Has the T&P valve itself worn out?
Fixable — replace the valve
If pressure is under 80 psi, the thermostat is at 120 °F, and it still drips around the clock, the valve has failed at the seat. Mineral scale and years of sitting untouched do this. Replacing it is a straightforward plumber's job.
How to tell it's this
- It drips constantly: overnight, at weekends, whether the heater has run or not.
- A pressure gauge reads under 80 psi cold and rises less than 20 psi after heating.
- The thermostat has been at 120 °F for a full day and nothing changed.
- The unit is more than about 5 years old and the valve has never been tested.
What to do
- Rule out the other three causes first. A new valve fixes nothing if pressure or heat is what is lifting it.
- Have a licensed plumber replace the valve. The part is $15–$50; the visit is what you are paying for.
- Ask them to check pressure and temperature at the same visit, so you find out if the valve was the cause or the symptom.
- If the heater is past about 12 years, weigh the repair against a replacement you will need soon anyway.
What it costs
With a plumber
Replace the T&P relief valve
$150–$350
installed
Part only
If a pro is already on site
$15–$50
the part itself
Typical US ranges for 2026, not quotes. Labour is 30–50% of an installed price, so where you live moves these numbers more than anything else does. Get two or three local estimates.
If that's not it
It only drips while the heater is running
Thermal expansion on a closed system →
The heater was fitted in the last few months
A newly installed heater →
Not sure which of these it is?
Answer four questions — where it's leaking, tank or tankless, gas or electric, and how
old it is — for a specific cause, a repair-or-replace call, and a cost.