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Water heater relief valve leaking or dripping

A relief valve that drips is opening because something made it open. When it drips tells you why, and a $10–$20 pressure gauge usually confirms it.

Written by Victoria StewartTechnical review by Phil Montoya, licensed plumber, October 6, 2026Updated October 4, 2026How we research
Inside a temperature and pressure relief valve Simplified cutaway of a T&P relief valve screwed into the top of a water heater tank. A spring holds a disc against a seat. A test lever on top can lift the disc by hand. A threaded shank goes into the tank, and a temperature probe below it sits in the top six inches of water. The outlet on the side feeds the discharge pipe. Two panels below show that pressure above the setting, usually 150 psi, lifts the disc slightly so the valve drips, while water at 210 degrees makes the probe push the disc wide open so very hot water gushes out. Inside a T&P relief valve Simplified cutaway, shown closed hot water in tank Test lever Spring Disc and seat Threaded shank Temperature probe lift it to test the valve holds the disc shut; its force sets the opening pressure, usually 150 psi scale or grit on the seat causes a steady drip screws into the tank must sit in the top 6 in. of water; pushes the disc open at 210°F Outlet Discharge pipe carries the water down to a safe, visible spot Pressure: opens a crack drip At the set pressure, water lifts the disc slightly: a weep or drip. Heat: opens wide gush At 210°F the probe lifts the disc fully: very hot water rushes out. Based on Watts, Cash Acme and A.O. Smith literature.
Inside a T&P relief valve. A spring holds a disc against the seat. Pressure above the setting (usually 150 psi) nudges the disc open and the valve weeps. Heat at 210°F makes the probe lift the disc wide open. Simplified from Watts and Cash Acme literature.

THE SHORT VERSION

  • Never cap, plug or remove the valve. It is the last thing standing between an overheated or overpressured tank and a burst. A drip means something else needs fixing.
  • Drips after the heater runs, often overnight: almost always thermal expansion in a closed system. A.O. Smith calls excess pressure the most common cause. The usual fix is an expansion tank.
  • Drips much of the time and your pressure reads above 80 psi: street pressure is too high or the pressure-reducing valve has failed. A plumber installs or replaces the regulator.
  • Bursts of very hot water, or water far hotter than normal at the taps: the heater is overheating. Turn it off and call a pro today.
  • Trickles nonstop at normal pressure, or only since you tested it: debris or wear on the valve seat. A new valve is about $15 as a part; installed prices run $20–$250 depending on whose survey you read.

A dripping relief valve is a symptom, not the problem

The temperature and pressure relief valve (T&P valve) is the brass valve on the top or upper side of a tank, with a pipe running down from it. If the water gets too hot or the pressure too high, it opens and dumps water through that pipe. Rheem’s service bulletin is blunt: without one, “the tank may rupture or explode.”

So water at the end of the pipe means the valve opened, and the question is why. The International Code Council says that with house pressure within code and a working valve “there should not be any water discharge,” so any discharge points to a plumbing problem, a heater problem or a failing valve.

Water from around the valve’s threads, not out of the pipe, is a seal leak instead. See where a water heater leak comes from.

How the valve opens: pressure, heat or your hand

Inside, a stainless steel spring presses a disc against a seat. Three things can lift it:

Pressure
At the valve’s setting, almost always 150 psi on a home heater, water pushes the disc up against the spring. A.O. Smith says it opens “slightly,” which you see as “dribbling” or “weeping.”
Temperature
A probe reaches into the top 6 inches of the tank, where the hottest water collects. At 210°F it lifts the disc and the valve opens “to full capacity,” in A.O. Smith’s words, until the water cools.
The test lever
Lifting it by hand pulls the disc off the seat to check that water flows.

The 2021 IRC requires the valve to open at no more than 150 psi (and never above the tank’s rated pressure) and 210°F (P2804.3, P2804.4). Watts and Cash Acme valves are certified to ANSI Z21.22/CSA 4.4, the standard for water heater relief valves, and reseat on their own after either kind of release.

Tankless heaters have a pressure relief valve too, and Rinnai’s manual says periodic discharge “may be due to thermal expansion in a closed water supply system.” The causes below apply to them as well.

Diagnose it by when it drips

Wipe the pipe end dry, put a cup under it, and check after showers, while the heater runs and first thing in the morning. The timing points to the cause.

