Hi--
I purchased an Enerworks flat panel with the external heat exchanger(see pictures)
It was purchased back in 2009 and after around two years I noticed that you couldn't hear the "clunk" of the check valve.
I monitored the temp gauge at the top of the tank and the temp gauge would dive when hot water was turned on meaning the cold water was shooting straight up the
thermosyphon loop and bypassing the hot water in the tank quite efficient I would imagine
I removed the check valve after draining the tank.(fun) I then soldered in a union mid pipe and a shut off valve for future service.
Check valve removed and the plastic housing was broken. This allowed the little check ball to shoot up and into the tank when the water was turned on, at some point.
The new check valve I received had a black housing and was made out of plastic which was more "heat resistant" from the company I purchased it from.
Hope this can help some people.
Part number I'm not sure of at the moment but I can probably look it up in a few days.