Bosch AquaStar 125FXKIT-NG 125FX Natural Gas Tankless Water Heater w/ Hood - Answered Questions & Fixed issues

I have the same machine first they said replace the micro switch i did then rebuild the water valve did not enough gas presure had it checked revented the unit the unit is brandnew relubed the water valve idk

check the thermostat, you will probly need someone that has worked on the same kind of unit, they are not all the same {tankless} I would suggest a repairman,, there is a little bit much to explain how to do it over the internet,

Ok the way this system works is from a pressure and flow switch. The pressure switch picks up the slight drop in pressure when the hot side water is turned on then turns on the burner. The flow switch measures the amount of water flowing and adjusts the size of the burner to the amount of water flow. So a couple different things might be happening. The flow switch might be sticking causing the water heater to throttle down the burner but not off when the water flow is off. Or the gas control valve might be by-passing gas when off causing the burner to stay on even when the demand is off. Run the hot water on high demand then turn the water off. If the burner is still on try banging on the water heater with your hand and see if the burner shuts off completely. If it does the gas control valve might be by-passing. You would also need to check and see if the pressure/flow switch is giving any voltage signal changes when the water flow is off. These tankless water heaters are more comlplicated to work on than a standard water heater and you may need a professional's help...good luck

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.

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.

The cold main water pressure is too low to maintain a constant flow. Make sure you have no other appliences on, or your not comprimised by the lowering of pressure

The water valve may be dirty and sticking, prohibiting it from fully closing the gas valve. This part needs to be periodically serviced (every 2-5 yrs depending on water quality and use) Purchase a water valve rebuild kit to service water valve.
This answer is off the Bosch site.
If this answer is not satisfactory, do not feel obliged to accept it!
Gary.

i have an aqua star 125 ng an the pilot lights and stays lit but the main burners wont fire up when i turn the water on i have good water pressure so i kind of stumped and i just rebuilt the diaphram in it also

I would first make absolutly sure that I had all air bleed out of gas line to water heater. If you are trying to bleed air out thru pilot assembly this can take a long time because the opening in pilot orifice is very small. To speed up process if you have a union close to unit or flex connector,you can slightly loosen fitting till you can hear a little hiss of escaping air/gas. When you first smell gas immedieatly tighten fitting. Before you do this make sure there are no standing piloy lights or other sources of flame in the vicinity. When you smell gas and tighten fitting back up try to relight pilot to water heater.If you still can't get pilot to light I would remove pilot line from gas valve and burner. Check to see if you can blow thru it, visually inspect pilot orifice, and if blocked I would get a orifice cleaning tool from hardware or welding supply store. Be careful to select the right size for your orifice and clean it. Reinstall and try to light pilot again.Hope this helps. Thanks

sounds like thermalcouple/heat sensor

The pilot light has a thermocouple that allows gas to the pilot light while the flame is burning. If flame goes out it shuts gas off to the pilot jet. It seems that your thermocouple is faulty and needs to be replaced if pilot light keeps going out as the outside air cools.

Probably excessive winds caused a draft to enter your boiler via the flue and blew your pilot out - not unknown - if the unit is now working okay - don't worry. As you seem to be servicing the unit you will probably get a good many years out of it yet.

You can get the instructions here:
http://www.boschhotwater.com/Portals/7/TechManuals/125FX_English_Two_Port_15_yr.pdf

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).

There are several sensors for this. The other possibility is that the solenoids for the gas are failing. Does it go to pilot at all?
Go to boschhotwater.com then find a technician. Look for the nearest Bosch Service Provider. Most will travel.

You have to fix this leak around the water valve before
it causes damage to your equipment.
1. Close the water supply valve going to your tankless
water heater.
2. Unplug or close the natural gas valve.
3. Now, you can work fixing the leak.
4. Once completed, you can proceed to open the
natural gas valve as well as the water supply valve.
5. Then, have it tested to get it done.

5.

proberly your fan is running on easy checked best of luck

overheat stat , fan, best of luck

may be overheat stat or fan check little hose at fan area for pinhole best of luck

Is the thermocouple in the pilot flame? & does it get hot [glow red] If not adjust position until it does & make sure it's clean
If thermocouple gets hot but no relight, replace couple