Relief valve symptoms, likely causes and what to do
What you seeLikely causeWhat to do
Drips during or after heating, especially overnight when no water is usedThermal expansion in a closed system (check valve, backflow preventer or pressure regulator on the supply)Measure pressure with a max-needle gauge; a plumber adds or services an expansion tank
Drips often, at any time; gauge reads above 80 psi even when the heater is idleStreet pressure too high, no regulator, or a failed pressure-reducing valve (PRV)Plumber installs, adjusts or replaces the PRV, plus expansion control
Pressure creeps up for hours with no water in use and the heater offPRV leaking through from debris or a worn sealPlumber cleans or replaces the PRV
Bursts of very hot water or steam; water far hotter than usual at the tapsOverheating: a thermostat that won’t shut off, and its high-limit backup not trippingTurn the heater off now and call a qualified technician
Constant trickle at normal, steady pressureValve not seating: scale, debris or corrosion on the seat, or an old valvePlumber replaces the valve
Started dripping right after you tested itSediment caught on the seatOperate the lever a few more times; if it won’t stop, close the cold supply and have the valve replaced
Water around the valve body or where it enters the tankThread seal leak, or a leak at the welded tank fittingReseal (plumber); a leaking weld means a new heater

Sources: A.O. Smith Technical Bulletins 45 and 52 and Use & Care Guide 100366722 (2023); Zurn Wilkins NR3XL PRV instructions; 2021 IRC P2903.3.2. Patterns overlap: a closed system with high street pressure drips both ways.

A.O. Smith’s troubleshooting chart adds stacking: repeated short draws layer very hot water at the top of the tank, right at the probe. Its fix is a lower thermostat setting; see water heater temperature.

Measure your water pressure with a $10–$20 gauge

This is the one diagnostic step a homeowner can safely do, and it separates the two most common causes. A gauge that screws onto a garden-hose faucet costs about $10–$20, according to a California water district. Get one with a red maximum needle; Watts sells its hose gauges with that needle “for detection of thermal expansion pressure surges.”

  1. Pick a faucet on the house side of the regulator

    The plumbing code lets outdoor faucets skip the pressure-reducing valve (IPC 604.8), so one may read street pressure. If unsure, also test at the washing machine’s cold hose valve.

  2. Read static pressure

    With no water running anywhere, screw the gauge on by hand, open the faucet fully and note the reading.

  3. Set the red needle and leave it

    Turn the red needle down to the black one and leave the faucet open overnight, ideally after evening showers so the heater runs while nobody uses water. Don’t leave a gauge outdoors in a freeze.

  4. Read it in the morning

    Check the red needle before anyone runs water, and compare with the patterns below. Your target: 40–60 psi, as A.O. Smith and Rheem recommend, and never above the 80 psi code maximum.

Reading house water pressure overnight to find why a relief valve drips A pressure gauge screwed onto a hose faucet, with a black needle showing current pressure and a red needle that stays at the highest reading. Below, three illustrative charts of pressure from 6 pm to noon. One: steady around 55 psi, under the 80 psi code maximum, which is normal. Two: pressure jumps to 150 psi each time the heater runs while no water is used, and the valve drips, which means thermal expansion in a closed system. Three: pressure sits around 105 psi all day, or creeps up for hours with no water used, which points to high street pressure or a failed pressure-reducing valve. Leave a gauge on overnight Illustrative readings, not measurements A hose-thread pressure gauge on a faucet on the house side of the pressure regulator, about $10–$20. Black needle: pressure now Red needle: highest since you set it Set the red needle at bedtime; read it at dawn. 1. Steady and under 80 psi 80 150 0 80 psi: code maximum 150 psi: valve opens Normal: open system or a working expansion tank If the valve still drips, the valve itself is the suspect. 2. Jumps to 150 psi when the heater runs 80 150 0 valve drips heater on Thermal expansion: the system is closed Spikes start with heating, while no water is used. Fix: expansion tank. 3. Above 80 psi, or creeping up with no use 80 150 0 6 pm midnight 6 am noon Street pressure or the pressure-reducing valve Solid: high all day. Dashed: regulator leaking through. 80 psi: IRC/IPC static maximum. 150 psi: the usual valve setting.
Three overnight patterns and what each means. Illustrative readings from a gauge with a red max needle. Steady pressure under 80 psi is normal. Spikes that start when the heater runs mean thermal expansion. Pressure that sits above 80 psi, or creeps up with no water in use, points to the street supply or the pressure-reducing valve.

Drips after heating: thermal expansion in a closed system

Water expands as it heats: Rheem says a 40-gallon tank grows to about 40.53 gallons. In an open system the extra pushes back into the city main. A check valve at the meter, a backflow preventer, a pressure-reducing valve or some water softeners block that path and make the system closed. Water won’t compress, so pressure soars: A.O. Smith says a closed tank at 50 psi reaches 250 psi after just 10 degrees of heating.

