What the Pressure Relief Valve Does
The pressure relief valve — also called the PRV, safety valve, or expansion relief valve — is a critical safety device. Its job is simple: if the pressure inside your sealed heating system rises to a dangerous level (typically around 3 bar), the valve opens and releases water to bring the pressure back down before anything can rupture.
The valve is connected to a discharge pipe that carries the released water safely away — usually to the outside of your home, where you may notice it dripping near the boiler or terminating low on an exterior wall. In a healthy system that pipe should be bone dry almost all the time.
So when water trickles from that discharge pipe, the valve is doing exactly what it was designed to do. The important question is not "why is the valve open" but "why is the pressure high enough to make it open?"
Why It's Leaking: High Pressure vs. a Failed Valve
There are really only two scenarios. Either system pressure is genuinely too high and the valve is correctly venting it, or the pressure is normal and the valve itself has failed and is weeping when it shouldn't.
Check your boiler's pressure gauge. If it reads well above 1.5 bar — climbing toward 2.5 or 3 — the pressure is too high and the valve is responding properly. The fault lies elsewhere (most often a waterlogged expansion vessel or an open filling loop).
If the gauge sits at a normal 1.0–1.5 bar but the valve still drips, the valve seat is likely scaled, worn, or has a piece of debris lodged in it. Once a PRV has lifted even once, a tiny grain of limescale can prevent it sealing fully again, causing a persistent weep. In that case the valve itself needs replacing.
The Usual Root Cause: A Waterlogged Expansion Vessel
In sealed systems, the most common reason for high pressure — and therefore relief valve discharge — is a failed expansion vessel (also called an expansion tank). The vessel contains a rubber diaphragm with a cushion of air behind it. As water heats and expands, that air cushion compresses to absorb the extra volume and keep pressure stable.
Over time the air charge leaks away or the diaphragm perishes. With no cushion left, every time the boiler fires the expanding water has nowhere to go, pressure spikes sharply, and the relief valve opens to dump the excess. You'll often see the gauge jump from normal when cold to well over 2.5 bar when hot.
The fix is to recharge the vessel's air pressure or, if the diaphragm has failed, replace the vessel. This is the proper repair — not just bleeding the system, which only resets the clock.
Can You Fix It Yourself?
There's one thing you can safely check: the filling loop. This is the small braided hose connecting your mains water to the heating system. If a valve on it has been left cracked open, mains pressure slowly over-pressurises the system and forces the relief valve open. Confirm both filling loop valves are fully closed.
Beyond that, PRV and expansion vessel work should be left to a licensed gas fitter. Replacing the valve requires draining down, isolating the boiler, and — under BC's Gas Safety Regulation — should be done by a qualified technician who can also confirm the discharge pipe is correctly routed and the new valve is properly rated for your boiler.
Do not cap, plug, or block the discharge pipe to stop the dripping. That defeats a safety device and can let dangerous pressure build inside the boiler.
Get It Diagnosed Properly
A dripping relief valve is your boiler telling you something is wrong — usually an easy-to-fix expansion vessel or a tired valve, occasionally something more. Either way it's worth resolving before it drains your pressure or floods your mechanical room.
GasBoilers.ca, a CanroHeat Division, services boilers across Greater Vancouver. We'll measure your cold and hot pressures, test the expansion vessel charge, and replace the valve or recharge the vessel as needed — properly, to code. Call 604-359-1081 to book.