Bosch aq 125 b manual - Bosch Water Heaters AquaStar 125 series Outdoor Gas Water Heater - 125BO

http://www.bosch-climate.us/files/201109162028550.250SX_English.pdf

sounds like pressure relief valve is stuck closed free of or replace

Check for crossover, aka cross connection
http://waterheatertimer.org/Crossover.html
Install ordinary tank electric water heater for $250 and solve problem quick, sure and cheap

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Clean water filter yearly or monthly. Page 16 example manual.
http://waterheatertimer.org/pdf/Bosch-125BO-NG-LP-outdoor.pdf
Shut off the installer supplied cold water supply valve.
Disconnect water pipe going to the back of the water
valve. Remove filter from back of water valve. Clean the
filter and inspect for damage. Replace if the filter is at all
damaged. Inspect the top of the water valve for signs of
a water leak or corrosion. If present, rebuild the water
valve.
Bosch service bulletins:
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/38b-125-series-bulletins.html
Descale heat exchanger yearly.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf

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Green can indicate a water leak .you would have to check for a leak to be sure

Natural Gas: Pilot burner or orfice may need to be cleaned. Pg 11 manual.
http://www.boschhotwater.com/Portals/7/TechManuals/125BL_English.pdf
Most tank and tankless gas water heaters today suggest annual cleaning and maintenance of burner.

Open following links for Bosch troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/125BLTechSupport/Troubleshoot/tabid/474/Default.aspx

Tankless should never be installed with plastic pipe, PVC or CPVC etc.

Before calling service technician, open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual.

Add a comment with your experience so others with same-similar problem can benefit.

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It sounds to me like your thermocouple is bad . See if you can trace it from the gas valve to pilot light. Remove thermocouple and try to get a exact replacement part. If the length varies don't worry about that. Install new thermocouple and see if that doesn't solve your problem. Thanks

there is a small dia copper pipe which has a thermal coupling on it the end of it sits in the flame of the pilot light,
if it fails which it does after about 25-15 months,it shuts down the gas burner

it will tell you right on the box if it for indoors or out doors either way they have to have a cover over them,,