The signs, per A.O. Smith’s Bulletin 45: the valve drips during heating when no water is being used, pipes creak while the heater recovers, and water surges when you first open a tap. Rheem’s rule of thumb is about one cup of water per 10 gallons of tank each heating cycle.

The usual fix is a properly sized thermal expansion tank on the cold supply; the IRC requires expansion control in a closed system, and A.O. Smith says the tanks “must be installed on all closed water systems.” An existing tank can be undersized or failing; Amtrol, a major maker, says its tanks need periodic professional inspection. See expansion tanks.

Drips much of the time: street pressure or a failed regulator

Utilities often deliver more than a house should see: one California district tells customers to expect 40 to 150 psi at the meter. The IRC and IPC cap static pressure indoors at 80 psi and require a pressure-reducing valve (PRV), a bell-shaped regulator where the main enters, when the supply is higher.

High pressure leaves the valve little margin. Watts says system pressure must stay under 75% of the valve’s setting, 112 psi for a 150-psi valve, and in a closed system every heating cycle adds more.

  • No regulator, or above 80 psi: a plumber installs a PRV, plus expansion control, because the regulator closes the system (IRC P2903.4.1).
  • Regulator present but pressure high: it may be set wrong or dead. The IPC requires PRVs to fail open, so a failed one passes full street pressure (604.8.1).
  • Pressure creeps up with no water in use: Zurn lists debris or a worn seal in the regulator as a cause, alongside thermal expansion. If it still creeps with the heater off, suspect the regulator.

Bursts of scalding water: the heater is overheating

A pressure release is a dribble. A temperature release is a gush of very hot water, sometimes steam. A.O. Smith’s Bulletin 52 says sporadic large discharges of hot water have one cause: “a malfunctioning thermostat.”

Gas and electric tanks also have a backup high-limit, the energy cut-off (ECO), meant to stop the heat first; a common electric one trips at 190°F, and many gas controls flash a four-blink code. A valve opening on temperature means the controls have failed to stop the heat.

Testing the valve: what manufacturers say, and the risks

Watts, A.O. Smith and Rheem all say to operate the lever at least once a year, because mineral deposits can stick the valve (Rheem’s older bulletin said every six months). Watts also wants a licensed plumber to remove and inspect the valve every two to four years, since corrosion inside can’t be seen. Their procedure and warnings:

  1. Check where the water will go

    Watts says to confirm first that a pipe carries the water somewhere safe, “otherwise personal injury may result.” No pipe, or one ending near a walkway or outlet: don’t test; call a plumber.

  2. Clear the area

    A.O. Smith: nobody in front of the pipe outlet; the water “may be extremely hot,” and so may the valve. Put a bucket under a pipe that doesn’t end at a drain.

  3. Lift the lever and hold it

    Rheem: lift far enough to get flow and hold 5–10 seconds, which also flushes sediment off the seat.

  4. Let it snap shut

    The flow should stop. A.O. Smith suggests lifting it several times; it “should seat properly and operate freely.”

The documented risk is that a valve left alone for years may not reseat. That is also the reason to test: a valve too scaled to close may be too scaled to open. If you’d rather not find out alone, have a plumber test it with a new valve on hand.

Replacing the valve: who, which one, and what it costs

Replacing the valve means draining hot water and resealing a fitting in the tank: a licensed plumber’s job. Rheem says repairing or altering a valve “is prohibited by national safety standards,” so a worn one is replaced, not rebuilt.

What the new valve must match

  • Certification: ANSI Z21.22/CSA 4.4 and ASME, per A.O. Smith’s manual
  • Pressure setting: no higher than the working pressure on the rating plate, generally 150 psi; A.O. Smith says a wrong-size valve voids the warranty
  • Capacity: BTU/hr rating at least the heater’s input; Watts’ ¾-inch LF100XL is rated to 105,000
  • Probe and shank length: enough to reach the top 6 inches of water; Watts and Cash Acme make extended versions for extra-thick insulation, so match the original
What a relief valve replacement costs
SourcePriceWhat it covers
Home Depot (Rheem PROTECH SP12574)$14.50The part only
HomeAdvisor$20–$200Installed, including labor
Fixr$100–$250Installed, including labor

Part price checked on homedepot.com October 4, 2026. HomeAdvisor (updated June 2026) and Fixr (updated August 2025) are lead-generation sites whose ranges come from their own surveys and databases, not measured local prices. None includes an expansion tank or regulator.

A new valve fixes a worn valve, not pressure: on a closed system without expansion control it will drip like the old one. If the heater is old, weigh replacing it; see repair or replace and how old is my water heater.

