Water hammer

[duraleigh]

I think Poconos is on the right track. I would believe "water hammer" to be caused by a blockage of the flow (like shutting off a faucet). You don't have that when the pump quits....just a loss of pressure.

I wonder if that internal bypass in the heater is coming in to play somehow?

As an aside, I would question the PB further on the external bypass. It seems it could at least be used to keep the sytem operating if you ever had to remove the heater....it may have another function as well.

[Waterworks]

The internal bypass in your heater is probably setup to make sure youre always getting about 50 GPM through the heat exchanger and sending the rest past. Auto-Pilot's bypass system for their SWG works in a similar fashion. Also, This bypass probably has some type of check valve, and you probably have another one between the heater and your chlorinator, and I'm guessing that you are hearing these valves when they slam shut. If you take a spare Jandy Check valve and manually open it and then just let go it pops closed with a very audible thud.

Brad
Waterworks Pools

[cygnusecks]

cleancloths, thanks for the excellent suggestion for a relief tube allowing the water to use air as a cushion during its deceleration. I may end up doing that.

Waterworks, also an excellent observation. I actually don't have all that many check valves in my system, I believe there are only two: one on the spa return, and one on the solar system. I'm going to do some more research into exactly where the hammer sound is coming from. It doesn't happen *all* the time, probably 80% of the time. But it's loud and so I am concerned that one day it will POP a PVC elbow or other welded joint, or the pipe itself (or worse, blow something up inside a piece of equipment!).