A chimney from a gnome's house??

My final answer is "I don't know". I am so glad you have hot water again and you didn't have to pay a plumber. Pam

Ummm no idea if I had to hazard a guess I would say lime, salt and maybe some tequila with a little umbrella from the beach in a shot glass. 😁

Congrats on the hot water! At first your "canned applause" note I read as canned applesauce and wondered why it was everywhere! I think it is time to get the old eyes examined!! Have no idea what that could be, but if it is a drink with mini umbrella - I don't want any part of that!!! LOL

LOL!!
I do appreciate the different guesses. 😀
I’m not even sure the people at Lowes or Home Depot will be able to put a name to it but I’ll try.

Bermuda, I've texted my husband, the plumber. We've been self employed for over 30 years. I just talked to him. He said the picture I texted wasn't too clear but he said it may be a check valve that is inside the supply line to conserve energy. If you PM me I would be happy to give you our business number and my husband would be glad to talk to you.

My advice instead of Lowes check with a plumbing supply house in your area. Generally the public can go in.

Bermuda,
Sorry...I miss typed. My husband corrected me when he got home. The check valve would have been in the water supply nipple. Jean

It looks like that silly little nub that I use to have at the end of my lawn chair arms

Modeler- thank you for asking your plumber DH about this little ‘brat’ of plastic. I will ask DH if it could be from that area.... what caused the problem is this ‘brat’ fit almost perfectly, blocking the hot water supply line (according to DH). DH trouble-shot the problem by ruling out where it could not have come from ie. ‘Brat’ is too big to be sucked up by our underground well pump, too small to get caught in water line from well to ‘water filtration’ container with sediment filter, etc. Just glad that we didn’t have to cut into any pipe. You are lucky to be married to a plumber.

Bermuda,
Sounds like your DH is quite the handyman and good at detective work. If he wants to talk to my husband just contact me. My husband talks to a lot of people to help them problem solve issues themselves. Jean