What never to do with a dripping relief valve

  • Cap or plug the valve or its pipe. The ICC says the code bans threads on the pipe end because they make it “easy for someone to screw on a cap or valve to stop the dripping,” which can lead to “injury or death.”
  • Put a valve or anything that can shut it in the line. The IRC forbids a shutoff or check valve between the tank and the pipe end (P2804.6).
  • Remove the valve, plug the opening, or fit a valve rated above the tank.
  • Shrink, kink or run the pipe uphill. See the discharge pipe rules in code requirements.
  • Live with the drip. A valve that weeps every night is doing the expansion tank’s job, and the tank takes the strain.

Common questions

Is a dripping relief valve dangerous?

The drip is the valve protecting you, but it means something is wrong. Drips after heating are a pressure problem to fix soon; A.O. Smith says thermal expansion can cause premature tank failure. Bursts of very hot water are an emergency: turn the heater off and call a technician.

Why does my relief valve drip at night?

The heater often runs after evening showers while nobody uses water. In a closed system, expanding water has nowhere to go and pressure climbs until the valve weeps. A pressure gauge with a red max needle left on overnight will show the spike.

Why did my relief valve start leaking after I tested it?

Sediment probably lodged on the seat. Rheem says holding the lever open 5–10 seconds flushes the seat, and A.O. Smith suggests lifting it several times. If it still won’t stop, A.O. Smith says to close the cold-water inlet and replace the valve.

Can I replace the T&P valve myself?

It’s a job for a licensed plumber. It means draining hot water, removing a fitting from the tank, sealing threads and choosing a valve with the right rating and probe length. A licensed plumber can also measure pressure and fix the cause, which a new valve alone won’t.

Will a new water heater stop the relief valve from dripping?

Not if pressure is the cause. A new tank on a closed system without expansion control will drip the same way, and A.O. Smith doesn’t cover that leakage under warranty. Fix the pressure first.

My tankless water heater’s relief valve drips. Why?

Usually the same reasons. Rinnai’s manual says periodic discharge “may be due to thermal expansion in a closed water supply system” and not to plug the valve. Check your pressure the same way.

Sources

Sources checked October 4, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.

  1. Technical Bulletin 52: Leaking temperature and pressure relief valve ↗ · A.O. Smith
  2. Technical Bulletin 45: Thermal expansion ↗ · A.O. Smith
  3. Installation Instruction and Use & Care Guide, residential power-vent gas water heaters, 100366722 Rev A (February 2023) ↗ · A.O. Smith, hosted by Lowe’s
  4. Series LF1L, LF1XL, LF10L and LF100XL T&P relief valves spec sheet (2014) ↗ · Watts, hosted on Cloudinary
  5. NCLX T&P relief valve specification BA001 (May 2014) ↗ · Cash Acme (Reliance Worldwide), hosted on Cloudinary
  6. Temperature & pressure relief valve instructions ↗ · Rheem, hosted by Home Depot
  7. Technical service bulletin: Thermal expansion ↗ · Rheem
  8. Water Heater Lifespan: When to Repair vs. Replace (March 2026) ↗ · Rheem
  9. CodeNotes: Water Heater Safety in the I-Codes ↗ · International Code Council
  10. 2021 IRC Chapter 28, Water Heaters (P2804 relief valves) ↗ · International Code Council, via UpCodes
  11. 2021 IRC Chapter 29, Water Supply and Distribution (P2903.3.2, P2903.4) ↗ · International Code Council, via UpCodes
  12. 2024 IPC Chapter 6, Water Supply and Distribution (604.8 pressure-reducing valves) ↗ · International Code Council, via UpCodes
  13. 2024 IPC Chapter 5, Water Heaters (504.4 relief valves) ↗ · International Code Council, via UpCodes
  14. Water pressure: pressure regulators and how to check your pressure ↗ · Jurupa Community Services District, California
  15. Series 276H300 and IWTG hose connection gauges spec sheet (2019) ↗ · Watts, hosted by Lowe’s
  16. NR3XL pressure-reducing valve installation and troubleshooting (Rev. C, 2016) ↗ · Zurn Wilkins, hosted by Consolidated Supply
  17. Therm-X-Trol expansion tank installation instructions ↗ · Amtrol, hosted by Stockpile Express
  18. 5600 Series electric water heater thermostats installation manual ↗ · Robertshaw
  19. Tankless water heater installation and operation manual 100000773 (SENSEI RSC) ↗ · Rinnai
  20. Rheem PROTECH T&P relief valve SP12574 (product page) ↗ · Home Depot
  21. Water heater repair cost guide (updated June 2026; lead-generation site) ↗ · HomeAdvisor
  22. Water heater repair cost guide (updated August 2025; lead-generation site) ↗ · Fixr
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