Question about Bosch AquaStar 125FXKIT-LP 125FX Liquid Propane Gas Tankless Water Heater w/ Hood

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Bosch AQ 125 FX LP model will not light or stay lit......

Bosch AQ 125 FX LP model
Sometimes it lights correctly, but usually the fan doesn't start, piezo ignites the burner and burns until over-temp switch shuts off gas. Replaced the control unit, but I still have original problem. If the fan does start it runs fine for about 10 to 15 seconds, then shuts off. Other times nothing happens, no fan, no piezo, no ignition. I use a whole house pre-filter, don't have hard water issues and have never found any debris in intake filter. Any suggestions? I've already spent almost 40% of the original cost on parts for a 7 year old unit. It has been flawless until the last 3 weeks.

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6 Suggested Answers

  • 2 Answers

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Posted on Jan 02, 2017

  • 334 Answers

SOURCE: pilot lights (not always) but gas will not flow into burner

First Clean the ignitor assembly and make sure the flame is reaching the flame sensor strongly.

You may need to replace the flame sensor.

Posted on Dec 06, 2008

  • 25 Answers

SOURCE: bosch gwh-425ef nat gas on demand heater

Actually this sounds more like a bad ECO on the outgoing hotwater pipe. Disconnect it and ohm it out for about .5 ohms with a digital multimeter.

Posted on Jan 19, 2009

  • 332 Answers

SOURCE: Looking for Outdoor Tankless propane water heaters

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,,

Posted on Jun 09, 2009

  • 518 Answers

SOURCE: I have a Bosch AquaStar AQ-125-BNG tankless water

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

Posted on Jul 05, 2010

SOURCE: aq 125 blp Bosch tankless water heater, pilot

Natural Gas: Pilot burner or orfice may need to be cleaned. Pg 11 manual.
Most tank and tankless gas water heaters today suggest annual cleaning and maintenance of burner.

Open following links for Bosch troubleshooting resources:

Posted on Jun 14, 2011

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