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Boiler Relief Valve Dripping Water Down the Discharge Pipe

Water dripping from the boiler relief valve discharge pipe means it's stuck open or system pressure is too high. Diagnose and fix it before it floods.

Category:HVAC

Difficulty:Moderate

Time:30-45 min

Success:50%

Updated:May 28, 2026

quick_referenceQuick Answer

For Boiler Relief Valve Dripping Water Down the Discharge Pipe, start with "Check the pressure gauge before you touch the valve": Do not cap, plug, or tighten the discharge pipe — that defeats a critical safety device. Look at the boiler's pressure-temperature gauge first. Normal cold pressure is 12-15 psi; normal hot operating pressure is 18-25 psi. If the gauge reads above 30 psi when the boiler is hot, your problem is overpressure, not a bad valve. The feed water pressure-reducing valve or the expansion tank is the real culprit. If pressure is normal and the valve still drips, the PRV itself has debris in the seat. Stop DIY if the boiler pressure gauge reads above 30 psi and is still climbing — shut down the boiler immediately and call a licensed hvac technician. a steam boiler overpressure situation can be extremely dangerous. This is listed as a moderate recovery and usually takes about 30-45 min.

verifiedGuide Snapshot

Repair areaHVAC

Estimated time30-45 min

DifficultyModerate

Stop conditions5

Last updated May 28, 2026. Review the stop conditions before continuing.

account_treeRecovery State

DeviceRadiator

Current statePressure Relief Valve Dripping

Specific stateStuck Open Or Overpressure

Failed stepBoiler Pressure Regulation

Likely failure typeLeak Path

DIY boundaryDIY recovery first

paymentsCost decision

help1. Understand the Problem

A boiler's pressure relief valve is a safety device that opens at a preset pressure (usually 30 psi) to prevent the boiler from becoming a bomb. It drips for two reasons: either the system pressure is genuinely too high, or the valve has sediment trapped in its seat and can't close fully. A dripping PRV is a warning — ignore it and the valve can stick permanently open or, worse, corrode shut and fail to open when actually needed.

build_circle2. Try This First

Best First Step

Check the pressure gauge before you touch the valve

Do not cap, plug, or tighten the discharge pipe — that defeats a critical safety device. Look at the boiler's pressure-temperature gauge first. Normal cold pressure is 12-15 psi; normal hot operating pressure is 18-25 psi. If the gauge reads above 30 psi when the boiler is hot, your problem is overpressure, not a bad valve. The feed water pressure-reducing valve or the expansion tank is the real culprit. If pressure is normal and the valve still drips, the PRV itself has debris in the seat.

visibility3. Visual Guidance

See what's happening and how to try the first recovery step.

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Check and repressurize the expansion tankIf system pressure is too high, the expansion tank is the most common cause. A waterlogged expansion tank can't absorb the thermal expansion of heated water, so pressure spikes and the PRV opens. Tap the tank — it should sound hollow on top and solid on the bottom where water sits. If it sounds full of water (solid throughout), the bladder has failed and the tank needs replacement. If it sounds hollow but pressure spikes, check the air charge with a tire gauge on the Schrader valve: it should match the system's cold fill pressure, typically 12-15 psi.

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Check the feed water pressure reducing valveIf the expansion tank is good but pressure creeps up even when the boiler is cold, the automatic feed valve may be letting too much water in. These valves are factory-set to 12-15 psi but can fail over time. Check the tag on the valve for its set pressure. If your gauge reads above that when cold, the feed valve is stuck open or out of adjustment. Try adjusting the set screw down; if it doesn't respond, replace the valve.

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Try to flush the PRV to clear debris from the seatIf pressure is normal but the valve drips, sediment in the valve seat is likely holding it cracked open. Place a bucket under the discharge pipe. Briefly lift the test lever on top of the PRV fully open for one second and snap it shut. The rush of water flushes debris off the seat. Do this two or three times. If it still drips after flushing, the seat is damaged and the valve must be replaced.

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Replace the pressure relief valve if flushing failsTurn the boiler off, let it cool below 100°F, shut the water feed valve, and drain enough water to drop the level below the PRV. Unscrew the old valve with a pipe wrench — it threads out of a tee or tapping on the boiler. Wrap the new valve's male threads with Teflon tape rated for hot water (not standard white tape), thread it in, and tighten. Refill the boiler, bleed air from radiators, and verify the new valve doesn't drip at operating pressure. Match the new valve's pressure and BTU ratings exactly to the boiler's data plate.

autorenew4. If That Doesn't Work

Try the next recovery options.

monitoring

Install a pressure gauge with a recording needleIf you suspect intermittent overpressure only at peak firing, install a gauge with a lazy/telltale hand that records the maximum pressure reached. This confirms whether the PRV is opening because of true overpressure or a failed seat — without having to watch the boiler for hours.

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Add a thermal expansion tank if you don't have oneSome older boiler systems never had an expansion tank installed, or it was removed during a renovation. If you recently had a backflow preventer or check valve installed on the water main, thermal expansion now has nowhere to go. Adding a properly sized expansion tank solves the overpressure problem at the source.

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Frequently Asked Questions

Is a small drip from the PRV discharge pipe normal?expand_more

No. A properly functioning boiler system should never discharge water from the PRV. Even a slow drip means something is wrong — either the valve seat is dirty or system pressure is out of spec. It won't fix itself and will only get worse.

Can I just put a bucket under the pipe and ignore it?expand_more

You can temporarily, but don't leave it long-term. A constantly dripping PRV will eventually corrode open fully and dump gallons on the floor. More importantly, minerals in the drip water build up on the valve seat, making it progressively harder for the valve to seal — or to open in a real emergency.

How do I know if my expansion tank needs replacement vs just air?expand_more

Tap the top and bottom. If the entire tank sounds like a dull thud (waterlogged), the internal bladder is ruptured and the tank is scrap. If it sounds hollow but can't hold air when you charge the Schrader valve, the bladder has a pinhole leak — also needs replacement.

warning5. Stop DIY If

Don't continue if any of these apply.

reportThe boiler pressure gauge reads above 30 psi and is still climbing — shut down the boiler immediately and call a licensed HVAC technician. A steam boiler overpressure situation can be extremely dangerous.

reportThe discharge pipe is hot to the touch or you see steam, not just water, coming out — this indicates the boiler is operating above design temperature and pressure.

reportThe PRV discharge pipe terminates somewhere that's not visible, making it impossible to verify whether the valve is actually closed or still leaking.

reportYou smell gas anywhere near the boiler — evacuate immediately and call the gas utility.

reportThe boiler is steam, not hot water — steam boiler PRVs operate at different pressures and require different servicing procedures. Call a professional.

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