Weird random pipes banging

Question

I have a strange problem with pipes banging at random times throughout the day. It seems to be related to our on-demand (gas) Rheem water heater, which was installed about seven years ago. The banging is softer than the kind of water hammer that you would hear when a washing machine valve gets turned on or off. But it's a similar kind of sound, a kind of soft thud. It starts and stops at random times, at all times of day or night. When it's occurring, the pipes will bang maybe once or twice per second, semi-regularly, and this will go on for about 10-15 minutes, and eventually stop by itself. If we run the hot water for a second or two while the pipes are banging, that always stops it. But a few hours later, it will start up again. This happens 5-10 times during the day, but we hear it most often in the morning - and I don't know if that's because the house is quieter in the mornings and that's when we notice it, or if it really does happen more often in the mornings. The heater is not connected to any kind of recirc timer, and it doesn't seem to have an internal clock.

Some other clues:

• When I go outside to listen, it sounds like the banging sound is coming from the pipes inside the house that are directly behind the water heater, and not the water heater itself.

• We never had this issue before we got the on-demand heater.

• The plumbing is copper, and reasonably well secured.

• The water heater is outdoors, in a mild climate with no freezing.

My original theory was the banging might be some kind of purge cycle trying to happen within the heater, or a sensor in the unit that was misfiring due to scale or sediment build-up. Flushing the water heater has been on my honey-do list for several years. I finally got around to flushing the heater with vinegar last week, for about an hour. And I would like to say that it might have helped the problem, and maybe the banging is happening less often. But I'm not sure that's true, we're still hearing the banging noises.

This problem sounds like a minor nuisance, but it's actually a sizable quality of life issue, in that we can't really sleep in on the weekends. I always have to get up out of bed to run the hot water, to make it stop. Our bedroom is right next to the bathroom that has the on demand heater, so we hear it the loudest.

Based on the description, do any of you have any theories about what might be causing this issue, and what I can do to stop it?

Answer 1

Thermal expansion/contraction seems a likely cause of the noise. If your household is not too large, or at least if you do not have young children at home, you could easily test whether the idea has any merit.

For a period of a day or two, after each use of hot water, flush cold water through the pipes. This could be accomplished by disabling power to the water heater and running water at the "hot" outlet until the water runs cold. Don't forget about automatic users like a clothes or dish washing machine. If you no longer hear the pipe noises while following this "thermal purge" protocol then we can conclude that the noises were caused by gradual cooling of the residual heated water in the pipes.

Once you're sure of the cause of the noises you can start devising solutions.