Pressure Relief Valve Replacement for Boilers | Vancouver BC

The pressure relief valve (PRV) is a mandatory safety component on every closed hydronic heating system and gas boiler in BC. Its function is to automatically discharge water if system pressure rises above a safe threshold — typically 30 PSI on residential boilers — preventing the boiler vessel from being exposed to dangerously high pressure. It is the last line of defence in a situation where the expansion tank has failed, the fill valve has stuck open, or the boiler controls have malfunctioned and overheated the system.

The PRV is a spring-loaded valve with a calibrated opening pressure. Under normal operating conditions — system pressure between 12 and 22 PSI in a typical residential system — the spring holds the valve closed and no water is discharged. If pressure rises to the valve's set point (30 PSI), the spring is overcome and water is discharged through the discharge pipe, typically directed to a floor drain or exterior discharge point.

Technical Safety BC regulations require that PRVs be installed on all boilers, that the discharge pipe terminates safely (within 6 inches of the floor, directed away from occupants), and that the valve be accessible for inspection and replacement. The PRV must be rated for the boiler's maximum allowable working pressure (MAWP) and for the boiler's maximum BTU output — not just for pressure.

The most common sign of PRV issues is water dripping from the discharge pipe during or after heating cycles. This dripping can have two different causes that require different responses: the valve itself has failed and is no longer seating properly (valve failure), or system pressure is genuinely exceeding the PRV set point and the valve is doing its job by discharging (overpressure condition). Treating these as identical and simply replacing the PRV without investigating the cause is a common mistake that leads to the new valve also dripping within days.

A valve that has degraded and weeps continuously — even when system pressure is normal — is exhibiting valve seat deterioration from repeated cycling or mineral deposit accumulation on the valve seat. Mineral deposits are common in Metro Vancouver water; calcium and magnesium build up on the valve seat and prevent it from closing fully even at normal pressure. This is a genuine valve failure requiring replacement.

Visible corrosion, mineral deposits, or white crust around the valve body and at the discharge pipe connection indicate the valve has been discharging repeatedly and should be inspected. A PRV that has been discharging under pressure for an extended period may have physically damaged its seat from the force of repeated water discharge events.

The answer depends on what is causing the discharge. A small drip that only occurs at the end of a heating cycle and stops when the boiler cools down is typically an overpressure condition caused by a waterlogged expansion tank — the boiler is working correctly, the expansion tank is not. This is not an immediate emergency, but it should be assessed within a day or two: repeated PRV cycling is degrading the valve seat, and continued introduction of make-up water is accelerating system corrosion.

A PRV discharging continuously — a steady stream of water rather than a drip — is an emergency requiring immediate boiler shutdown. Continuous discharge indicates either a stuck-open PRV or, more seriously, an uncontrolled overpressure condition. In either case, operating the boiler with a continuously discharging PRV risks running the system dry, which can cause boiler overheating and heat exchanger failure, or indicates a pressure condition that represents a risk of vessel damage.

Never plug or obstruct the discharge pipe of a PRV. This is illegal under BC plumbing code and removes the boiler's primary safety protection. If the discharge is nuisance or damage-causing, the correct action is to address the cause of the discharge — not to block it.

Pressure relief valves for residential boilers are available in various pressure ratings (typically 15, 30, or 45 PSI) and BTU capacities. The correct replacement must match the boiler's MAWP and BTU rating — a PRV with insufficient BTU capacity will not discharge fast enough to protect the boiler in a genuine overpressure event. Parts cost for a correct-specification residential PRV is typically $40–$100.

Labour for PRV replacement is typically 30–60 minutes including diagnosis, valve replacement, and system pressure check after replacement. At current Metro Vancouver gas fitter and plumber rates, this represents $100–$175 in labour. Total cost for PRV replacement is therefore approximately $140–$275 for a standard residential installation.

We strongly recommend combining PRV replacement with expansion tank assessment if there are any signs of overpressure. As noted, a PRV discharging due to a waterlogged expansion tank will simply discharge again with the new valve. Addressing both in the same visit costs less than two separate calls.