What Causes a Heating Boiler to Make Noise When Firing?


How a boiler works

  • Boilers use a fuel source: oil, gas, propane, or electricity to heat water to just below a boil between 160 degrees and 180 degrees. This heated water is sent through pipes throughout the building into radiators built in all of the rooms. As the hot water passes through the pipes in the radiators the pipes get warmer and emit the extra heat into the room. Now spent of its heat, the cooled water returns to the boiler to begin the cycle again.

Possible cause of boiler noise: mineral deposits

  • Hard water used by a boiler has a higher proportion of calcium salts (lime) in it than soft water. As the water is heated, these calcium salts become insoluble in the water and they settle at the bottom of the container in which they were heated. This can be a pot on a stove or the kettle for a boiler. The noise heard with boiler heating is known as kettling. When the water flows too slowly through the pipes, it can come to a rolling boil. The bubbles from this bang against the sides of the pipes and create a pinging or banging sound. A whole home water softener might help this issue.

Possible mechanical causes of noise

  • Boilers which are too large for the heating space or those with incorrect settings for the return flow rate can cause noise during heating. Both of these can cause bubbling and boiling in the pipes of the boiler system, which leads to the commonly heard knocking sounds. Ensuring that the heating boiler is the correct size for the job and that the temperature of the return water flow is only 52 degrees Fahrenheit different from the temperature to which the water is heated.


  • Photo Credit Wikicommons: public domain: http://commons.wikimedia.org/wiki/File:Gasboiler.JPG
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