# Water heater T&P relief valve dripping: the four causes in order, and why a new valve is usually the wrong first move > A T&P relief valve opens at 150 psi or 210 F. A drip after each heating cycle is usually thermal expansion or supply pressure over 80 psi, not a bad valve. | | | |---|---| | Source | https://ownerspec.com/plumbing/guides/water-heater-tp-relief-valve-dripping/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-27 | | Updated | 2026-09-27 | | Facts verified | 2026-09-27 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer A temperature and pressure (T&P) relief valve on a residential tank water heater opens at 150 psi or 210 F. Watts lists those two settings for its 100XL-150, and IRC Section P2804 caps the pressure setting at 150 psi and the temperature setting at 210 F. A valve that drips a little after each heating cycle is usually not a bad valve. A.O. Smith's manual names thermal expansion on a closed supply as a cause of intermittent discharge, and Rheem's manual says replacing the valve for that cause will not correct the problem. The order to check is an overnight gauge reading first (the hotwater.com bulletin calls for a pressure-reducing valve set to 50 to 60 psi when static pressure passes 80 psi), then an expansion tank, then the valve itself. On an electric heater, several gallons of water at the discharge pipe is not thermal expansion: turn the breaker off and call a technician. Never cap or plug the discharge line. ## Fact sheet - **Answer:** A residential tank water heater's T&P relief valve is set to open at 150 psi or 210 F: IRC Section P2804 caps both figures and Watts lists them for its 100XL-150. A drip after heating cycles points to thermal expansion or supply pressure above 80 psi; Rheem's manual says a new valve will not correct an expansion drip. Only a valve that fails to reseat after the annual lift-lever test is replaced. - **Basis:** IRC 2024 Sections P2804.3, P2804.4 and P2804.6.1 as printed in the Texas Windstorm Insurance Association's adoption on up.codes; the manufacturer manuals from A.O. Smith (185762) and Rheem (AP23657). The model codes are adopted locally and vary. - **Assumptions:** A residential storage (tank) water heater with a factory-fitted combination temperature and pressure relief valve on a 3/4 inch port.; The 24-hour pressure test and the 80 psi threshold follow the hotwater.com Res Tech Support bulletin; other makers may print other thresholds.; The overheating case is documented by the maker for electric heaters. No equivalent gas-heater text was read this session. - **Method:** Code text read from the up.codes mirror of the Texas 2024 IRC adoption on 27 September 2026. Valve settings from Watts' 100XL-150 product page. Test interval, thermal expansion wording, reseat failure and discharge line rules read from the A.O. Smith 185762 and Rheem AP23657 manuals. The 24-hour gauge test, the 80 psi and 50 to 60 psi figures and the electric overheating instruction from the hotwater.com Rev2A bulletin. Cash Acme's site returned 403 on every attempt, so no Cash Acme document is cited. - **Primary source:** [IRC 2024, Section P2804 Relief Valves (Texas Windstorm Insurance Association adoption, up.codes viewer)](https://up.codes/viewer/texas/irc-2024/chapter/28/water-heaters) (manufacturer or vendor document) - **Facts verified:** 2026-09-27 - **Last updated:** 2026-09-27 - **Canonical URL (cite this):** https://ownerspec.com/plumbing/guides/water-heater-tp-relief-valve-dripping/ If the discharge pipe is pouring rather than dripping, stop reading and go to the breaker: the maker's own bulletin says that is an overheating fault, not a valve fault. ## What does the drip pattern tell you? Timing and volume point at the cause before any tool comes out. | Symptom | Likely cause | Confirm with | |---|---|---| | Drips after the heater runs, then stops | Thermal expansion on a closed supply | Gauge climbs overnight with no water drawn | | Drips at all hours, faucets spit when opened | Static supply pressure too high | Gauge reads above 80 psi at rest | | Several gallons on the floor, electric heater | Overheating: thermostat or element fault | Do not test anything. Breaker off, call a technician | | Steady weep that started right after a lift-lever test | Valve did not reseat | Lift the lever once more; if it still runs, replace the valve | A.O. Smith's troubleshooting table ties water leaking at the relief valve to excessive water pressure and a closed system. The [thermal expansion tank guide](/plumbing/guides/thermal-expansion-tank-when-required/) covers how to tell whether your supply is closed. ## What are the valve's ratings? A T&P relief valve is a spring-loaded valve with a temperature probe in the tank that opens on either of two limits. Watts' page