Question about Bosch AquaStar 125BO-NG 125BO Outdoor Natural Gas Tankless Water Heater

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One of the burner jets on my bosch 125bo unit is yellow (vs. blue) so I assume I have something clogging it, how do I get at it? Is there a repair manual for these units?

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They make a long stem wire brush for this,, you will need to go to a plumbing supply house to get one turn it on and gently rub it over the burner make sure it is lit so the pressure from the gas will blow out what is there good luck

Posted on Jun 09, 2009


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The other day our Aquastar 125 pilot went out. No problem, I relit it. It has gone out several times since then. But when I relight it, it often turns the burner on as if the faucet is running. AF

Safety valve is overheated water.
Sounds like the flow control sensor or other sensors, or pc board are bad.
You need service technician to diagnose problem and replace part.

Or replace whole thing with tank-type heater.


Oct 10, 2014 | Bosch Water Heaters AquaStar 125 series...

1 Answer

How to light the Bosch TF250 pilot light

I believe this is a hydro-sparker pilot ignition then burner on when T-Couple/Flame Sensor acknowledges pilot lit.
W/O the signal feedback the Gas Valve will not open and light burner.
This is a Technician Service Job as you have insufficient knowledge to DIY this potentially dangerous LP/NG Control Valve/Sparker Assembly
I assume that lack of knowledge by your lack of a Manual for the Unit
You cannot force ignition on a hydro-sparker manually.

Sep 08, 2014 | Bosch Water Heaters

1 Answer

Bosch gas heater is firing up and hot water is coming through but then cuts out after 1min water pressure drops and the burner cuts out

Ignoring the pressure drop which often is normal when the burner goes out, note that the pressure may also go up when burner is lit.
After 1 min. - suggest the flame detector needs cleaning. It's situate in the combustion chamber, usually at the rear, and looks like a right angled 1 inch pin above the gas jets. Clean with 240 grit emery cloth and also clean the nearby gas jets with a tooth brush

Apr 24, 2014 | Bosch Water Heaters

1 Answer

Bosch aq 125 no water preasure

Clean water filter yearly or monthly. Page 16 example manual.
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
Bosch service bulletins:
Descale heat exchanger yearly.

Sep 29, 2012 | Bosch Water Heaters AquaStar 125 series...

1 Answer

Water takes 30 sec to cover 10m from heater to tap

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1/2" pipe has inside diameter of .54" and holds 1.2 gallons per 100 feet
10 meters is 32 feet and holds .4 gallons
Average faucet puts out 2 gallons per minute, so 30 seconds you should get 1 gallon of water.
Put a bucket under faucet and check amount of water flow.

Assuming you draw just hot water from the faucet, then 30 seconds is slow for receiving hot water.
If you have single-handle faucet and raise handle straight up in middle, then you are drawing half hot and half cold, so it would take 30 seconds for hot to arrive.

If hot water is slow, the problem might be crossover.
or low water pressure due to clog in water line
Other problems could include clogged water filter, clogged water softener, defective pressure valve, and low water pressure.


Jul 10, 2011 | Bosch Outdoor Gas Tankless Water Heater...

1 Answer

Black soot on outside wall above hot air exhaust. Excess flame, incomplete combustion? How to adjust/correct?

All gas water heaters, tankless and tank-type, require cleaning burner and combustion parts.
Tankless manuals suggest once per year.
Tank-type once every 2-3 years.
Before calling service technician, open following links for Bosch troubleshooting resources:
Burners do not operate cleanly:

Jun 26, 2011 | Bosch 125BO NG AquaStar Outdoor Natural...

1 Answer

Water not hot enough

Check the temperature control knob first. It is located under the main cover near the bottom. Some of these water heaters can be very sensitive to water pressure also, Is your pressure steady or erratic?. There is a owner's manual pdf on the internet if you do not have one. Just type in Bosch water heater aquastar 125bo and when you find the correct site, click on the words that say literature. Take care.

Feb 23, 2011 | Bosch Water Heaters AquaStar 125 series...

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