Make sure during your maintenance that the burners where pulled and cleaned, We have had similar issues with the same age and model numbers. Even though the flame sensor was clean and the the flames appeared to hit the flame sensor we still needed to clean the burners. Also look for corrosion at the outlet sensor.
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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.
Could it be an electric power intermittency (loose wire). Turning on several taps (or even one) should reliably start it up, so flow sensor intermittency could be an issue.
If it's detecting overtemp, mis-sensing flame-out or something and inhibiting flame is should be throwing up a code.
It's not the flame sensor issue. IIRC you'd have to cycle the power to clear those errors.
Call Takagi's help line- they should be able to tell you the remaining likely causes, and how to rule them out.
If I were you I’d have been on the phone with Takagi Tech support a long time ago. I never used them but know that Rinnai and Noritz tech support will walk you through anything.
It is starting to sound like a malfunction of the servo valve. You should be able to unplug it without unplugging the flow sensor. If so, you won’t be able to control the water flow to keep from overshooting it, but you should not have the other problem. See what happens then.
Again, I’d get tech support on the line if possible while you are at the heater.
Thanks for the suggestions - I am going to contact Takagi - It is hard right now based on my schedule so it may take me a little while.
Thanks Again everyone!
Quick Follow Up:
After contacting Takagi and getting the problem to occur with the cover off, with a good flow of water going through the heater, and no error codes - the Tech told me I needed to remove the flow sensor and soak it in CLR for a couple of hours - I haven't yet been able to do that but hope to soon.
A new repalcement Flow Sensor is only $28 so if I still have problems I will buy a new one...
I was finally able to soak the flow sensor in CLR for a few hours. With this back in place a new problem surfaced - I finally started to get an error code (4 LED blinks)
I worked with Takagi Technical Support on the phone and completely cleaned the burners, heat exchanger, fan, pressure switch, and exhaust piping. There was a very small amount of dust on most items which I cleaned with an air compressor. I also cleaned/sanded the igniter and flame detector, and reassembled the unit. So far it is working great, and I am keeping my fingers crossed.
Thanks again to everyone here for their help - I now am far too much of an expert on Takagi tankless heaters!