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Bosch AquaStar 125b suddenly goes cold

This tankless water heater has worked awesome for years... About 15+ years! It's NON electric, strictly pressure-based. Now suddenly it just goes cold in the middle of a shower, but only in that sweet spot where I like the temp. And this is with the downstairs shower too, so I doubt it's a handle issue. It'll stay hot or cold forever, bit no middle grounds. So I replaced both the gas AND water valves as they were corroded and leaked anyhow. But now, even with no leak, SAME ISSUE! This swap also included the hot sensor and all the "guts". The only things NOT done were the copper flow/ heat areas with the fins. Maybe the pilot sensor?? We even got a submersible pump with vinegar and cleaned out the whole system last year, and even then it was surprisingly clean. I'm close to just trashing this thing, not sure what else it could be???

Posted by Elizabeth Atkins on

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john h

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Your not making sense with you question --state it goes cold in middle of shower --but state it will stay hot or cold forever ----if its the latter continued hot till you shut off water that normal working if the unit shuts down during shower then you have a problem ----unit is set at one temp and runs at that temp and you use shower 1 handle / 2 valve what ever you own to adjust temp their are really only 4 reasons stops making hot water ----2 are usually well water supply --incoming water pressure /flow or to cold ----other are gas valve -or sensor so i would look into your shower handles since almost all have adjustments and anti scold safety

Posted on Sep 28, 2021

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This web site has Aquastar 125B parts. It appears your unit was manufactured 7/7/02, so that is what I pulled up for you. At least this will be a start for you to locate the part. I hope these links work.






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http://waterheatertimer.org/Crossover.html
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

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I know that this is a great answer but with out being there its hard to answer you call there tech support they are great, I have had to call they from jobs before they can help
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