Question about Bosch AquaStar 125FXKIT-NG 125FX Natural Gas Tankless Water Heater w/ Hood

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On demand water heater won't stay lit.

Pilot lights array momentarily when there is demand for hot water, continues trying to light but it seems there is not enough gas continuing to come out of the array, or maybe something is telling the gas to shut off? Bad sensor? Regulator needs to be turned up?

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Your thermocoupler is bad..

Posted on May 22, 2009

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Reliance 606 natural gas water heater - model #640YORT - water does not stay hot. pilot has to be clicked to heat and then u can use hot water til tank empty and it will not continue to heat water.


If I understand correctly, the pilot light is not staying lit.
You light the pilot and water in the tank reaches temperature.
Once hot water is gone, then pilot light has to be lit again.

Gas pressure is good, and gas is flowing.
Thermostat is working at high end because it cuts off heating.
Vent is working because tank heats up.
Air intake is working.

Gas control thermostat might not be reading low temperature. Replace gas control valve.
Pilot tube might be dirty. Clean burner parts and combustion chamber, and air intake, and vent.
Backdraft might be blowing out pilot. Check for constant updraft by running water heater 5 minutes, and then light match under vent hood and see if smoke goes up.
Very hot air can put out pilot, especially when water heater located in attic. Add ventilation.
Improper vent might have water dripping back down into combustion chamber and putting out pilot light and rusting out the parts. Inspect for rusted parts and water.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

Aug 09, 2011 | Reliance Natural Gas Water Heater Control

1 Answer

No hot water coming out. ignition comes on ,but no lighting of gas. There is gas in bottle and it's on as normal.Works one day , doesn't the next.I cannot remember if there was a pilot light before!


check for pilot but im sure that the pilot is on demand thats what makes these things so cool and inexpensive to operate. try having someone turn on the hot water while looking for ignition. if there is nothing it may be a relay. im not sure but if you open it up check for a 3 amp fuse its green. all the new furnaces with mother boards have a fuse. I am not really a heating guy but i have been looking into on demand systems and sat in a training session at the plumbing supply. never hurts to try and help.
if its not a fuse or the ingniter isnt functioning you may need to call a heating person who knows on demand or check Bosch on line maybe they have the trouble shooting guide if you dont.

sorry that all i can suggest hope it helps

Dec 04, 2010 | Bosch & Tankless LP Hot Water Heater

1 Answer

I have a Bosch GWH 425 HN Tankless Hot Water Heater...On 2/28/2010, it began intermittant exploding when demand for hot water occurred. Local technicians worked on the unit several times, changed and...


I have the same problem with the same model. The problem has been identified by a friend of mine, correctly, I think.
This is a very dangerous problem involving Bosch's much touted safety feature, ironically enough. IT IS THE FLAME SENSOR IN THE GAS VALVE. The flame sensor is supposed to confirm that the pilot has lighted BEFORE the main gas line sends gas to the main burners, after which the pilot shuts off. The pilot light is not coming on, the sensor that shouldn't let gas fto the main burners is failing and the main gas is flowing and ignighting from the spark that is supposed to light only the pilot in a big explosion.
This is the 2nd unit of this model that has failed. I think Bosch should do a recall

Apr 25, 2010 | Bosch & Tankless LP Hot Water Heater

1 Answer

Tankless on demand water heater.


Tankless water heaters normally take a little longer than a regular heater to provide hot water because they have to start and get warmed up. I'm guessing the heater uses a pilot light and that the short amount of hot water when you turn on the tap is hot because when the water is sitting still in the heater the heat from the pilot light is enough to get it hot. The tub faucet will get hot a lot faster than the shower because it lets a lot more water through and empties out the pipes faster than the shower. You could probably speed things up by letting the tub faucet run on completely hot for like 10-15 seconds and then turning on the shower.

Oct 24, 2009 | RTO Takagi Jr. TK-JR-N Tankless Water...

1 Answer

My Bosch on demand water heater won't stay lit


DO NOT try to adjust gas pressure unless you have the appropriate tools and gauges to do it right. You don't want to burn your house down.

First make sure all of the gas valves are fully opened. If that checks out and the problems continue, call an HVAC technician or your gas supplier to have them evaluate the adequacy of the supply line as well as the gas pressure at your appliance. Go with whatever they say or ask you to do.

May 19, 2009 | Bosch AquaStar 125FXKIT-NG 125FX Natural...

1 Answer

Trying to find out if my new hot water heater is shot


The red button you pushed on the hot water heater was probably what is called a "piezo-electric" ignitor. It's normal for it to make a snapping sound and then pop back out. It sounds like you just lost the pilot light.

That being the case, do this:

Turn the gas valve on the hot water tank to the "off" position - it should be silver in color and down low on the tank. You may have to push it down to do this
Leave it off for about two minutes or so
Rotate the valve back on to the "Pilot" position
Hold down the pilot gas valve - it should be red and immediately to the left of the main gas valve.

While holding the pilot gas valve down, push the other red button to the right of the main gas valve - this is the piezo ignitor - push it several times in a row quickly

The gas pilot light should light
You can stop pushing the ignitor now, but continue to hold down the gas pilot valve (the one just left of the main gas valve). Let this valve up after about another 20 seconds or so.

Now rotate the main gas valve to the "on" position

Listen carefully - you should hear the main gas light and feel heat at the water heater flue outlet on top - don't touch it with your hand - you could get burned

If no luck, repeat the process until the heater re-lights

TIP - the instructions for re-lighting the pilot should be written on the heater

Just a potential diagnosis - the thunder clap, if as close as you describe, may have been intense enough to create what is known as a "draft inversion" that may have snuffed out your pilot light. The air pulse came down the flue pipe.


Apr 08, 2009 | American Standard Water Heaters

2 Answers

Tankless water heater pilot light will not stay on


It is most likely the water valve stuck open, telling the controls to heat water, that it thinks is flowing through it, but without actual flow it overheats. Depending on how comfortable you are with taking it apart yourself, you could open the valve to see if it just needs cleaned out, or if you have a ruptured diaphragm or some other cause.

Jan 25, 2009 | Bosch AquaStar 125FXKIT-NG 125FX Natural...

1 Answer

Pilot Light will not remain lit


Try this, turn the black gas ****, on top of the control box, to the Pilot position. Set the temp. control on the front of the box to the desired temp. Then, hold a match or whatever else you use to light it, next to the end of the pilot, then, while holding the match to the pilot, press the reset button on top. When the pilot lights, continue to hold the reset button down for about 1 minute. Then, after a minute, turn the gas **** to the ON position, then it should remain on.

Jan 20, 2009 | Water Heaters

2 Answers

Gas hot water tank, pilot light


Look inside the water heater when it is heating the water, the flame should be blue. If it has yellow tips on the flame, it needs an adjustment to the air shutter. There is a nut on this air shutter to make this adjustment. This adjusts the air gas mixture. You may first try just simply tapping on the burner lightly to loosen any debris that may have built up and vacuum it out. MAKE SURE YOU TURN OFF THE GAS BEFORE CLEANING THE BURNER!! Also, check for drafts that may be blowing out your pilot light, Furnace register or outside drafts on windy nights. Some water heaters have an automatic shut down if the water heater gets too hot, so don't have it set to its hottest setting. Most water heaters only last about 10 years or so, It could be you are ready for a new one....Joe

Dec 16, 2008 | Water Heaters

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