Why Steam Boiler Pipes Knock


Boiler pipes can produce a range of unpleasant noises from gurgling sounds caused by trapped air in the central heating system to humming noises caused by inefficient pumps. Another common annoyance is a knocking or hammering sound coming from the boiler pipes when hot water passes through them. Knocking noises are usually not indicative of major boiler problems and can usually be remedied without professional help.


  • Water heats up as it passes through boiler pipes. Metal is an efficient conductor of heat, which results in the pipes expanding when exposed to this heat and contracting when no hot water is present in the pipes. Boiler pipes are usually constructed so there is space around them to allow for this expansion and contraction without them contacting any surrounding materials. However, if there is insufficient space around the pipes, this can lead to a knocking sound from the walls or floorboards of houses.

Surrounding Joists

  • You can fix boiler pipes fitted too near wooden joists by cutting away some of the wood on one side of the joist. Avoid cutting away too much wood, however, as this may weaken the strength of the joist. Installing padding insulation around the boiler pipes can also help prevent the knocking noise from boiler pipes.

Loose Pipes

  • Some boiler pipes create a knocking noise even when there is sufficient space around them to allow for normal expansion and contraction. This is usually due to inadequate support and loosely fitted pipes. Secure loose pipes by attaching a batten between the wooden joists and fixing it in place using pipe clips.


  • Knocking noises in boiler pipes are sometimes caused by turning taps on and off in a quick, aggressive manner. This motion causes the supply of water to suddenly cut off from the pipes, resulting in a reverberation along the length of the pipes. This effect is often referred to as water hammer and can be due to factors such as a faulty ball valve in the boiler storage tanks or worn out jumpers in the taps. Installing shock absorbers in the mains system usually remedies this issue.

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