Sta Rite SR 400LP Vibration and Noise

Hello all - - my first post. I have a Sta Rite SR400LP pool/ spa heater. Approx 4 1/2 years old. The heater is only used 20-30 times a year to heat a hot tub attached to the pool. When I turned it on yesterday, all indications were initially normal. The blower activated, water temperature illuminated on led, and approx 20 seconds later heard a "click" followed by the propane burner igniting. Then the PROBLEM: Unit began to vibrate with accompanying "banging" sound. The water temperature display then began to climb rapidly and the burner shut down. The blower remained on (normal). There were no codes displayed on the exterior LED screen. I assume the banging was caused by water in coil assembly overheating and becoming steam. I removed the thermal regulator from beside the intake/ exhaust water connections. It is clean and doesn't appear damaged. But, I don't know how to check it operationally. Any ideas on how to proceed?

Hi, and welcome to the group. I cannot answer your question, we don't use a heater down here in balmy Louisiana, but just wanted to let you know that you are not being ignored, and to bump this post up so maybe you can get some useful responses.

Might have to be a little patient because forum traffic slows WAY down this time of year, but don't give up on us!!

A quick Update.
Installed a new Thermal regulator (kit). The start up sequence remains normal. Approximately 5 to 8 seconds after the burner lights, the unit begins to vibrate and the "rock" banging begins - - but not to the extent previously noted. The temperature will increase a few degrees but not enough to shut the system down. The noise and vibration stop shortly afterward. The burner remains for a few minutes and then shuts off (normal operation). The burner will come on automatically several minutes later and continue the cycle.
I believe the rock banging noise and vibration is from steam being generated in the heating coils. With a new thermal regulator installed, the next culprit should be the Manifold bypass valve. On this early unit, the manifold must be removed to replace the valve. I'm waiting on the FedEx courier for the o-ring kit, which I want to install since I'm removing the manifold.
I believe the manifold valve is partially open and diverting some of the cold water from entering the heating coils. There is insufficient water volume/ flow to keep some of the water in the coils from turning into steam. I'm also suspicious that there may be air in the system. I recently installed a two speed Pentair 1 1/2 HP pump. I'm having a tough time maintaining a closed system (no air) especially in the low speed mode. I am NOT using the low speed side of the pump when operating the gas heater.
I'll update you as I proceed. Hope the manifold valve solves the problem.

In my (limited) experience with gas heaters, the sort of banging you describe results from inadequate flow through the heat exchanger. I don't know your heater model, much less your piping, but I'd recommend you verify adequate flow.

OK - - It's fixed. As I suspected, the MANIFOLD VALVE was at fault. The "piston" in the valve had broken off and left a permanent opening for cold, inlet water to bypass the heating coils and go directly to the outlet pipe. This didn't leave a sufficient amount of water in the heating coils to avoid steam from forming - - which is the banging noise I was hearing. Additionally, the broken piece had been flushed out of the unit; downstream, towards the pool where it snagged at the next (Jandy style) valve. This was causing a (back) pressure build up at the pump/ filter. Amazingly, there was still plenty of flow to the pool. I have attached a couple of pictures. The first is a shot of the loose piston, next to the jandy valve which was opened to gain access to the pipe. The next is a photo of the new manifold kit on the left. Next to the damaged piece, missing the piston.
http://i1002.photobucket.com/albums/...t/IMG_0944.jpg
http://i1002.photobucket.com/albums/...t/IMG_0937.jpg