Leaking Temperature Pressure Relief Valve: Causes and Fixes

The T&P (temperature and pressure) relief valve is a safety device — its job is to release water if the tank reaches dangerous pressure or temperature. A dripping T&P valve is either doing its job correctly (something is driving it open) or has failed (stuck partially open). Either way, it needs attention. Here’s how to tell which situation you’re in.

Why Is My Water Heater Pressure Relief Valve Leaking?

The T&P valve has two triggers: temperature (typically set at 210°F) and pressure (typically set at 150 PSI for residential water heaters). When either exceeds the set point, the valve opens and releases water.

Thermal expansion (most common cause): When water heats, it expands. In a “closed system” — where a pressure-reducing valve (PRV) prevents expanded water from pushing back into the street main — the expanding water has nowhere to go except up in pressure. If the pressure exceeds the T&P valve’s set point, the valve opens.

Whether your system is closed: Most Seattle homes have a PRV on the main supply line (it’s required when street pressure exceeds 80 PSI). If you have a PRV, your system is closed. A closed system without an expansion tank is the most common cause of T&P valve leaking in Seattle.

Temperature set too high: If the thermostat is set above 140°F or has failed in the “always on” position, the water temperature can approach the T&P valve’s thermal trigger. Check the thermostat setting and verify the actual temperature at the tap.

Faulty valve: T&P valves contain a spring-loaded mechanism that can fail due to corrosion, scale buildup, or age. A valve that’s been in service for 10+ years or that’s been manually tested (lifting the lever) may not reseal properly. If pressure and temperature are within normal range but the valve still drips, the valve itself has failed.

Is a Dripping Pressure Relief Valve Dangerous?

The drip itself: A slowly dripping T&P valve is not an immediate danger — it’s releasing pressure as designed. However, it signals a condition that should be diagnosed.

The underlying cause may be:
– High system pressure (thermal expansion without an expansion tank) — addresses itself each cycle but indicates a condition that stresses the system over time
– A failing thermostat heating water beyond setpoint — this is a condition that could progress to an actual overpressure event
– A failed valve — a T&P valve that won’t fully close is no longer a reliable safety device

The real danger is NOT having a working T&P valve. Water heater tanks can develop dangerously high internal pressure if the thermostat fails and there’s no functional T&P valve. The result — a catastrophic tank failure — releases enormous energy and can be destructive.

Don’t:
– Cap the discharge pipe
– Tape or plug the valve outlet
– Replace the valve with one rated at a higher pressure or temperature than the original

If the valve is dripping and you don’t know why, have it diagnosed rather than blocked.

How to Replace a Water Heater Pressure Relief Valve

Required tools: Pipe wrench or large adjustable wrench, thread sealant (Teflon tape or pipe dope), bucket, towels.

Prerequisites: Confirm you have the replacement valve with the correct pressure rating (matching the original — the rating is stamped on the valve body, typically 150 PSI/210°F for residential).

Steps:

1. Turn off the water heater. Gas: thermostat to “Pilot.” Electric: turn off circuit breaker.

2. Turn off the cold water supply to the heater.

3. Open a hot water tap in the house to relieve pressure in the system.

4. Attach a hose to the drain valve and drain 2–3 gallons from the tank to lower the water level below the T&P valve port.

5. Disconnect the discharge pipe from the T&P valve. The discharge pipe is typically a rigid pipe or tube running down from the valve to within 6 inches of the floor.

6. Unscrew the T&P valve from the tank using a pipe wrench. It’s threaded into the tank port — turn counterclockwise.

7. Apply thread sealant to the threads of the new valve. Screw the new valve into the port by hand, then tighten with a wrench — do not overtighten.

8. Reconnect the discharge pipe to the new valve outlet.

9. Restore water supply and energy. Allow the tank to refill and reheat. Verify the new valve doesn’t drip.

Cost: T&P valve: $15–$40. Labor if done by a plumber: $75–$150 including the valve.

Water Heater Relief Valve Leaking Constantly — What to Do

A valve that drips continuously (not just during heating cycles) is either stuck open due to internal failure or responding to a continuous high-pressure condition.

Step 1: Test supply pressure. Use a gauge at an outdoor hose bib (typically available for $12 at hardware stores). Normal residential pressure: 50–80 PSI. Above 80 PSI and the system is consistently over the comfortable range. If pressure is 100+ PSI, a PRV adjustment or replacement may be needed.

Step 2: Check if you have an expansion tank. Look at the cold water inlet pipe at the top of the water heater — a properly installed expansion tank is a small tank (typically 2–4 gallons) connected to the cold water inlet line, usually with a T-fitting. If you have a PRV and no expansion tank, thermal expansion is likely the cause.

Step 3: Check the thermostat. Run hot water at a tap for 2 minutes and measure temperature. If it’s approaching 140°F or above, the thermostat setting or a failing thermostat is driving the valve toward its thermal trigger.

Step 4: Replace the valve if pressure and temperature are normal. A valve that drips with normal pressure and temperature is faulty.

What Causes a T&P Valve to Keep Opening?

A T&P valve that opens repeatedly — not just a constant drip, but a significant release of water during each heating cycle — indicates thermal expansion in a closed system.

Thermal expansion in a closed system:
1. Cold water enters the tank (at 50°F, say)
2. The heater brings it to 120°F
3. Water expands as it heats — roughly 2% in volume
4. In a closed system, this expansion pushes pressure up in the tank
5. If the pressure exceeds the T&P valve set point, the valve opens briefly and releases hot water through the discharge pipe

The expansion tank solution: A properly sized thermal expansion tank connects to the cold water inlet and provides a cushion — a sealed air chamber that compresses to absorb the expanded water volume. With an expansion tank, pressure during thermal expansion is absorbed by the tank rather than driving the T&P valve open.

