In the past day I started hearing a banging behind the wall of my house it seems when the heat comes on. We have water heated baseboards. We lived in this house nearly two years and never experienced this. No work has been done to the plumbing in the house in year when a pipe froze and burst last year in the garage and had to be replaced. What could be the cause of the sudden noise and what are possible ways to test and identify the problem?

The one thing that always bothered me about the system was you could hear the water traveling through the baseboards and lines when the heat comes on. Could both of these issues be related? Could air be in the system and does a baseboard circulation heating system require air to be removed from it?

2 Answers 2


What you describe is called water hammer and can have a number of causes. Air in the system could be among the causes. The system you describe should have all air removed. I assume the system has an expansion tank... I would also ensure that the tank is in proper working condition.

  • Can you point me to a resource that shows how to check if the expansion tank is in proper condition? Commented Jan 6, 2015 at 21:16
  • Just tap or hit it on the top and the bottom. The bottom part should sound "hollow", because it is filled with air. If it sounds as if filled with water, this means the membrane has been damaged and you should replace the expansion tank. Commented Jan 7, 2015 at 10:28
  • Taped top and bottom of tank, but can not hear the difference. Commented Jan 8, 2015 at 4:55

When pipes get hot (metal or PVC) they expand. If they are running on a wall or through holes in the structure there is a good chance that they would make noise based on the fact that they are expanding. A lot of time this happens when a pipe is braced tight in one location but not another - it stays tight at the one location while rubbing against the wood in another.

In one of my bathrooms if you leave the hot running for 3-4 minutes it will make an annoying rubbing/knocking sound for about 10 minutes.

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