A weeping pressure relief valve on a Vaillant boiler is rarely the real problem on its own — it's usually a warning sign pointing somewhere else in the system.
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Quick answer
The pressure relief valve on a Vaillant boiler is a safety device that opens automatically to release water if system pressure climbs above a safe limit, protecting the pressure vessel from damage. A valve that's continuously dripping or weeping usually means either the valve's seat has worn out or the system pressure is genuinely too high, often due to a waterlogged expansion tank rather than a fault in the valve itself. A licensed gas fitter checks both the valve and the expansion tank before replacing anything, since replacing the valve alone doesn't fix a root pressure problem.
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A boiler is a sealed, pressurized system, and pressure inside it rises as the water heats up. The pressure relief valve exists purely as a safety backstop: if pressure climbs past a safe threshold for any reason, the valve opens and discharges water to bring the system back down before anything is damaged or, in a worst case, fails catastrophically. It's not a part that's supposed to do anything under normal operation — a well-behaved system should almost never trigger it.
A relief valve that drips or weeps continuously, even a small amount, is telling you something is off. Sometimes it's the valve itself — mineral deposits on the seat, or general wear after years of service, can stop it sealing fully even at normal pressure. Just as often, though, the valve is doing exactly what it's designed to do because the system pressure really is elevated, commonly because the expansion tank has lost its air charge and can no longer absorb the water's normal expansion as it heats. Replacing the valve without addressing a waterlogged tank usually just means the new valve starts weeping too.
The opposite failure mode — a valve that won't open at all when tested, often stuck from corrosion — is arguably more serious, since it means the safety backstop isn't there if it's ever actually needed.
The system is drained down enough to work on the valve safely, the old valve is removed, and a new one rated correctly for the boiler is installed in its place, with the discharge piped to an appropriate location at floor level as required by code. If the root cause was a failed expansion tank, that gets addressed at the same time — installing a new relief valve without fixing a waterlogged tank is treating the symptom, not the cause.
It can be tempting to just tighten a fitting or, worse, cap a weeping discharge line to stop the drip. Don't. That valve is the one thing standing between elevated system pressure and a much more serious failure. If it's weeping, get it assessed rather than silenced — this is one of the few boiler symptoms where waiting genuinely isn't the safer option.
Homes on Fraser Valley well water tend to see mineral buildup accumulate on valve seats faster than homes on Metro Vancouver's softer municipal supply, which can make seat-related weeping show up sooner in a system's life on well water. It's one more reason regular servicing matters more in those areas than it might elsewhere in the region.
Safety note
A boiler is a sealed, pressurized system, and its safety devices exist to prevent that pressure from becoming dangerous. Never cap, plug, or ignore a weeping relief valve, and never attempt to open the boiler's pressure vessel yourself. If you smell gas at any point, leave the home and call FortisBC at 1-800-663-9911. Work on pressure-bearing or gas-fired components should only be done by a Technical Safety BC licensed gas fitter.
It's not an immediate gas safety emergency, but it shouldn't be ignored either — it's a signal that pressure in the system needs attention. Book a service visit rather than letting it drip indefinitely.
No. Never cap, plug, or block a relief valve's discharge. It's a safety device, and disabling it removes the system's protection against dangerous pressure buildup.
A waterlogged expansion tank that's lost its air charge is one of the most common causes, since it can no longer absorb the water's normal expansion as the system heats.
It's a reasonable item to include in an annual boiler service, along with checking system pressure and the expansion tank's air charge, since these three things are closely related.
No, a relief valve is a high-pressure safety device, not something that manages low pressure. Low pressure has different causes, like a slow leak elsewhere in the system, and is diagnosed separately.
Only if the underlying cause, such as a failed expansion tank, is also addressed. Replacing the valve alone while ignoring a root pressure problem often just delays the same symptom returning.
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