Radiator or pipes banging when heat comes on

Symptoms

• Loud bang or knock when the heating first turns on
• Sound may travel through walls or floors
• May repeat with each heating cycle

Likely causes

• CommonPipes expanding against loose brackets or joists as they heatMetal pipes expand slightly with heat and can knock against wood framing.
• CommonTrapped air in the pipes or radiatorsAir pockets moving through the system can create banging or knocking sounds.
• PossibleWater hammer from a valve closing quicklyA sudden pressure change when a valve shuts can cause a bang in the pipes.
• PossibleLoose radiator or pipe not properly securedA radiator that can shift slightly will knock against the wall as it heats.

What to do

1. Bleed all radiators to remove trapped air, starting with the ones closest to the boiler.
2. Check that radiators are firmly mounted to the wall and not able to shift or vibrate.
3. Where pipes are visible, check that they're properly clipped and not resting tightly against joists or framing.
4. Add a small amount of pipe insulation or padding where a pipe touches wood framing tightly.
5. If banging continues after bleeding and securing, it may point to a pressure or valve issue needing a technician.
6. Note whether the noise is a single bang at startup or ongoing knocking, since this helps a technician diagnose faster.

Tools and parts

• Radiator bleed key
• Pipe insulation

When to call a pro

Call a heating technician if bleeding and securing pipes doesn't stop the banging; it may be a pressure or valve issue.

Safety