My problems continue, but I have narrowed things down quite a bit.
I have intermittent cold start ups and cold drops - During these the water heater doesn't run (the fan doesn't run). This happens on occasion and turning on several other hot taps in the house doesn't rectify the problem - so it isn't a low water flow problem.
What fixes it is turning the water off at say the shower or tub, waiting a few seconds and turning it back on - Magically the heater turns on almost every time. Plus most times it works without fail.
My heater has been de-scaled. The Gas line was just checked by PG&E and shows no issues (though I didn't get the exact ouput pressure).
My water pressure is now very consistent with my new pressure regulator set at round 65psi.
I live in relatively warm coastal climate of Santa Cruz, CA and the incoming water temp, even in the winter, is not very low at all.
I have checked all valves, screens, filters everything to see if something is creating a flow problem and can't find any.
My heater shows no LED error codes - and mostly works just fine - It rarely is running at the High Flow rate when it is heating for a shower or tub fill.
My next option is to test the flow sensor (is this possible) - Check the flame detector even though this shouldn't be the problem. Get a remote and see if anything shows up on that.
Other than that I am stumped...I would love to modify the heater to turn on with a lower flow (under the .75g/m) but since I can turn on several taps and it still won't turn on I don't see that solving my problems.