Expansion tank sizing: For a 50-gallon water heater in Seattle (where PRV-regulated street pressure is typically 65–75 PSI), a 2-gallon expansion tank is standard. Your plumber will calculate the correct size based on tank volume and system pressure.

How Much Does It Cost to Replace a Pressure Relief Valve?

Parts:
– Standard 3/4-inch 150 PSI/210°F T&P valve: $15–$40
– Expansion tank (if needed): $30–$80 for the tank

Labor:
– T&P valve replacement only: $75–$150 (typically a 30–60 minute job)
– T&P valve plus expansion tank installation: $150–$300

Total cost:
– Valve only: $90–$190
– Valve plus expansion tank: $180–$380

When a plumber is worth calling for this:
– You’re not comfortable working on gas connections or draining the tank
– The discharge pipe needs to be modified to meet code (not long enough, improperly sloped, reduced in diameter)
– An expansion tank installation requires pipe work beyond a simple T-fitting

Can I Cap a Leaking Pressure Relief Valve Temporarily?

No. This is one of the most dangerous DIY mistakes on a water heater.

Why capping is dangerous: The T&P valve is the primary safety device preventing tank overpressure. If the thermostat fails and continuously heats the water with no functional pressure relief, tank pressure builds until the tank fails catastrophically. A water heater tank failure at high pressure releases enormous energy — the “BLEVE” (Boiling Liquid Expanding Vapor Explosion) risk is why these safety devices are code-required.

What to do instead:
– If the valve is dripping but you can’t immediately address it: leave it dripping, place a bucket under the discharge pipe, and schedule service within a few days
– If the valve is releasing significant amounts of water: shut off the water heater and cold water supply and call a plumber promptly

If someone told you to just cap it: They’re wrong. A capped T&P outlet is a code violation and a serious safety issue.

Pressure Relief Valve Dripping After New Water Heater Install

A new T&P valve dripping shortly after a water heater installation is almost always thermal expansion in a closed system.

What happened:
– The new heater is properly installed
– The PRV on the supply prevents expanded water from backing into the main
– No expansion tank was installed (or the old one was too small for the new heater)
– Each heating cycle pushes pressure up, the T&P valve briefly opens

This is the most common oversight in water heater replacement: Not installing or verifying the expansion tank when replacing a heater in a closed system. Code requires expansion tanks in closed systems, and it’s an inspection item — but some installations miss it.

The fix: Install an appropriately sized expansion tank on the cold water inlet. This should have been part of the original installation cost quote. If it wasn’t, ask the installing plumber to return and install it — it should be covered under their work warranty.

Do I Need an Expansion Tank If My Relief Valve Keeps Leaking?

If the T&P valve is opening during heating cycles (not a constant drip from a failed valve), yes — an expansion tank is almost certainly the right fix.

The diagnosis:
– T&P valve leaks only during heating cycles (heater runs, valve opens briefly, then closes when heating stops) → thermal expansion, expansion tank needed
– T&P valve drips constantly regardless of heating cycle → either high system pressure (PRV issue) or failed valve

Expansion tank installation: A 2–4 gallon expansion tank connects to the cold water inlet with a T-fitting. It’s a small, permanent addition that absorbs thermal expansion and eliminates the T&P valve cycling.

Cost: $30–$80 for the tank; $100–$200 installed by a plumber. Code required in closed systems. If you have a PRV (most Seattle homes do) and no expansion tank, adding one is not optional — it’s a code requirement.

How Often Should a Pressure Relief Valve Be Replaced?

Recommended interval: Every 5–6 years as preventive maintenance, regardless of whether the valve is dripping. Alternatively, replace whenever the water heater itself is replaced.

Why: T&P valves contain a spring, seat, and disc that can corrode or calcify over years of exposure to hot water and minerals. A valve that hasn’t been tested in 10 years may not open correctly when needed — it could be stuck closed from scale buildup.

The testing risk: Manually lifting the T&P valve test lever to verify operation can cause the valve to not reseal properly (particularly on older valves with mineral deposits on the seat). If you test a valve and it continues to drip afterward, plan to replace it.

Replacement on water heater replacement: A good plumber replaces the T&P valve when installing a new water heater. Confirm this is included in the installation scope — it’s a $15–$40 part that should be standard practice.

FAQ

Q: Why is my water heater pressure relief valve leaking?
A: The three most common causes are thermal expansion in a closed system (no expansion tank, PRV on the supply line), water temperature set too high, or a faulty valve that won’t fully close. Test system pressure and verify the thermostat setting before replacing the valve.

Q: Is a dripping pressure relief valve dangerous?
A: The drip itself isn’t immediately dangerous — the valve is functioning as designed. But the underlying cause needs diagnosis. More importantly, don’t cap the valve — a non-functional T&P valve leaves the heater with no overpressure protection, which is genuinely dangerous.

Q: How do I replace a water heater T&P valve?
A: Turn off the heater and water supply, drain a few gallons from the tank, disconnect the discharge pipe, unscrew the old valve, apply thread sealant to the new valve, thread it in, reconnect the discharge pipe, restore water and energy. Cost: $15–$40 for the valve; $75–$150 if a plumber does it.

Q: Do I need an expansion tank if my T&P valve keeps opening?
A: Almost certainly yes, if the valve opens during heating cycles. A closed system (with a PRV and no expansion tank) creates thermal expansion pressure that the T&P valve must relieve. An expansion tank absorbs the expansion and eliminates the cycling.

Q: How often should I replace the T&P valve?
A: Every 5–6 years as preventive maintenance. Replace it whenever you replace the water heater. A T&P valve more than 10 years old may not open reliably when needed — scale and corrosion can cause it to stick closed.