What causes an airlock in a boiler system?

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What Is an Airlock in a Boiler System?

A boiler heats water and circulates it through a closed loop of pipes, radiators, or baseboard heaters, then returns the cooled water to be reheated. That loop is designed to be completely full of water with no air in it. When a pocket of air gets trapped somewhere in the system, usually at a high point, a bend, or inside a radiator, it blocks water from flowing past it. The result is a radiator or zone that stays cold or only warms partially, even though the boiler itself is firing and heating water normally. Airlocks are one of the most common reasons Metro Vancouver homeowners call about a boiler that seems to be running but isn't actually heating certain rooms.

Common Causes of Airlocks

Air gets into a sealed hydronic system in a handful of predictable ways. Understanding the cause often points to whether it's a one-time nuisance or a sign of a recurring issue.

• Refilling or repressurizing the system after maintenance, a repair, or a part replacement, which can introduce air if not bled out properly afterward
• Draining and refilling a boiler or radiator loop for any reason, including seasonal shutdowns or replacing a pump or valve
• Low system pressure, which allows dissolved air to come out of solution and form pockets, especially in older systems that lose pressure gradually over months
• A failing or clogged automatic air vent that should be releasing trapped air but isn't
• A small pinhole leak or loose fitting that lets air seep in even as it lets a small amount of water seep out
• High points in the pipe layout, common in multi-level Metro Vancouver homes and townhomes, where air naturally rises and collects
• A recently installed radiator, baseboard loop, or expansion tank that hasn't been fully purged of air

What You Can Safely Check Yourself

Bleeding a radiator is a routine homeowner task that doesn't require a licence. Here's a simple checklist:

1. Let the heating system cool down or turn it off first, since bleeding a hot, pressurized radiator can release hot water
2. Locate the bleed valve, usually a small square or slotted fitting near the top of the radiator
3. Hold a cloth or small container underneath, then use a radiator bleed key or flathead screwdriver to turn the valve slowly counter-clockwise
4. Listen for a hissing sound as air escapes, then close the valve as soon as a steady stream of water appears with no more air
5. Check your boiler's pressure gauge afterward; bleeding radiators lowers system pressure, and most residential boilers need to sit within the manufacturer's marked range, often topped up through a filling loop

If bleeding brings back a warm radiator and the pressure holds steady afterward, the airlock is likely resolved. If pressure drops quickly again, or the same radiator goes cold again within days, that points to an underlying issue rather than a one-off airlock.

When to Call a Licensed Technician

Some situations go beyond a simple bleed-the-radiator fix and should be left to a licensed technician:

• Airlocks that return repeatedly within days or weeks despite bleeding
• Multiple radiators or an entire zone staying cold, which can point to a pump, valve, or circulation problem rather than trapped air alone
• Persistent pressure loss that requires frequent refilling, which may indicate a leak somewhere in the system
• Any need to drain, refill, or purge the boiler itself, adjust the expansion tank, or work on the boiler's internal components, gas line, or venting
• Unusual noises like continuous gurgling, banging, or knocking that don't resolve after a normal bleed
• Baseboard or in-floor hydronic loops, which often need a purge valve and a proper flushing procedure rather than a simple radiator bleed key

In BC, any work on a boiler's gas connection, venting, or internal gas components legally requires a Technical Safety BC-licensed gas fitter. CANRO's technicians are Red Seal certified and hold Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, so any repair that goes beyond a homeowner-safe radiator bleed is handled to code.

What to Expect From a Professional Visit

When a technician comes out for a boiler that's airlocking repeatedly, the visit typically includes checking system pressure and the expansion tank, inspecting the automatic air vent and bleed points throughout the system, pressure-testing for leaks if pressure keeps dropping, checking the circulator pump for proper operation, and, if needed, fully purging and refilling the loop through a proper procedure that removes trapped air from every branch rather than just the radiators that are obviously cold. On older Metro Vancouver homes with cast-iron radiators or galvanized piping, a technician will also check for scale or debris that could be trapping air at restrictions in the pipe. CANRO provides upfront pricing before any work begins, and for anything beyond a routine service call, a free written quote so there are no surprises.

Airlocks in Older vs. Newer Metro Vancouver Homes

Frequently asked questions

Reviewed by the CANRO Heating & Cooling service team — July 02, 2026. CANRO holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, employs Red Seal certified technicians, and carries WorkSafeBC and liability insurance. 4.9★ from 85+ Google reviews · serving Metro Vancouver since 1999 · (604) 359-1081 · info@canroheat.ca.