A drip from the discharge tube of your water heater’s temperature and pressure relief valve almost always points to a real problem you cannot ignore. A t&p valve leaking is the safety valve doing exactly what it is supposed to do: vent water when tank pressure or temperature exceeds safe limits, typically 150 PSI or 210 degrees Fahrenheit. The leak itself is not the issue. The condition that caused the valve to open is the issue, and ignoring it can lead to water heater rupture or catastrophic flooding when the valve eventually fails closed.
Understanding What the T&P Valve Does
Every water heater has a temperature and pressure relief valve (T&P) plumbed into the top or upper side of the tank with a discharge tube running down to within 6 inches of the floor or to a drain. The valve has two functions in one body. It opens if water temperature exceeds 210 degrees Fahrenheit, which prevents the tank from converting to a steam bomb. It also opens if internal pressure exceeds 150 PSI, releasing water before the tank ruptures from over-pressurization.
When the valve opens for either reason, water exits through the discharge tube. If the cause persists, the valve continues to drip. Even an intermittent drip indicates the tank is approaching the safety threshold somewhere in its operation. Tracking down which threshold (and why) is the diagnosis you need.
Thermal Expansion: The Most Common Cause
Modern homes with backflow preventers, pressure-reducing valves, or check valves on the main water line create a closed plumbing system. When the water heater heats cold water, the water expands by about 4 percent. Without somewhere to go, that expansion pushes system pressure up rapidly, often hitting the 150 PSI relief threshold on a single heating cycle.
Diagnose this by checking your water pressure with a gauge attached to an outdoor hose bib. Pressure between 40 and 80 PSI is normal. If static pressure reads 70 to 80 PSI and you see the gauge climb to over 100 PSI during a water heater cycle, thermal expansion is your culprit.
The fix is an expansion tank. A 2-gallon Watts PLT-5 (around 45 dollars) or Amtrol ST-5 plumbed onto the cold water line near the water heater absorbs the expansion volume and prevents pressure spikes. Pre-charge the tank to match your static water pressure (typically 50 to 60 PSI) before installing.
High Incoming Water Pressure
Some municipalities deliver water at 90 to 130 PSI, which exceeds the safe range for residential plumbing. The water heater is just one of many appliances that suffer; faucet cartridges and dishwasher valves also wear out fast under high pressure.
Install a pressure-reducing valve (PRV) on the main supply line where it enters the house. Watts N45B and Zurn Wilkins NR3XL are common choices in the 100 to 200 dollar range. Set the outlet to 60 to 70 PSI. If you already have a PRV that is failing, replace it; PRVs last about 7 to 12 years before the rubber diaphragm fails.
Failed or Stuck Open Valve
T&P valves themselves wear out. Mineral scale deposits on the seat prevent the valve from sealing fully, and a small drip starts even at normal operating pressure. Test by lifting the test lever on the valve briefly; water should flow strongly when held, and the lever should snap shut when released. If the valve continues dripping after the test, the seat is compromised.
Replace the T&P valve. A Watts 100XL or Cash Acme F-30 costs 25 to 45 dollars at any hardware store. Shut off the water heater (turn off gas at the unit, or breaker for electric), close the cold water inlet, open a hot water faucet to relieve pressure, drain a few gallons from the tank, then unscrew the old valve with a pipe wrench. Wrap new threads with PTFE tape, thread in the new valve hand-tight plus 1 to 1.5 turns with the wrench. Reconnect the discharge tube. Restore power and water; refill the tank before re-energizing electric elements.
Temperature Too High
If the valve trips on temperature rather than pressure, the water heater is overheating. The thermostat could be stuck, the dip tube could have failed (allowing cold water to mix at the top instead of the bottom), or the thermostat setting could simply be too aggressive.
Check the temperature setting on the thermostat. 120 degrees Fahrenheit is the standard recommendation. If the dial is set to 140 or higher, drop it to 120 and observe. If temperature creeps above 130 degrees despite the lower setting, the thermostat itself is failing and needs replacement. For electric water heaters, the upper or lower thermostat replacement runs 15 to 30 dollars and takes 30 minutes; for gas units, the gas control valve (which contains the thermostat) costs 150 to 300 dollars.
How to Tell Which Cause You Have
Touch the discharge tube outlet to feel the water temperature. If the water is hot, the valve is opening on temperature. If the water is cold or warm, the valve is opening on pressure. Pressure-driven leaks tend to be intermittent (just after a heating cycle when expansion peaks). Temperature-driven leaks come during longer hot-water use when the thermostat is calling for heat continuously.
- Drip occurs right after heating cycle and stops within an hour: thermal expansion
- Constant drip with hot discharge water: thermostat or dip tube failure
- Drip occurs at any time with cold or warm discharge: high system pressure or failed PRV
- Heavy stream rather than drip: valve has failed open, replace immediately
- No water visible but valve assembly looks wet: condensation, not a leak
What Not to Do
Never plug, cap, or otherwise restrict the T&P discharge tube. The valve must be free to vent at all times. Plugging the tube turns the water heater into a pressure vessel without protection, and tank rupture under steam pressure can launch the heater through a roof. This is not hyperbole; documented incidents exist.
Never replace a T&P valve with one of different pressure or temperature rating. Match the new valve to the BTU rating on the water heater nameplate and the pressure rating stamped on the old valve (150 PSI is universal for residential). Never install a T&P valve on the side outlet if the manufacturer requires top installation; the valve must be in contact with the hottest water in the tank.
When to Call a Plumber
DIY-friendly fixes include installing an expansion tank, replacing a T&P valve, and adjusting thermostat settings. Call a licensed plumber if the water heater is over 10 years old (replacement may be more cost-effective), if you have a gas water heater and suspect the gas control valve, if you need to install or replace a PRV on the main supply, or if any signs of corrosion show on the tank itself.
A water heater leaking from the tank (rather than the valve) is end-of-life and needs replacement. A 50-gallon gas water heater runs 1,400 to 2,200 dollars installed. Tankless units run 3,500 to 5,500. Catching the T&P symptom early and fixing the root cause typically buys you 3 to 7 more years on a heater that would otherwise fail expensively.
