Thank you for posting your question here on Fixya.com.
I have never heard of the plug popping out before. The problem is whether it was due to poor thread makeup or a much more serious problem of a stuck UG and water boiling in the exchanger. If no one was present to see or hear the unit as it blew you will need to look deeper for your answer. Start with boiling some water in a pan then remove it from heat and drop the UG in. It should pop open showing that it is working properly. If it doesn't the UG has failed and likely led to your mishap. You will need to carefully realign the plug into the header and try to salvage the threads if you can. If you're lucky the plug threads are gone if there is still a problem. If not, the header can be replaced as well but it cost more. A new complete header assembly should come with the plug already installed. The UG part number is 006719F The complete header assembly is 006827F
If this answer does not fix your problem, please comment with additional details prior to rating the answer. You may also contact me thru our website at www.arrowpools.net. Positive feedback is appreciated once your problem is solved!
John
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