Why is my boiler leaking from the pressure valve?
If water is dripping from the pipe connected to your boiler’s pressure relief valve, the valve is doing its job: releasing water to protect the system from dangerous over-pressure. The question is why the pressure got high enough to trigger it.
This is one of the most common boiler complaints we see across Metro Vancouver and the Fraser Valley. Sometimes it’s a simple pressure adjustment; other times a component has failed.
HeatLand has serviced pressurized boiler systems since 1999. Below we explain how the pressure relief valve works, why it leaks, and when you need a licensed technician to correct the underlying cause.
How the pressure relief valve works
The pressure relief valve (also called a PRV or safety valve) is a critical safety device. It’s set to open at a specific pressure, commonly around 3 bar, to release water and prevent the sealed heating system from becoming dangerously over-pressurized. When it opens, water discharges through a dedicated pipe, often routed outside or to a drain.
Because the valve only opens when pressure is too high, a leak from it is almost always a symptom, not the root problem. The valve is telling you the system pressure has climbed beyond safe limits, or that the valve itself is no longer sealing properly.
Understanding this matters: simply replacing a dripping valve without fixing high pressure will not solve the issue, as the new valve will keep opening. A technician diagnoses whether the fault is upstream pressure or a worn valve seat.
Why the valve leaks: high pressure vs. a failed valve
There are two main scenarios. In the first, system pressure is genuinely too high. This commonly happens when the expansion vessel loses its air charge and can no longer absorb the expansion of heated water, so pressure spikes and the relief valve opens. Overfilling the system when topping up pressure, or a faulty filling loop that lets mains water leak in, produces the same result.
In the second scenario, the valve itself has failed. Over time, debris or scale can lodge on the valve seat, or the seal can wear, so the valve weeps continuously even at normal pressure. Once a relief valve has passed water, it sometimes won’t reseat cleanly and needs replacing.
Distinguishing between these requires checking pressure both cold and hot, testing the expansion vessel, and inspecting the valve. That combination of tests is why this is a technician-level diagnosis rather than a DIY fix.
What you can safely check
You can gather useful information before calling. Look at the pressure gauge when the system is cold; it should read roughly 1 to 1.5 bar. Then watch what happens as the boiler heats up. If pressure climbs well above 2.5 to 3 bar during operation, high pressure is likely triggering the valve.
Note whether the valve drips only when the boiler is hot (points to expansion/pressure) or continuously even when cold (points to a failed valve). Check whether you recently topped up the system, which can cause temporary overfilling.
Do not attempt to open the boiler, adjust the expansion vessel, or replace the relief valve yourself. These involve draining, repressurizing, and in some cases gas-side work. In BC, gas appliance servicing must be done by a TSBC-certified gas fitter. Share your pressure observations with HeatLand and we’ll arrive ready to correct the cause.
When to call a licensed technician
Call a licensed technician whenever the pressure relief valve is leaking, because it signals either unsafe pressure or a compromised safety device, neither of which should be left. Continuous discharge also wastes water and can corrode the surrounding area over time.
Treat it as more urgent if the discharge is heavy, if pressure climbs rapidly, or if you notice other warning signs such as a gas smell, unusual noises, or a carbon monoxide alarm. A safety valve that keeps opening should never be capped or blocked to stop the drip, as that defeats its protective purpose and is dangerous.
HeatLand’s licensed, insured technicians measure system pressure, test and recharge or replace the expansion vessel, set correct fill pressure, and replace the relief valve if it has failed, on all major boiler brands. We serve Metro Vancouver and the Fraser Valley, with emergency service and financing available.
Quick Checklist
- Read the pressure gauge when the boiler is cold (target ~1 to 1.5 bar)
- Watch whether pressure climbs above ~2.5 to 3 bar when hot
- Note if the valve drips only when hot or also when cold
- Recall whether you recently topped up system pressure
- Never cap or block the relief valve to stop the drip
- Call a licensed technician to test the vessel and valve
| Symptom | Likely cause | Fix |
|---|---|---|
| Drips only when boiler is hot | Waterlogged expansion vessel | Recharge or replace vessel |
| Pressure over 3 bar in operation | High system pressure | Correct pressure, check vessel |
| Weeps continuously even when cold | Failed / worn relief valve | Replace relief valve |
| Started after topping up | System overfilled | Bleed pressure to correct level |
Frequently Asked Questions
Is it safe to keep using a boiler that leaks from the pressure valve?
It’s best not to. While the valve is protecting the system, continuous leaking means pressure is unsafe or the valve has failed, and both should be corrected. Never block the valve to stop the drip. Have a licensed technician diagnose the cause promptly to avoid water damage and keep the safety device functional.
Why does my pressure relief valve only leak when the heating is on?
When heating is on, water expands and pressure rises. If the expansion vessel has lost its air charge, it can no longer absorb that expansion, so pressure exceeds the valve’s set point and it opens. Once the system cools, pressure drops and the leak stops, a classic sign of a failing expansion vessel.
Can I just replace the pressure relief valve myself?
It’s not recommended. Replacing the valve without fixing the underlying high pressure means the new valve will keep leaking. The job also requires draining and repressurizing the system, and boiler servicing in BC must be done by a licensed gas fitter. A technician fixes both the valve and the root cause.
How much does this repair cost?
Cost depends on whether it’s a simple pressure adjustment, an expansion vessel recharge or replacement, or a new relief valve. Because parts and labour vary by brand and model, an exact figure requires an on-site check. HeatLand quotes clearly before any work, with financing available for larger repairs.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.