Water Heater Overflow Pipe Dripping: Causes & Fixes

1. Water Temperature Set Too High

If your water heater thermostat is set above 120°F (49°C), the increased heat raises system pressure, causing the T&P relief valve to open and drip water through the overflow pipe.

1. Turn off power to the water heater at the breaker (electric) or set the gas valve to “Pilot” (gas).
2. Locate the thermostat dial on the heater. On gas units it is on the gas valve; on electric units, remove the access panel.
3. Lower the temperature setting to 120°F (the recommended safe level).
4. Wait 1–2 hours for the water to cool and pressure to stabilize.
5. Check whether the overflow pipe has stopped dripping. If it has, monitor over the next 24 hours.

2. High Water Supply Pressure

Incoming municipal water pressure above 80 PSI can push system pressure beyond the T&P valve’s threshold, forcing it to discharge water continuously through the overflow pipe.

1. Attach a water pressure gauge to an outdoor hose bib or laundry faucet to measure your supply pressure.
2. If pressure exceeds 80 PSI, locate the pressure reducing valve (PRV) on the main supply line near where it enters your home.
3. Turn the PRV adjustment screw clockwise in small increments to lower pressure, rechecking with the gauge after each adjustment.
4. Aim for a supply pressure of 60–80 PSI.
5. If no PRV is installed, have one fitted by a licensed plumber.
6. Once pressure is corrected, monitor the overflow pipe for 24 hours.

3. Thermal Expansion in a Closed Plumbing System

Homes with a backflow preventer or check valve on the supply line create a “closed system,” trapping expanded hot water with nowhere to go—this forces the T&P valve to relieve pressure through the overflow pipe.

1. Confirm whether your home has a check valve or backflow preventer on the cold supply line; these are common in newer homes and some municipal systems.
2. If confirmed, the solution is to install a thermal expansion tank on the cold-water supply line feeding the water heater.
3. Shut off the cold water supply to the water heater.
4. Install the expansion tank on the cold supply line using a tee fitting, following the tank manufacturer’s instructions.
5. Pre-charge the tank’s air bladder to match your supply pressure using a bicycle pump or air compressor, then install.
6. Restore water supply and check that the overflow pipe stops dripping.

4. Faulty or Worn T&P Relief Valve

The temperature and pressure relief valve can wear out over time, develop mineral buildup, or fail to reseat properly after opening—causing a persistent drip even when pressure and temperature are normal.

1. Turn off the power or gas supply to the water heater and allow the water to cool for at least an hour.
2. Place a bucket under the overflow pipe discharge point.
3. Carefully lift the T&P valve’s test lever for 2–3 seconds, then release it. This can flush debris and allow the valve to reseat.
4. Wait 10 minutes and check whether dripping has stopped.
5. If the valve continues to drip, it must be replaced. Turn off the cold water supply to the heater and open a hot water faucet to relieve pressure.
6. Unscrew the old T&P valve using a pipe wrench. Wrap the threads of the new valve with PTFE (plumber’s) tape.
7. Thread in the new T&P valve by hand, then tighten with a wrench (do not over-tighten).
8. Reattach the overflow pipe, restore the water supply, and turn the heater back on.
9. Test the new valve using the lever after 24 hours to confirm it reseats correctly.

5. Sediment Buildup Inside the Tank

Mineral sediment accumulating at the bottom of the tank can cause uneven heating and hot spots that raise localized temperature and pressure, contributing to the T&P valve opening and the overflow pipe dripping.

1. Turn off the power or gas and let the water cool for an hour.
2. Connect a garden hose to the tank’s drain valve at the bottom of the heater and run the hose to a floor drain or outside.
3. Open a hot water faucet in the house to allow air into the system.
4. Open the drain valve and allow the tank to fully drain. Watch for significant sediment (gritty or discolored water).
5. Briefly open the cold water supply to flush remaining sediment out through the drain hose. Repeat until the water runs clear.
6. Close the drain valve, remove the hose, and refill the tank before restoring power or gas.
7. To prevent future buildup, flush your water heater annually.

6. Corroded or Incorrectly Sized Overflow Pipe

If the discharge pipe connected to the T&P valve is corroded, kinked, or has accumulated mineral deposits, it can trap residual water that drips out slowly even when the valve itself is functioning correctly.

1. Inspect the overflow (discharge) pipe visually along its full length for corrosion, kinks, or scale buildup.
2. Confirm the pipe runs downward at all points so water drains fully—no upward loops that can trap water.
3. If corroded or blocked, shut off water to the heater and disconnect the discharge pipe from the T&P valve outlet.
4. Replace with a new section of the correct pipe material (CPVC or copper are standard; check local codes).
5. Ensure the pipe terminates no more than 6 inches from the floor or into a drain, per code, and is not threaded or capped at the end.
6. Reconnect and check for leaks after restoring water supply.

Frequently asked questions

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