Is thermal expansion making the T&P valve leak?
Fixable — but not by replacing the valve
The valve is working correctly. Water expands about 2% when you heat it. If a check valve, backflow preventer, or pressure-reducing valve seals your plumbing off from the street main, that extra volume has nowhere to go — pressure climbs until the T&P valve lifts near 150 psi and lets some out.
How to tell it's this
- The drip starts a few minutes after the burner or element fires, and stops once the tank is up to temperature.
- It never drips overnight when nobody has used hot water.
- Your main has a check valve, backflow preventer, or pressure-reducing valve — common on any house on a water meter fitted since roughly 2000.
- Pressure measures under 80 psi cold, then climbs 20 psi or more after the heater runs.
What to do
- Screw a pressure gauge onto an outside spigot. They cost about $12 at any hardware store.
- Read it before the heater has run. Under 80 psi is normal.
- Run a lot of hot water, wait for the heater to reheat, then read it again. A rise of 20 psi or more confirms thermal expansion.
- Have a plumber fit a thermal expansion tank on the cold inlet.
- Do not just replace the relief valve. A new valve will lift on exactly the same schedule, and you will have paid twice.
What it costs
With a plumber
Add a thermal-expansion tank
$200–$450
installed
Part only
If a pro is already on site
$40–$80
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
The gauge reads over 80 psi before the heater has even run
House pressure is too high →
It drips constantly — heating or not, day and night
The valve itself has failed →
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.