for the 100XL-150 3/4 lists a 150 psi pressure setting, a 210 F (99 C) temperature setting, a 112 psi maximum working pressure, 3/4 inch NPT and a test lever. The code sets the same ceilings. IRC Section P2804.3 requires a pressure relief setting not less than 25 psi above the system pressure and not greater than 150 psi, and compliance with ANSI Z21.22. Section P2804.4 caps the temperature relief setting at 210 F. The joint standard reference comes from the heater makers. Rheem's manual AP23657 says the factory-installed valve complies with ANSI Z21.22/CSA 4.4, and A.O. Smith's manual 185762 prints the same pair. Watts' page says ANSI Z21.22 certified and CSA Listed, without the 4.4 number. The code text here is the 2024 IRC as adopted by the Texas Windstorm Insurance Association; the model codes are adopted locally and vary, and no IPC section was read. ## Which fix matches which cause? The valve is the last item on the list, not the first. | Cause | Fix | Who does it | |---|---|---| | Thermal expansion | Expansion tank on the cold line. Rheem: a new relief valve will not correct this | Homeowner or plumber | | Static pressure over 80 psi | Pressure-reducing valve on the main, set to 50 to 60 psi per the hotwater.com bulletin | Plumber, in most cases | | Overheating (electric) | Thermostats and elements checked by a service technician | Technician, power off first | | Valve fails to reseat | New T&P valve of the same rating, after draining per the manual | Homeowner where local rules allow, otherwise a licensed plumber | The hotwater.com bulletin gives the test: a recording pressure gauge left on for 24 hours. Over 80 psi means a pressure-reducing valve set to 50 to 60 psi, covered in the [80 psi PRV guide](/plumbing/guides/high-water-pressure-80-psi-when-you-need-a-prv/). A climb overnight from a normal resting reading means a closed supply, and the [expansion tank size calculator](/plumbing/calculators/expansion-tank-size-calculator/) picks the tank. The same bulletin lists a faucet that surges or drips after 30 minutes of no use as a companion symptom. **Before you buy a valve** Put a gauge with a drag needle on a hose bib and leave it overnight with no water used. The SENCTRL 0-200 psi gauge parks its second needle at the night-time peak. Under 80 psi and steady means the valve itself is the suspect; a climb means an expansion tank; over 80 psi at rest means a pressure-reducing valve. The [PRV and gauge review](/plumbing/reviews/best-water-pressure-regulator-prv-and-gauge/) lists the alternatives. ## How do you test the valve, and how often? At least once a year, with the lever on the valve body. A.O. Smith's manual says the valve must be manually operated at least once a year. Rheem's manual says to lift and release the lever handle at least once a year to make certain the valve operates freely, and to allow several gallons to flush through the discharge line to an open drain. Rheem warns the discharge can scald. The test is also the diagnosis for the fourth cause. A.O. Smith's manual says that if the valve fails to completely reset after manual operation and continues to release water, close the cold water inlet, follow the draining instructions, and replace the valve. That is the only case on this page where the Cash Acme NCLX-5 or its equivalent is the answer. The Cash Acme listing title prints 150 PSI and a 3/4 inch size. Cash Acme's own site returned 403 to every request this session, so the 210 F setting and the ANSI Z21.22/CSA 4.4 mark are not confirmed here for that model: read the tag on the valve in the box and match the rating and probe length on your old valve. Many jurisdictions class water heater work as licensed plumbing, so check local rules before starting. ## What are the discharge pipe rules? IRC Section P2804.6.1 sets these rules for the discharge pipe: - Not smaller than the diameter of the valve outlet, and full size to the air gap. - Terminates not more than 6 inches, and not less than two pipe diameters, above the floor or the receptor. - No threaded connection at the end of the pipe. - No valves or tee fittings in the line, and gravity flow only. Both makers add their own. Rheem's manual says the line must pitch downward for complete gravity drainage, carry no reducer, restriction or shutoff valve, and end unthreaded and unconcealed. A.O. Smith's manual carries a DISCHARGE PIPE (DO NOT CAP OR PLUG) label on its diagram, says the valve must not be removed from its opening nor plugged, and notes that runs over 30 feet or with more than four elbows can restrict discharge capacity. ## What should you do first? 1. Electric heater with gallons on the floor: breaker off, call a technician. 2. Otherwise fit a drag-needle gauge to a hose bib and read the peak after a night with no water drawn. 