Is household water pressure making the T&P valve leak?
Fixable — at the main, not the heater
If your incoming water pressure is above 80 psi, the relief valve is not faulty — it is the first thing in the house weak enough to complain. Most US plumbing codes cap residential pressure at 80 psi for exactly this reason.
How to tell it's this
- A gauge reads over 80 psi with every tap closed and the heater cold.
- Other fixtures misbehave too: banging pipes, dripping taps, toilet fill valves that run on.
- It drips at any hour, not only after the heater runs.
- You are near the bottom of a hill, or on a street with a tall water tower.
What to do
- Fit a $12 gauge to an outside spigot with everything else in the house turned off.
- Read it. Over 80 psi is the problem. Over 100 psi is damaging every appliance you own.
- Check whether you already have a pressure-reducing valve at the main — a bell-shaped brass body near the shutoff. If it is there, it has probably failed.
- Have a plumber fit or replace it, then re-read the gauge to confirm it settles between 50 and 70 psi.
What it costs
With a plumber
Fit or replace the pressure-reducing valve
$300–$600
installed
Part only
If a pro is already on site
$60–$150
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
Pressure is normal cold but jumps after the heater runs
Thermal expansion on a closed system →
The hot water scalds and the dial is set high
The thermostat is set too high →
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.