Why Is My Boiler Leaking? Causes and Fixes

A leaking boiler should be addressed immediately — leaks can cause major property damage, void the manufacturer warranty, and signal serious heat-exchanger problems. Here are the common causes, what you can check, and when to call CANRO.

First: shut off and check

1. Turn off the boiler at the front panel
2. Locate the source of the leak — note exactly where the water is coming from
3. If the leak is large or worsening, shut off the water supply to the boiler (isolation valve usually under or beside the boiler)
4. Photograph the leak source for diagnosis

1. Pressure relief valve weeping

Most common cause. The PRV opens when boiler pressure exceeds ~3 bar to protect against over-pressure. If it’s weeping when pressure is normal, the valve seat is damaged or there’s an upstream issue.

Root causes: Failed expansion vessel (water can’t expand), system over-filled, faulty PRV.

Fix: Verify pressure (should be 1.0-1.5 bar cold). Check expansion vessel pre-charge. PRV replacement if seat is damaged.

2. Condensate drain backup or detached

Condensing boilers produce 1-2 litres of acidic condensate per hour. If the drain line is blocked or has come loose, condensate overflows from the boiler.

Root causes: Clogged condensate trap, frozen line (winter), disconnected drain.

Fix: Clear the trap, inspect drain line, replace neutraliser cartridge if depleted.

3. Pump seal failure

Circulator pumps have a seal between the wet (water) and dry (motor) sections. Seal wear allows water to drip from the motor end.

Root causes: Pump age (typical seal life 8-12 years), water-chemistry damage.

Fix: Replace pump or pump cartridge. CANRO carries common pumps in the truck.

4. Heat exchanger leak

Serious. Indicates the primary or secondary HX has corroded or cracked. Water visible inside the cabinet (not from a fitting) or pressure dropping rapidly are tell-tale signs.

Root causes: Water-chemistry corrosion, age, scale damage, freeze damage.

Fix: Heat-exchanger replacement (very expensive, often $1,500-$3,000+ in parts). Frequently a "time to replace the boiler" trigger.

5. Loose or failed fittings

Water dripping from specific connection points — usually a fitting that needs tightening or a gasket replacement.

Root causes: Vibration loosening, gasket age, freeze damage.

Fix: Tighten fitting or replace gasket. Easy first-visit repair.

6. Auto air vent stuck open

Auto air vents (typically at the top of the boiler or radiators) bleed trapped air during normal operation. A stuck-open vent drips water continuously.

Fix: Replace vent. Cheap, quick first-visit repair.

7. Expansion vessel failure

The expansion vessel absorbs thermal expansion (water expands ~4% when heated from cold to hot). A waterlogged vessel can’t absorb the expansion → relief valve opens → leak.

Fix: Re-pressurise the vessel air side, or replace the vessel.

Frequently asked questions

CANRO Heating & Cooling — TSBC Class-A gas-fitting licence · Red Seal HVAC certified · WorkSafeBC and liability insured · 4.9★ from 85+ Google reviews · Serving Metro Vancouver since 1999.