How to Fix a Loose Home Radiator Cap

Updated February 21, 2017

Radiators warm your home by heating the surrounding air. Water is circulated from a central boiler through the radiator system to distribute this heat. Most radiators have a bleed screw that allows trapped air to be bled from the system. In many older radiator systems, this is also called a radiator cap. These are less sophisticated than bleed screws, but the principle is still the same. Fixing a loose home radiator cap is important to keep your system operating efficiently.

Turn off the power for the radiator boiler. You can turn off the power switch on the boiler, or shut off the boiler power at the main circuit.

Place newspapers or towels on the ground or carpet underneath the radiator cap you are repairing. Unscrew the radiator cap.

Clean off the threads of the radiator cap with a piece of steel wool. Remove any calcium deposits or silt build-up that may be present.

Wrap the threads on the radiator cap with plumber's tape. Make at least two turns around the cap and trim off any excess with the scissors.

Insert the cap back into the radiator. Tighten with the wrench.

Turn the central boiler and thermostat on. Allow the water to circulate throughout the system for about an hour. Shut the system down again. Open the bleed screw on the radiator where you fixed the loose cap. Allow the air inside to escape. Close the bleed screw again when you see water start to seep out.


If the radiator cap still doesn't fit into the socket after cleaning and adding additional sealant, it may be stripped and cannot be fixed. In that case, the radiator cap should be replaced with a new one.


Allow the water in the heating system to cool for at least 30 minutes after shutting off the power to the boiler to avoid burns.

Things You'll Need

  • Towels
  • Newspaper
  • Wrench
  • Pliers
  • Plumber's tape
  • Scissors
  • New radiator cap (if needed)
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About the Author

Nathan McGinty started writing in 1995. He has a Bachelor of Science in communications from the University of Texas at Austin and a Master of Arts in international journalism from City University, London. He has worked in the technology industry for more than 20 years, in positions ranging from tech support to marketing.