump to match the house static pressure before the water goes back on, because a tank left at its factory charge in a higher-pressure house is half-used from day one.
- Over-temperature: the thermostat or gas control is replaced and the tank temperature verified before the heater is left running.
These repairs sit inside the usual $150–$800 range for water heater work, after a $90–$150 diagnostic visit that is credited when you go ahead, and the written quote separates the valve from the expansion tank. On a tank already near the end of an 8 to 12 year life, ask for the replacement price too.
ZK Mechanical does relief valve replacement and expansion tank installation as part of water heater service from its Burlington shop. Call (647) 801-1252 or book online.