Troubleshooting Boiler Pressure Relief Valve (PRV) Leaks

A boiler relief valve that is "weeping" or blowing off is rarely a faulty valve; it is usually a symptom of a system-wide pressure imbalance. Understanding why these valves trip is essential for protecting the boiler's heat exchanger and surrounding components.

The Role of the Relief Valve

Pressure relief valves are safety devices designed to discharge when system pressure exceeds a specific threshold—most commonly 30 PSI for residential hot water boilers.

Pressure in a closed-loop system is generated by thermal expansion. As water temperature increases, it expands in volume. Without a functional "buffer," this expansion creates immense hydraulic pressure that can rupture cast iron or copper components.

The Two Primary Causes of High Pressure

If a relief valve is active, you are likely facing one of two issues: Loss of Expansion Space or Uncontrolled Water Infeed.

1. Expansion Tank Failure (Waterlogging)

The expansion tank contains a rubber bladder with air on one side and system water on the other.

The Symptom: Pressure rises rapidly as soon as the burner fires but drops quickly when the pump runs or the system cools.
The Cause: If the bladder ruptures or loses its air charge, the tank becomes "waterlogged." Since water cannot be compressed, the expanding heated water has nowhere to go but out the relief valve.
The Fix: Check the "Schrader" valve on the tank. If water squirts out when pressed, the bladder is compromised, and the tank must be replaced.

2. Faulty Auto-Fill Valve (Pressure Reducing Valve)

The auto-fill valve is designed to maintain a minimum "fill pressure" (typically 12–15 PSI).

The Symptom: Pressure rises slowly and steadily, even when the boiler is not firing, eventually reaching the 30 PSI limit.
The Cause: Debris in the valve seat or a failed internal seal allows domestic water to "creep" into the boiler loop continuously.
The Fix: Close the manual shut-off valve leading to the auto-fill. If the pressure stops rising, the fill valve is leaking internally and needs service or replacement.

Technical Specifications & Diagnostics

For the HVAC Marksman, we don't guess—we measure. Use this table to diagnose the fault quickly:

Observation	Likely Culprit	Action Step
Pressure spikes ONLY during heat cycle	Expansion Tank	Check air pre-charge (PSI should match system fill).
Pressure rises while boiler is OFF	Auto-Fill Valve	Isolate the water supply; monitor for "Pressure Creep."
Relief valve leaks at < 25 PSI	Faulty PRV	Replace the valve; the spring is likely weakened or scaled.
Water from air vent on tank	Ruptured Bladder	Replace Expansion Tank immediately.

Pro-Tip: Sizing and Support

An undersized expansion tank is just as dangerous as a failed one. If you are working on a high-volume system or a large commercial loop, standard tanks may not suffice.

Need a Calculation? Don't eyeball it. Reach out to our design team for precise expansion tank sizing at hydronics@jswmi.com.