Rinnai code 14 help

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Bsperr

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I hope that someone can help me with this problem, because it's extremely annoying. I have a Rinnai RC98hpe natural gas, condensing tankless unit that is heating water for our 1500sf, two bath home. I bought it off Ebay for a steal (looks like a customer return that was never put in service) and self-installed. The water heater has worked perfectly for the last couple years except during the winter months when the water supply gets colder. If I open the hot water for the tub on full blast, which is around 5 feet from the heater, then I get a code 14 (the resettable bi-metal switch, not the one-shot fuse). If I open the hot water to the tub gradually, then I don't have this problem. I also get a code 14 if our washing machine, which is around 30 feet from the heater, is set to hot or warm water. I don't get a code 14 if I just open the hot water at the tap full blast or run the dishwasher.

The gas pipe to the heater is sized properly, and I'm getting 7.8 in WC of pressure at the heater. I've set the water pressure to 70 psi. I've flushed the heat exchanger with vinegar, which seems to help temporarily. I can set the forced low fire manifold pressure on the heater just fine, but if I try to set the forced high fire manifold pressure I get a code 14 immediately (this is while the hot taps for the tub and sink are open). Our water is pretty soft, so I don't think it could be scale in the heat exchanger. I also don't see any signs of overheating on the heat exchanger.

I'm thinking I should try to test the temperature at the bi-metal switch as I try to recreate a code 14 (I've read it supposed to pop at 207 F) to make sure it's not malfunctioning. Does anyone else have any ideas about what could be causing this problem. I've tried calling Rinnai, but all they will do is give me the numbers of certified repair techs in the area.
 

Bsperr

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I just wanted to follow up with how I fixed this problem in case it ever starts happening to another Rinnai owner. I finally got around to taking the water heater apart and cleaning it out. Around 4 of the burners had gotten clogged with debris, so I blew them out with compressed air. The clogged burners were all on the side furthest from the bimetal switch that was triggering the code 14. I'm guessing that all of the gas fed into the chamber was burning on the side closest to the bimetal switch, so the temperature was spiking too quickly. I also cleaned some verdigris of the coils in the heat exchanger with a weak acid based cleaner (I don't know if that was necessary, but I thought I might as well while I had it torn down). No more code 14, and the water heater is running like a top now .
 

Reach4

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I finally got around to taking the water heater apart and cleaning it out. Around 4 of the burners had gotten clogged with debris,
Wow. I wonder where the material comes from. Do you have a sediment trap/drip leg on the incoming gas? I read those are being used less these days. I guess the combustion air might be a more likely source of solids.
 

Bsperr

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I'm not sure how the burners got clogged. I saw some dead flies and other bugs in the burner chamber, so the fan might be pulling them into the chamber and their little ashes were just piling up. I do have a drip leg on the gas line.
 
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