3. Over 80 psi at rest: a pressure-reducing valve set to 50 to 60 psi. 4. A climb overnight from a normal reading: an expansion tank on the cold line. 5. Steady and under 80 psi: lift the lever, let it flush, and replace the valve only if it will not reseat. ## The picks - **SENCTRL 0-200 psi Lead-Free Water Pressure Gauge with Lazy Hand**: The overnight reading that separates expansion from high pressure (0-200 psi dial on a hose thread, second drag needle that stays at the highest pressure seen until reset; the same gauge picked on the 80 psi PRV guide) Amazon: https://www.amazon.com/dp/B0BFCN2XWZ/?tag=ownerspec-20 - **Cash Acme NCLX-5 3/4 in Temperature and Pressure Relief Valve, 150 PSI**: Only if the valve failed to reseat after the lift-lever test (3/4 inch valve; the listing title prints 150 PSI. Cash Acme's own site would not load this session, so the 210 F setting and the ANSI Z21.22/CSA 4.4 mark are not confirmed here for this model) Amazon: https://www.amazon.com/dp/B00I2VVPJO/?tag=ownerspec-20 ## Frequently asked questions ### Can I just replace the T&P valve to stop the drip? Not as the first step. Rheem's manual says that when the drip comes from thermal expansion, replacing the relief valve will not correct the problem, and recommends an expansion tank instead. A.O. Smith's manual says the valve is not intended for constant relief of thermal expansion. Read the supply pressure over 24 hours first; replace the valve only if it fails to reseat after a lift-lever test. ### How often should the T&P valve be tested? At least once a year, by both makers read for this page. A.O. Smith's manual says the temperature-pressure relief valve must be manually operated at least once a year. Rheem's manual says to lift and release the lever handle at least once a year and let several gallons flush through the discharge line to an open drain. Rheem warns the water can scald. ### Can I cap the T&P discharge pipe to stop the mess? No. A.O. Smith's manual labels the discharge pipe DO NOT CAP OR PLUG and says the relief valve must not be removed from its opening nor plugged. Rheem's manual says DO NOT plug the relief valve outlet. IRC Section P2804.6.1 also forbids a threaded end on the pipe and any valve or tee in the line. A capped valve cannot relieve a 150 psi or 210 F event. ### Why does the T&P valve only drip after showers or overnight? Because the tank is reheating a fresh charge of cold water and the water expands as it warms. On a closed supply, one with a check valve, backflow preventer or pressure-reducing valve, the expanded water has nowhere to go and pressure climbs until the valve relieves it. A.O. Smith's manual names this intermittent discharge from thermal expansion; the fix is an expansion tank, not a new valve. ### Water is pouring, not dripping, from the T&P pipe on my electric heater. What now? Stop and cut the power. The hotwater.com Res Tech Support bulletin says that on an electric water heater, several gallons of water on the floor around the T&P discharge pipe is not thermal expansion, and instructs turning off the circuit breaker or removing the fuses and contacting a service technician to check the thermostats and elements. That is an overheating fault, and the valve is doing its job. ## Sources - [IRC 2024, Section P2804 Relief Valves (Texas Windstorm Insurance Association adoption, up.codes viewer)](https://up.codes/viewer/texas/irc-2024/chapter/28/water-heaters) (manufacturer or vendor document) - [Watts, 100XL-150 3/4 Temperature and Pressure Relief Valve product page](https://www.watts.com/products/plumbing-flow-control-solutions/relief-valves/temperature-and-pressure-relief-valves/100xl/100xl-150-34) (manufacturer or vendor document) - [A.O. Smith, Instruction Manual, Residential Electric Water Heaters, document 185762 (PDF)](https://assets.hotwater.com/damroot/Original/10001/185762.pdf) (manufacturer or vendor document) - [Rheem, Electric Residential Hybrid Water Heater Use and Care Manual AP23657 Rev 04 (PDF)](https://media.rheem.com/blobazrheem/wp-content/uploads/sites/36/2025/01/AP23657-Use-Care-Manual.pdf) (manufacturer or vendor document) - [hotwater.com Res Tech Support, Operation and Maintenance bulletin: High Water Pressure, Thermal Expansion, and T&P Relief Valve Dripping, Rev2A (PDF)](https://university.hotwater.com/wp-content/uploads/sites/2/2019/02/High-Water-Pressure-Thermal-Expansion-T-P-Relief-Valve-Dripping-Rev2A.pdf) (manufacturer or vendor document) --- Citation: OwnerSpec, "Water heater T&P relief valve dripping: the four causes in order, and why a new valve is usually the wrong first move", https://ownerspec.com/plumbing/guides/water-heater-tp-relief-valve-dripping/, facts verified 27 September 2026. 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