Disaster opening pool for season

[marhan]

Our 20K in-ground pool has a Hayward pump and sand filter. Had a heater installed last year and all the pool house piping was replaced. We uncovered the pool yesterday, drained some excess water so the skimmer would work, threw in some chlorine tablets so algae wouldn't bloom, and figured on adjusting the chemistry starting today. We left the pump on continuously - pressure was normal. The heater was not on and won't be until the chemistry is correct.
In the morning the pool house was flooded and 30% of my water gone from the pool - down about a foot. The output pipe at the top of the filter had separated from the threaded hole in the filter. I can easily slide the threaded pipe in and out of the opening - it looks to my untrained eye that the original PVC pipe thread was 1-2 mm too small. There is no damage or irregularity to the threads on either the pipe or the filter. I was furious, thinking the contractor used the wrong size pipe and jerry-rigged it with a gasket fitting.
When I called the contractor, he says "oh yeah - the system had a blockage and the water in the pipe overheated - the PVC shrank and the fitting popped off." This sounds like total B.S. to me. If there was a blockage, why did the blockage magically go away and the pump continue to empty 30% of my pool? Why would the filter output coupling fail, instead of the pump fitting which is where the heat would collect? Has this happened to anyone, and was the explanation plausible? I'm thinking this is just a fabrication to cover poor installation technique.

 

[macgulley]

Was this the first time the pump has been run since the install?

 

[JasonLion]

Welcome to TFP!

If there is a complete blockage of water flow, the pump will get quite hot, possibly hot enough to melt a pipe or even it's self. The filter will not get hot. The pressure inside the filter will get quite high, and it is quite possible for something to fail, even though the filter is most likely rated for the highest pressure it could have reached. So it is plausible that there was a blockage, and the pressure inside the filter blew out the top pipe. At that point the pump could well still have been running and proceeded to pump 30% of your pool out onto the equipment pad.

One thing you are not taking into account is that those pipe threads are tapered. The threads are a smaller diameter near the end of the pipe and a larger diameter back towards the middle of the pipe. I suspect that the larger threads further along the pipe were stripped, while the smaller threads closer to the end remained intact.

Both the filter and the pipe should be rated for the highest pressure the pump could possibly have produced. So either the joint was not properly put together, or the wrong kind of pipe was used, or something was defective in some other way. Determining which of those happened will be difficult, proving it most likely impossible.