It’s common to hear knocking sounds from a radiator, especially when the heating switches on or off. In many cases, the noise isn’t a serious problem. It’s usually caused by expansion in the pipes or changes in water flow inside the heating system.
While knocking can be louder than ticking, it’s often manageable once the cause is identified.
The Most Common Reasons This Happens
1. Pipes Expanding Against Floorboards or Walls
As hot water flows through the system, pipes expand slightly. If they’re tightly fitted through floorboards or wall openings, they can knock as they move.
If the noise is lighter and more frequent, see Why Is My Radiator Ticking?
2. Sudden Changes in Water Flow
When the boiler turns on or off, water flow can change quickly. This shift in pressure sometimes creates knocking sounds inside the radiator or pipework.
If you’ve noticed pressure changes recently, check Why Is My Boiler Pressure Low?
3. Sludge or Partial Blockages
Buildup inside the radiator can restrict water flow. As water forces its way through, it may create knocking or tapping sounds.
If the radiator is also cold at the bottom, see Why Is My Radiator Cold at the Bottom?
4. Air Pockets in the System
Air trapped inside a radiator can cause uneven heating and unusual sounds. While air more commonly causes bubbling or ticking, it can sometimes contribute to knocking.
If the top of your radiator feels cold, see Why Is My Radiator Cold at the Top?
5. Loose Brackets or Mountings
If the radiator isn’t firmly secured, small movements during heating cycles can produce knocking noises against the wall.
This is less common but easy to check.
Is This Normal?
Light knocking during heating cycles is relatively common.
It may be normal if:
• The noise only happens when heating starts or stops
• The radiator heats properly
• There are no leaks
It may not be normal if:
• The knocking is very loud or constant
• Multiple radiators are affected
• Heating performance has declined
If your radiator also struggles to heat up fully, see Why Is My Radiator Not Getting Hot?
When Should You Worry?
You should consider further checks if:
• The noise becomes louder over time
• You notice leaks around pipes or valves
• Boiler pressure drops repeatedly
• The heating system shuts down unexpectedly
Persistent knocking may indicate circulation or pipe fitting issues.
What You Can Do
If your radiator is knocking, you can:
• Check that radiator brackets are secure
• Ensure pipes aren’t tightly wedged through flooring
• Bleed the radiator to remove air
• Monitor boiler pressure
If the knocking continues or worsens, a heating engineer can inspect the pipework and system flow.
In most situations, a knocking radiator is caused by pipe expansion or flow changes rather than a serious fault. If the sound becomes louder or heating performance drops, professional advice can help resolve it.