Is A Hissing Or Kettling Boiler Dangerous?

If you’re experiencing a new high-pitched hiss or kettling sound, it’s worth investigating immediately

Some boilers are near-silent, whereas others are quite noisy. After all, boilers and central heating systems work by passing water and gas through a series of pipes, which causes some noise. 

If, however, you’re experiencing a new high-pitched hiss or kettling sound, it’s worth investigating immediately. Despite the name, boilers do not actually boil water. In fact, at no point does the water inside a typical boiler reach boiling point.

Whistling sounds indicate boiling or very hot water escaping from a narrow gap. In the context of a boiler’s usual operation, this is not normal. 

So, is a hissing or kettling boiler dangerous? 

Causes of hissing boiler 

If your boiler is making a high-pitched whistling or kettling noise, you should investigate it immediately. The first thing to do is listen for the noise to hear where it’s coming from. 

Here is a summary of common causes of a hissing boiler: 

  • Limescale build-up around the heat exchanger
  • An airlock near the boiler
  • Overheating
  • Pump issues
  • Faulty thermostat
  • Other internal mechanical or electrical boiler issues

To determine the source of the noise, you need to pinpoint its location. For example, the noise may come from the boiler, the pump, the valves, or any other internal components. 

Once you’ve identified the source of the noise, you can begin to diagnose the cause of the issue.

High water temperature 

As mentioned, boilers don’t boil water. If the water temperature becomes excessively high, pressure increases, and some may escape, making a whistling sound. 

Boilers generally have multiple protective mechanisms to prevent overheating. The thermostat measures the water temperature and monitors heating to ensure it reaches the target temperature without exceeding it. 

If the thermostat breaks, perhaps from a short circuit, then the boiler may overheat. In this scenario, it should automatically shut down. 


Despite the boiler’s internal temperature not reaching 100C, water forced through small pinhole leaks can cause a whistling sound at practically any temperature. Small leaks combined with water pressure might cause a faint whistling sound. 

Leaking boiler
Above: Leaks could cause kettling sounds if the water is under sufficient pressure 

Sediment and rust build-up

Sediment and rust building up around the boiler’s components can a) cause leaks and b) force water through smaller gaps. Both could theoretically cause a whistling sound, though it’d likely be faint. 

Pump issues 

If the pump is set too high, water pressure might also be too high, causing whistling noises. 

Pumps may also be set too low, which causes water to remain stationary for too long, which causes it to overheat. 

Air build-up 

If air builds up in and around the boiler and its neighbouring pipes, some may escape causing a whistle sound. In addition, air locks may increase the reading on the boiler’s pressure gauge. 

Causes of a hissing boiler when running hot water 

If the boiler hisses when running hot water, the water might be overheating. This could be caused by:

  • A faulty thermostat
  • Faulty pump 
  • Other internal electrical or mechanical issues

If your water is overheating, then shut your boiler off and call a heating engineer. This could be the sign of an electrical fault or short interfering with the cut-off. Boilers shouldn’t overheat, and the water certainly shouldn’t boil - so this is worthy of immediate inspection! 

Causes of a hissing when turned off

If your boiler hisses when turned off, there could still be a leak or other issue with neighbouring pipework. 

If the hiss is accompanied by the scent of gas, this is an emergency, and you should evacuate the building and call the emergency gas line at 0800 111 999. 

Is a hissing boiler dangerous?

If the hissing noise is caused by very hot or boiling water, that’s a strong sign of an electrical fault. For example, the thermostat could be faulty, causing water to overheat, or there could be some other issue preventing the boiler from shutting off to prevent overheating. 

If you suspect your boiler of overheating, simply shut it off at the mains and allow it to cool down. 

So long as there’s no scent of gas, it’s probably not dangerous, but do call a heating engineer as soon as possible. 

Causes of kettling boiler

Kettling noises may or may not be linked to boiling or hot water. Cold water can cause a whistling noise, too, if it’s escaping at sufficient pressure. 

Some key causes of a kettling boiler include:


Sludge buildup causes turbulent water flow, which can be noisy. For example, sludge might force water through gaps at high enough pressure to cause audible noise. It would likely be very faint, however. 

Boiler overheating 

A louder whistle is probably caused by hot water escaping in some way. This is possible if the boiler thermostat fails, causing overheating. This is one of the more alarming causes of boiler kettling. If you suspect overheating, shut the boiler down and call a heating engineer as soon as possible. 

Frozen condensate pipe

If the condensate pipe freezes in cold weather, it might cause a whistling sound as wastewater is forced through a smaller gap than usual. 

Mains water pressure

If the mains water pressure is too high, it might make whooshing, whining and whistling sounds. 

This should be inspected by a heating engineer. 

What to do with a kettling/hissing boiler?

If you are experiencing banging, gurgling, or any other similar noises in your central heating system, it is important to check if the system needs bleeding. Start by bleeding your radiators in sequence. If that does not work, try bleeding the pump. 

Next, check the water pressure of your system. Look at the pressure gauge on the front of a combi or system boiler to check the water pressure. If you have a conventional system, you may need to add water manually to increase the pressure. 

Low water pressure may signal an airlock, limescale, sludge, or leak. High water pressure may indicate a faulty pressure relief valve (PRV). 

In cold weather, it is also important to check for frozen pipes. The pipe most likely to freeze is the condensate pipe, which runs on the external wall at the rear of a condensing boiler.

Before you start fiddling with your boiler get in touch with our friendly engineers.

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