The temperature and pressure relief valve, usually called the T&P valve, is the most important safety device on a tank water heater. It's a spring-loaded valve with a temperature-sensing probe that extends into the tank. If either temperature or pressure exceeds its set point, it opens and lets water out through a discharge pipe until conditions are safe again. Almost nobody thinks about it until it drips.
What the valve protects against
A water heater has controls that should keep water at a set temperature. If a thermostat or gas control failed and kept heating, temperature and pressure inside the sealed tank would keep rising. The relief valve is the backstop: it opens before conditions become dangerous, and releases water so fresh, cooler water enters. That's why it has to stay unobstructed, and why its discharge line has to carry water somewhere safe.
Why relief valves drip or discharge
Thermal expansion. In a closed plumbing system, heating water raises pressure until the valve lets a little out. Occasional drips after the heater has run are the classic sign. An expansion tank is often the fix.
High supply pressure. If the water coming into the home is at high pressure, the valve may be pushed close to its set point before heating even starts. A pressure-reducing valve may be missing or failing.
Overheating. A stuck thermostat or faulty gas control can let water get too hot. This is the scenario the valve exists for, and it needs prompt attention.
A worn valve. Mineral deposits, corrosion or debris on the seat can stop the valve sealing properly, so it weeps even when conditions are normal. Very common in hard water and on older valves.
California factors
Hard water in much of the inland south and the Central Valley builds scale on valve seats, making weeping valves common. Hardness varies by district; ask your water provider about yours.
Closed systems created by pressure regulators and check valves are widespread, making thermal expansion a frequent cause of drips.
Location of the discharge. In garages, lines often run toward the floor; in exterior closets, outside; in interior closets and upper floors, the route matters more because a discharge lands in living space. Rules for discharge routing vary by jurisdiction and we don't state them; the installer confirms what applies.
Seismic strapping. Strapping and discharge lines sit close together on many installations. When a heater is secured or replaced, the installer makes sure the discharge line stays clear and supported.
A drip today can mean a pressure problem throughout the house.
What not to do
Please don't
- Cap, plug, tie off or block the valve or its pipe
- Lift the lever to test or "flush" the valve
- Try to replace the valve yourself; the tank must be depressurized and partly drained
- Turn up the heater's temperature to compensate for a drip
Do keep people clear of the outlet, and put a bucket under a drip if it's safe to reach without touching the heater.
How the technician deals with it
First they establish why the valve is releasing. They measure incoming water pressure and watch how it behaves during a heating cycle, check for a pressure-reducing valve and whether it's working, look for devices that close the system, and check that the heater isn't overheating by testing its controls. Then they fix the cause: an expansion tank, a new regulator, a thermostat or control repair. If the valve itself is worn, it's replaced with one correctly rated for the heater. The discharge line is checked for a clear, unobstructed path.
What the discharge tells you
How the valve releases water is itself a clue. A few drops after the heater has been running point toward thermal expansion. A steady trickle regardless of what the heater is doing suggests a worn seat or high supply pressure. Hot water or steam released in bursts suggests the water inside is getting too hot, which needs prompt attention. Mineral crust around the outlet shows a valve that has been weeping for a long time. Passing these observations on helps the technician.
When the valve should be replaced
A relief valve that weeps under normal pressure and temperature has usually lost its seal and should be replaced. So should one that is heavily corroded or crusted with minerals, or one that has discharged hot water repeatedly. Many technicians also fit a new valve whenever a heater is replaced, since the old one is matched to the old tank and has aged alongside it. The new valve must suit the heater's rating, which the technician checks.
What affects the cost
| Cause | Typical fix |
|---|---|
| Worn valve | Valve replacement; a modest repair. |
| Thermal expansion | Expansion tank installation. |
| High supply pressure | Pressure-reducing valve installation or replacement. |
| Overheating | Thermostat or gas control repair. |
| Discharge line problems | Rerouting or replacing the line. |
How long it takes
Diagnosis is usually quick. Valve replacement and expansion tank installation are often completed in one visit. A regulator or control repair may add time. You'll know the plan and the price before work starts.
Find the cause of the drip before it becomes something bigger.
Relief valve questions
It's a safety valve on the top or side of a tank water heater. If the water inside gets too hot or the pressure gets too high, the valve opens and releases water through its discharge line, bringing conditions back to a safe level. It protects the tank from the dangerous build-up that could otherwise occur if controls failed.
The most common reasons are thermal expansion in a closed plumbing system, high incoming water pressure, a thermostat or control fault letting the water get too hot, or a valve that has worn, corroded or collected debris so it no longer seals. The technician works out which, because a new valve alone won't fix the first three.
No, never. Capping, plugging or blocking the valve or its discharge line removes the tank's protection against dangerous temperature and pressure. A dripping valve is a symptom to diagnose, not a leak to seal. Keep the outlet clear and call for a technician.
We don't give instructions for operating the valve. Manufacturers provide guidance on testing, but on an older valve, lifting the lever can release scalding water and sometimes leaves the valve unable to reseal. A technician can check it safely as part of a maintenance visit.
Keep everyone away from the discharge outlet, because the water can be scalding. If water is spreading, close the home's main water shutoff if you know where it is. Don't touch the heater's controls. Call right away and describe what's happening. If you smell gas, leave the building and call your gas utility and 911 from outside.
The discharge line carries released water to a safe location. Where it may terminate and what it's made of depend on the jurisdiction and the manufacturer, and we don't state those rules. If yours has been cut short, capped, or runs somewhere that worries you, mention it and the technician will look.
It varies with water quality and conditions. In hard water, mineral deposits can build up on the valve seat. A valve that has been weeping for a long time is often worn. Many technicians replace the valve when replacing the heater and inspect it at maintenance visits.
Tankless units are typically installed with a pressure relief valve on the water connections, often built into the isolation valve set. It serves a similar protective purpose. The same rule applies: never cap or block it. The installer routes its discharge to a spot where any release is easy to see and won't cause damage.
An occasional drip usually isn't an emergency, but it should be looked at soon because it signals a problem. A steady discharge of hot water, steam, or a valve that opens repeatedly is more urgent. Call and describe it, and we'll help you judge.