Why is my boiler making banging noises?

Banging from inside a hot-water boiler can be kettling. Water overheats in the heat exchanger and forms steam bubbles, and the pressure they build can cause a rumble or a bang. Something is stopping water from carrying heat away fast enough, or letting it run too hot: scale, low pressure, trapped air, a pump problem or a faulty thermostat. Banging out in the pipes, away from the boiler, usually points to the piping instead. Either way, it is a job for a licensed technician.

Kettling: water overheating in the boiler

Vaillant, a boiler maker, describes kettling as a noise like a kettle boiling, often described as rumbling, banging or gurgling, and typically caused by water overheating inside the boiler^[1]. As the water heats, steam bubbles form, and the pressure they create makes the noise^[1]. The causes Vaillant lists come down to water not flowing well enough through the heat exchanger, or a thermostat letting the water get too hot^[1].

Why the water overheats

Scale in the heat exchanger

When hard water is heated, scale on the heat exchanger insulates it and obstructs the flow of water, which leads to overheating and bubbles^[1]. Continual fresh make-up water shortens a boiler's life, because its minerals build up in the heat exchanger, reduce heat transfer and overheat it^[2]. A sealed heating loop that holds the same water for years stays fairly clean. One that keeps topping itself up through a slow leak keeps bringing in new minerals.

That matters for Virginia homes on well or hard municipal water. Virginia Cooperative Extension explains that calcium and magnesium dissolve into groundwater as it passes through limestone and other rocks^[5], and hard water leaves mineral scale inside pipes and in appliances such as water heaters^[5]. Navien adds that in a hard-water area, a scale prevention system can extend a heating boiler's life by preventing lime and calcium deposits^[4].

Low pressure or trapped air

If system pressure is too low, water cannot circulate well through the heat exchanger, which creates hot spots and steam^[1]. Air in a boiler system can cause it to overheat or keep the pumps from circulating water^[3], which is why every boiler system needs an air eliminator in the piping to remove air as water passes through^[3]. If your gauge keeps dropping, our guide to why a boiler loses pressure covers the leaks and valves behind it.

A pump, valve or control problem

Blockages in the pipes or a problem with the pump restrict flow and overheat parts of the boiler^[1], and a faulty thermostat or control can let the boiler heat water hotter than it should^[1]. Modern boilers often catch this before you hear it. On one Navien model, the heat exchanger overheat codes tell the technician to check for proper system flow, and one of them also to clean the system filters^[2].

Banging in the pipes, not the boiler

If the noise comes from the walls, floors or radiators rather than the boiler cabinet, the cause may be in the piping rather than the boiler. In hot-water systems, pipes often tick or knock as they expand and contract against joists or straps when they heat and cool. Hammering in a steam system is a different problem and needs someone who knows steam heating. Do not assume a noise from the pipes is harmless: describe where and when it happens and let a licensed technician look.

Why it is worth fixing soon

Kettling is not just noise. The high temperature and pressure of kettling strain the heat exchanger and can cause micro-cracks, corrosion and wear^[1], and the boiler works harder to reach temperature, so it uses more energy^[1]. Replacing a heat exchanger can be costly^[1], and ignoring kettling can lead to repairs, inefficient operation and even safety risks^[1]. See whether to repair or replace a boiler if yours is older.

What a technician does about it

You can help the diagnosis by noting when the noise happens (as the burner lights, mid-cycle or after it shuts off), where it comes from, and what the pressure gauge reads. Navien's manual asks owners to check the pressure and temperature gauge daily for signs of higher than normal pressure^[2].

The technician then checks pressure, air removal and flow. Pressure that climbs too high as the boiler heats can come from an expansion tank that is waterlogged or undersized^[2], and a relief valve that weeps may be reacting to over-pressure from a waterlogged or undersized expansion tank rather than being faulty itself^[2]. If scale is the cause, Vaillant's advice is flushing the system with a descaling solution, done by an installer^[1]. Navien notes that flushing the heat exchanger is a somewhat complicated procedure, done when combustion or performance shows a need^[2], so it is a job for the technician. Afterwards, annual maintenance by a qualified technician can prevent kettling^[1]; see how often a boiler should be serviced.

When to call a licensed pro

Call promptly if the banging is new or getting louder, the pressure gauge reads higher than usual, the relief valve is dripping or discharging, the boiler keeps locking out, or the bang comes from the burner as it lights. If you smell gas, do not try to light any appliance, do not touch electric switches or use a phone in the building, and call your gas supplier from a neighbor's phone, or the fire department if you cannot reach the supplier^[2]. Worn or poorly adjusted boilers, and flues that are blocked, disconnected or leaking, are sources of carbon monoxide, an odorless and colorless gas^[6]. If a CO alarm goes off, EPA's advice is to get anyone with symptoms out of the house immediately and seek medical attention; if no one has symptoms, ventilate the home with fresh air, turn off all potential sources of CO and have a qualified technician inspect your fuel-burning appliances and chimneys^[7].

Virginia's tradesman rules define HVAC work to include the repair and maintenance of "steam and hot water heating systems, boilers"^[8], and no one may work as a tradesman unless licensed^[9]. Check any business on DPOR's License Lookup^[10]. You can also browse more HVAC repair answers or find licensed heating businesses in Blacksburg.

If your boiler is banging, describe the noise once and get quotes from licensed boiler technicians.