Question about Bosch AquaStar Indoor Natural Gas Tankless Water Heater with Vent Hood #125

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Bosch 125 Tankless Water Heater - Water not hot enough

I've had a Bosch 125 Tankless water for four years or so. Can't remember the exact model, but it has a standing pilot light. It has been working perfectly until this fall. Now all of a sudden the water is not near hot enough. The water just barely gets warm regardless of which faucet is used. Any ideas?

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  • Anonymous Feb 17, 2009

    how do I find the venturi valve to clean it?

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Sounds more like the venturi in the watervalve of the tankless water heater is clogged. I would recommed rebuilding it and paying close attention to properly relubing the pushrod and cleaning the venturi.

Posted on Jan 19, 2009

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Sounds like your mixing valve or tempering valve not unit.Somewhere there is a unit that prevents scalding temperatures- I expect this is your issue find and adjust this first- there are directions on it when you find it.

Posted on Dec 01, 2008

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1 Answer

Tankless Water Heater


Yes you need to delime tankless each year.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

Contact Bosch for exact repair information. My records show following number for tankless gas: 800-642-3111
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Sep 20, 2012 | Bosch Water Heaters

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Aquastar125WP is overheating and the strainer has been removed. We need to fix it. Have any ideas? Thanks for your support, Martina


Before calling service technician, open following links for troubleshoot resources:
The problem could be bad gas control solenoid, or bad sensor, or bad wiring or bad PC-board, etc, or combination of problems. Fixing one problem may not completely fix problem.
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/38b-125-series-bulletins.html
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Before installing tankless water heater, read the service manuals and product manual to make sure you have testing skills and equipment needed to repair tankless gas water heater, and that you are prepared to buy expensive parts and do the recommended yearly and monthly maintenance.
Also check that area has qualified service technician, and that person carries parts on-hand, and is certified to service your model number. Keep Tankless manufacturer contact information on-hand, and be skeptical of promises of unlimited hot water, and effortless long-lasting water heating, and saving money. The only folks saying this are the folks selling the product, and not their customers.

Fix most tankless gas water heaters yourself, same day, all brads for about $350.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Sep 21, 2011 | Water Heaters

1 Answer

Pilot flame is not large enough to contact thermo couple


Before calling service technician, open following link for Bosch troubleshoot resources, including manuals and Bosch contact information:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Each gas water heater manual recommends yearly maintenance that includes cleaning the burner and combustion parts, which includes check of connections, sensors, venting, gas pressure and air filter, etc.
Yearly maintenance covers most common problems that cause tankless failure.

Jul 28, 2011 | Bosch AquaStar Indoor Natural Gas Tankless...

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Replaced thermocouple on Bosch TF250. Now the pilot flame is so big that it heats and boils the water in the heat exchanger when hot water is not required. When we turn the tap on, we get dirty water,...


Before calling service technician, open follow links for troubleshooting resources.
Bosch site does not show TF250, but shows 250SX and 250SXO.
Gas Products Technical Support: 800-642-3111
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/tabid/370/Default.aspx

At no time when visiting tankless website have I read suggestion that people fix tankless as a do-it-yourself project. In fact each site says repeatedly to call service technician for installation and repairs.
When reading service bulletins, the repair protocols are generally long and very sequential, even if a thermocouple seems straightforward there are many interrelated parts and sensors that must communicate exactly with each other.
Example: following pdf outlines operation sequence for non-standing pilot model
http://waterheatertimer.org/pdf/Tankless-gas-operation-overview.pdf

The dirty water also suggest that tankless has not gotten regular maintenance that is required for continued operation. Tankless are not install-it-and-run-flawless-for-years type of machine.
Read manual for specific maintenance required for your model. For example yearly flushing the heat exchanger with vinegar to avoid lime build-up is suggest most models. Tankless are succeptible to lime build-up.
All costs are energy. Maintenance alone can easily eat up the promised $1-8 monthly savings from tankless vrs tank-type water heater. One service call can wipe away years of expected savings, plus replacement part can sometimes cost as much as whole new tank-type water heater, installed.

Apr 17, 2011 | Bosch AquaStar 125FXKIT-NG 125FX Natural...

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I have a Bosch AquaStar AQ-125-BNG tankless water heater, and for some reason when it is heating the water, it seems to get exremely hot and the heat is radiated back into the cold water supply line and...


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.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

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1 Answer

Water doesn't get hot enough, I did check Water pressure and it is over 60 psi, clean up strainers, Burners are clean , but I don't see the flame hight enough to warm up the water. seem to me that the gas...


1) I would contact BOSCH, or a service technician:
Tankless are affected by many internal and external factors and can be a combination of factors:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

2) Google search 'bosch tankless water heater fixya' to see cross section of problems/solutions: .
For example:
http://www.fixya.com/support/p949576-bosch_1600h_lp_liquid_propane_aquastar_a/solutions

3) Read manual for yearly maintenance protocol.
For example, tankless should be de-limed each year.
a) U-tube search for 'de-lime tankless water heater'
b) Open following link with photos of de-liming in downloadable pdf format
http://waterheatertimer.org/pdf/Delime-tankless-water-heater.pdf

5) All costs are energy.
You can also add simple tank-type water heater that can be turned on when tankless is offline.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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That particular tankless water heater has hydro-generated ignition. In order for it to function, it requires a certain water pressure and velocity. You can try turning on the hot water in two location in your house to see if that gets it started. If it does, try getting a higher flow shower head, or removing the limiters in your faucets.

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1 Answer

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Sounds like hard water. Service the water valve. Parts are available from your local Bosch Service Provider (Contact Bosch if you don't know who the service provider is).

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