A relief valve discharging is a safety device doing exactly what it was designed to do, which means something upstream is pushing pressure or heat too high.
Tell us what you need—we’ll call you back fast.
Quick answer
A gas boiler's pressure relief valve or combination temperature-and-pressure relief valve discharges when system pressure or water temperature exceeds its set threshold, which is its intended safety function. The usual causes are a waterlogged expansion tank, an overfilled system, a fill valve passing water and continuing to add pressure, or in the case of a temperature-and-pressure valve, an overheating condition from a failed high-limit control. The valve itself may also simply need replacing if mineral buildup is preventing it from reseating cleanly after discharging.
Need a licensed gas fitter for your gas boiler? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.
A relief valve exists specifically to release pressure or excess heat before it reaches a level that could damage the system or, in an extreme case, cause a dangerous failure. When it discharges, it is telling you that pressure or temperature reached its set threshold, not that the valve itself is broken. The correct response is to find out why pressure or temperature got that high in the first place, since capping, plugging, or otherwise disabling the valve to stop the dripping removes the one safeguard designed to prevent a much more serious outcome.
A waterlogged expansion tank, which has lost its air cushion either from a leaked charge or a failed diaphragm, can no longer absorb the normal expansion of heating water, so that expansion pushes system pressure high enough to trip the relief valve during heating cycles. An overfilled system, often the result of topping up pressure a bit too generously during a repair or refill, starts closer to the valve's threshold and only needs a modest amount of thermal expansion to push it over. A fill valve or pressure-reducing valve that is passing water continues adding to the system even after it should have stopped, gradually pushing pressure up until the relief valve intervenes.
If the relief valve is a combination temperature-and-pressure type and the discharge is accompanied by noticeably hotter than normal water, an overheating condition caused by a failed or maladjusted high-limit control, or restricted water flow through the heat exchanger, can be the trigger rather than pressure alone, which is a more serious finding that points toward the boiler's control system rather than just the expansion tank or fill valve.
A relief valve that has discharged even once can develop mineral deposits on its seat that prevent it from sealing perfectly afterward, leading to a slow, persistent drip that continues even if the original overpressure cause has been fixed. In that situation, the valve itself needs to be replaced rather than continuing to investigate the rest of the system, since a valve that will not reseat cleanly will keep dripping regardless of what triggered the original discharge.
Note whether the discharge happens specifically during heating cycles, which points toward the expansion tank, or continuously regardless of heating, which points more toward the fill valve or overfilling. Check whether the water coming from the discharge pipe feels noticeably hotter than typical hot tap water, which is worth mentioning to the technician since it can indicate an overheating issue rather than a pure pressure issue. Never cap, plug, or tie off the discharge pipe yourself to stop the dripping, since that defeats the valve's safety purpose entirely.
Safety note
Never plug, cap, or otherwise block a relief valve or its discharge pipe to stop it from dripping, since this removes the boiler's protection against dangerous overpressure or overheating. If discharge is accompanied by a gas smell, leave the home and call FortisBC at 1-800-663-9911.
No, this removes a critical safety function and should never be done. If the valve is dripping, the underlying cause needs to be found and fixed, or the valve itself replaced if it will not reseat.
A slow, occasional drip is not an emergency but should be scheduled for service soon. A relief valve actively discharging steam, or discharging continuously and heavily, warrants a more urgent call.
Intermittent discharge, particularly during heating cycles, often points toward a waterlogged expansion tank that only fails to absorb expansion once the water heats up enough, rather than a constant overpressure condition.
A technician checks water present at the valve's schrader-style test point and the overall system pressure trend; if the underlying pressure and temperature are normal and the valve is still dripping, the valve itself needs replacement.
It is generally an affordable, contained repair on its own; cost depends on the specific valve and boiler model and is best confirmed by the technician.
A relief valve doing its job by discharging is a sign the safety system is working, not an immediate danger, but it should not be ignored, since it indicates an upstream condition that will keep recurring until addressed.
Family-run, TSBC-licensed (gas LGA0043870 · boiler LBP0046450), serving Metro Vancouver homes since 1999. Residential, natural gas only.
Call (604) 359-1081 Email info@canroheat.ca“Canro does an amazing job! Super professional and skilled with everything—boilers, furnaces, and AC. Always a pleasure working with them!”
“Five stars for Canro. We needed a replacement of a 30-year-old boiler. A new high-efficiency unit was installed—clean, professional work.”
“I can’t speak highly enough about George, who was recommended for a boiler service call for my parents’ home. George is excellent.”
25+ years keeping Metro Vancouver homes comfortable. Honest advice, upfront pricing, work that lasts.