Can you have someone come out to do a temp rise test at the header. If water is pushed to fast thru the heater you will not heat efficentlty. Do you have a bypass installed in the plumbing at the heater? Also there is a built in bypass at in out header in the heater. If the spring in there is stuck open water is bypassing the heat exchanger but it will make a rattling sound. Has anyone done a test on gas pressure at the gas valve and while it is running?
Everything has been done...heat rise test, spring was checked(no rattle either by the way), they even sent Jandy out to look at it and all I get is the same old story...the heater works fine.......but, clearly its NOT fine. I have a customer of mine who has the exact same heater and I felt the water coming out.....holy crap, it was like jacuzzi water.
I have also messed witht he heat rise screw....turn it all the way down, all the way up, never any change at all....water consistently comes out the same temp. maybe 3-4º warmer than pool water.
I am aware of the built in bypass, I dont know about an external one.....but, as you say, if the water is moving to fast, it wont heat up, thats been my gut feeling of whats occuring all along.