Pentair Heater Knocking Sounds?

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Q: I have a pentair 400 heater.  It has been making a knocking sound like their is a rock hitting . I know it is not air. It still heats but when it starts it knock I turn it off.  Some one subjected I take off the cover take it could be a rock in the fan area.

A:pool heater ask the pool guy

The knocking sound could be not enough water flowing through the heater, possible water chemistry issues like calcium buildup in the pipes, or a valve issue. It could be serious enough to need get a pool guy out to take a look.

Good luck!