A hot-water or steam heating system makes some normal sounds. Pipes tick as they expand, and you may hear the burner light or a pump hum. A new or louder noise usually means something changed. The type of sound helps narrow down the cause.
Kettling: a rumbling or boiling sound from the boiler
Kettling sounds like a kettle coming to a boil. It comes from water overheating and forming steam bubbles inside the heat exchanger of a hot-water boiler. Boiler manufacturer Vaillant lists these common causes in its Vaillant guide to boiler kettling:
- Scale buildup. Minerals from hard water coat the heat exchanger. The scale insulates the metal, so spots overheat.
- Low system pressure. Low pressure can let the boiler overheat.
- Control problems. A faulty thermostat or control can heat water higher than it should.
- Poor circulation. A failing pump, a blocked pipe or a closed valve restricts water flow through the boiler.
Kettling is not something to ignore. Overheating puts stress on the heat exchanger and can shorten its life. A technician can check pressure, pump operation and controls, and flush or descale the heat exchanger if needed.
Banging or hammering in pipes and radiators
Banging is most common in steam systems, which are still found in many older homes in St. Paul and the east metro. Steam hits water that is sitting in the pipes and drives it into fittings with force. This is called water hammer. Common causes, as described in the HeatingHelp guide to water hammer in steam systems, include:
- Radiators or pipes that lost their pitch, so condensate cannot drain back to the boiler
- One-pipe radiator valves that are only partly open
- Uninsulated steam mains that create more condensate than the piping can drain
- Clogged returns or failed steam traps
- A boiler that is overfired or has dirty water, which carries water up into the piping
In hot-water systems, a single sharp knock when the system starts or stops is often expansion of pipes rubbing against framing or hangers. Repeated banging during operation can come from kettling, a pump problem or a stuck valve.
Gurgling, trickling or sloshing sounds
In a hot-water system, gurgling usually means air is trapped in the pipes or radiators. Air also blocks heat, so you may notice a radiator that is warm at the bottom and cool at the top. Causes include:
- Air that entered after a repair or when the system was refilled
- Low system pressure, which lets air collect at high points
- An automatic air vent that has stopped working
The Department of Energy HVAC guide for homeowners recommends bleeding air from each radiator at the start of each heating season on hot-water systems. If gurgling keeps coming back soon after you bleed the radiators, the system may be losing water or pressure, and a technician should find out why.
Other sounds and what they can mean
- Hissing from a steam radiator vent: Normal while air escapes at startup. Steam or water spitting out of the vent is not normal. The vent may be stuck or the radiator may need re-pitching.
- Whistling: Can come from a steam vent, a partly closed valve, or air in a hot-water system.
- Humming or grinding near the boiler: Often the circulator pump or its motor. Grinding can mean worn bearings.
- A loud bang or rumble when the burner lights: Called delayed ignition. Gas builds up before it lights. This is a safety concern and needs a technician promptly.
What you can safely check yourself
- Note when the noise happens: at startup, while running, or at shutdown. Note which room or pipe it comes from.
- On a hot-water boiler, read the pressure and temperature gauge and write down the numbers. Your technician can compare them to the correct range for your system.
- Bleed air from hot-water radiators with a radiator key, with the system off and cool. Hold a rag and cup under the valve and close it when water flows steadily.
- On a steam boiler, look at the sight glass. If you cannot see water in it, turn the boiler off at its service switch and call for service. Do not add water to a hot steam boiler.
- On one-pipe steam radiators, make sure the supply valve is fully open or fully closed, never partly open.
Do not open burner panels, adjust gas valves or try to relight a boiler that keeps shutting off. Do not cap, plug or remove the pressure relief valve. If you smell gas, leave the house right away, do not use light switches or phones inside, and call 911 and your gas utility from outside. If a carbon monoxide alarm sounds or anyone feels dizzy, nauseated or has a headache that clears up outside, get everyone out and call 911.
When to call a professional
Book a boiler repair visit if the noise is new and does not stop after you bleed the radiators, if you hear kettling, if steam pipes bang every cycle, or if the pressure gauge reading has changed. Call right away for a loud bang at ignition, water leaking from the boiler or relief valve, or no heat during a cold snap. Krinkie's offers boiler repair and 24-hour emergency heating service. Many noise problems start with deposits or low pressure that routine boiler maintenance can catch early.
Common questions
What is boiler kettling?
Kettling is a rumbling or boiling sound caused by water overheating and forming steam bubbles in the heat exchanger. Scale buildup, low pressure, control problems and poor circulation are common causes.
Why do my steam radiators bang?
Banging in steam systems is usually water hammer: steam meets water trapped in a pipe or radiator. Lost pitch, partly open valves and failed traps are common reasons the water does not drain.
Is a noisy boiler dangerous?
Most noises point to wear or maintenance needs, not immediate danger. A loud bang when the burner lights, a gas smell or a carbon monoxide alarm needs immediate action.