Last updated: July 2026 · Reviewed against Watts and Cash Acme valve documentation and plumbing code guidance
A leaking T&P (temperature & pressure) relief valve is either doing its job or worn out — and telling the difference matters, because one is a safety system working correctly and the other is a simple part replacement. The valve leaks for one of three reasons: excess pressure in your plumbing, excess temperature, or a failed valve. Here is how to diagnose which, and why you should never just cap it.
⚠️ Never plug, cap, or block a T&P valve to stop it leaking. It is the one component preventing your water heater from becoming a pressure hazard. If it discharges, something needs fixing — not silencing.
What the T&P Valve Actually Does
The T&P valve is a safety device that opens automatically if the water temperature exceeds about 210°F or pressure exceeds 150 psi. It releases water through a discharge tube pointed at the floor, relieving the dangerous condition. A valve that opens is often responding to a real problem — so a “leaking” valve is frequently a symptom, not the disease.
Cause 1: Thermal Expansion (Most Common)
When water heats, it expands. In a “closed” plumbing system (one with a backflow preventer or pressure-reducing valve at the main), that expanding water has nowhere to go, so pressure spikes and the T&P valve weeps to relieve it — typically right after a heating cycle. The fix is not the valve; it is installing a thermal expansion tank ($40–$60 part, $150–$300 installed) to give the water somewhere to expand. If your valve drips only after hot water use, this is almost certainly your cause.
Cause 2: Excess Water Pressure
If your home’s water pressure is above 80 psi, the T&P valve may leak continuously. Test it with a $10 pressure gauge screwed onto an outdoor spigot. If pressure is high, you need a pressure-reducing valve (PRV) on your main line — a plumber job, but it protects your whole plumbing system, not just the heater.
Cause 3: Excess Temperature
If the thermostat is set too high (above 140°F) or has failed, the water can overheat and trip the valve. Check your thermostat setting first — 120°F is the recommended setting. If the water is scalding and the valve discharges, and lowering the setting does not help, the thermostat itself may have failed and needs replacement.
Cause 4: A Failed Valve
Sometimes the valve itself is simply worn: debris under the seat, or a weak spring after years of service. Test it by lifting the valve’s test lever and letting it snap back — it should seat and stop. If it keeps dripping afterward, replace it. A new T&P valve costs $15–$30 and is a straightforward swap once the tank pressure is released, though many homeowners have a plumber do it ($100–$200).
How to Diagnose It in 2 Minutes
- Drips only after hot water use → thermal expansion → needs expansion tank
- Drips constantly → test water pressure → high pressure needs a PRV
- Water is scalding hot → thermostat too high or failed
- Still drips after lifting/reseating the lever → valve is worn → replace it
This valve leak often shows up as water pooling at the base, which is why it is a common cause behind both bottom leaks and top leaks.
FAQ
Is a leaking T&P valve dangerous?
A leaking valve itself is a safety feature working. The danger is ignoring it or capping it — that removes your protection against a pressure or temperature failure. Diagnose the underlying cause, but never block the valve.
Why does my relief valve only leak sometimes?
Intermittent leaking right after the heater runs is the classic sign of thermal expansion in a closed plumbing system. The fix is a thermal expansion tank, not a new valve.
Can I replace a T&P valve myself?
Yes, if you are comfortable shutting off power and water, draining a few gallons to relieve pressure, and using a wrench and thread tape. The valve is inexpensive. If high pressure or a failed thermostat is the real cause, address that too or the new valve will leak as well.
Sources: U.S. Department of Energy — Water Heating · Watts and Cash Acme T&P valve documentation · model plumbing code guidance on thermal expansion.