Why Timing Is a Clue
When a leak appears only while the heating is running — and stops once the system cools — it tells a technician a lot. The common thread is heat and pressure: as the boiler fires, the water heats, expands, and the system pressure rises. A leak that shows up under those conditions and disappears when cold is almost always linked to that heat-and-pressure cycle.
This is genuinely helpful, because it rules out faults that would leak constantly (like a corroded joint or a failed pump seal that weeps at all times). Instead it points toward expansion, pressure relief, or components that only seep when hot and pressurised.
If the leak ever coincides with a gas smell or a CO alarm, the diagnostic detective work stops — leave the home, call FortisBC at 1-800-663-9911 or 911 from outside, then call us at 604-359-1081.
The Expansion Vessel Connection
The number one cause of a heating-only leak is a waterlogged expansion vessel paired with the pressure relief valve. Here's the chain of events: when the boiler fires, water expands. In a healthy system, the expansion vessel's air cushion absorbs that extra volume. When the vessel has lost its air charge, there's nowhere for the expanding water to go, so pressure spikes — and the relief valve opens to vent the excess.
The result is a leak (really, a discharge) that appears every time the heating runs and stops when the system cools and pressure drops back. You'll often watch the pressure gauge climb from a normal 1.2 bar cold to well over 2.5 bar hot, then see water at the outside discharge pipe.
The fix is to recharge the expansion vessel's air pressure, or replace the vessel if its internal diaphragm has failed. This addresses the root cause; simply bleeding off pressure only postpones the next discharge.
Seals and Joints That Open When Hot
Heat makes metal expand and rubber soften. Some seals, gaskets, and threaded joints are perfectly tight when cold but open just enough to weep once everything warms up and pressure rises. These are intermittent leaks that can be frustrating to catch precisely because they vanish by the time you go looking.
Union fittings, pump head gaskets, and auto air vent seals are typical offenders. The repair is usually to re-seal or replace the affected gasket or valve. Because the leak is heat-dependent, our technician will often run the system up to temperature deliberately to reproduce and locate it.
This is exactly why a photo or note of when and where you see the water helps — "only when the heating's been on for 20 minutes" tells us where to focus.
Heat Exchanger Warning Signs
Less commonly, a heating-only leak can signal early heat exchanger trouble. A hairline crack can open slightly under thermal expansion and pressure, weeping when hot and sealing when cold. Warning signs to mention to us include rust-coloured water, a pressure that keeps dropping over days, and the boiler occasionally locking out.
A heat exchanger fault is the most serious cause in this category and, on an older boiler, frequently points toward replacement rather than repair. We'll confirm whether it's genuinely the heat exchanger or something more affordable like the expansion vessel before recommending anything — many homeowners fear the worst when the real fix is far simpler.
The only way to know for certain is a proper diagnosis with the system brought up to operating temperature.
What to Do
Note your cold and hot pressure readings if you can, watch where the water appears and how long after the heating starts, and check the outside discharge pipe. Don't keep topping up pressure on a system that vents every time it heats — you're feeding the leak.
Then call GasBoilers.ca at 604-359-1081. As a CanroHeat Division serving all of Greater Vancouver, we'll run your system up to temperature, reproduce the leak, test the expansion vessel, and fix the true cause. Same-day and weekend service is available where possible.