Boiler Making Noise? Banging, Kettling & Gurgling Explained

A loud bang or clunk when the boiler fires up — sometimes described as a "water hammer" sound — is typically caused by thermal shock. Cold return water hitting a very hot heat exchanger causes rapid expansion that sounds like a knock. This is common in systems where the pump runs too briefly or where cold mains water is admitted to an overheated system.

Banging can also come from pipe movement as copper pipes expand and contract through heating cycles. If pipes run through tight clips or joists, they can tick or bang against the structure. This type of noise is harmless cosmetically but worth insulating to prevent long-term wear on pipe joints.

Kettling is the most common noise complaint in Metro Vancouver's hard-water zones — particularly areas supplied from the Capilano and Seymour reservoirs. The sound is a rumbling, bubbling, or low-pitched kettle boiling sound that occurs continuously when the boiler is running. It indicates limescale or sludge accumulation inside the heat exchanger, which creates localised hot spots where water flashes to steam.

Kettling reduces efficiency (sometimes by 15–20%), causes premature heat exchanger wear, and can eventually lead to cracking. A chemical descaling flush (power flush) or a magnetic system filter installation can address the cause. Many Metro Vancouver homeowners benefit from installing inline scale inhibitors as preventive measures.

A gurgling sound — particularly from radiators or the pipework — almost always means air is trapped in the system. Air can enter during filling, after maintenance, or through micro-leaks at fittings. Air pockets prevent full circulation, creating cold spots in radiators and causing the gurgling as water forces past the trapped air.

Gurgling from the boiler itself (not the pipes or radiators) can indicate a partially blocked or failing circulator pump, where the impeller is cavitating. This is more serious and requires professional service — a cavitating pump is working harder than it should and will fail prematurely.

A high-pitched whistle typically indicates water or steam being forced through a restriction — a partially closed valve, a stuck automatic air vent, or a failing pressure relief valve that's beginning to open. On condensing boilers, a whistle can also come from the combustion air intake or flue if a bird's nest, insect nest, or debris is partially blocking the vent terminals.

Whistling related to the PRV beginning to open is a serious warning sign. Do not ignore it — if system pressure is climbing high enough to open the relief valve, the expansion tank likely needs replacement.