Bosch 125BNG burners not firing

Tankless?... call service technician

Tank? clean arrestor screen, replace gas valve thermostat
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Bosch 1600H NG??? I suppose. If so you need a digital manometer.
Take a static (off) reading then leave it hooked up and start flow. Watch pressure draw down and you should see the heater draw Water Column down below 5" as the pocket of NG in the pipe just before the unit draws down to minimal to keep heater lite. In that case problem in gas line size/gas line run length, Meter pressure.
If you have and maintain in operation 5" WC then is probably the water valve needs rebuilt along with a new diaphgram and also the water filter should be replaced too. This occurs esp after 2-3 year as Bosch specs rebuild at 3 yrs on the Maint Schedule. Also clean all the faucet aerators for good flow above 1.25 GPM Min at all faucets.
Under a single faucet flow (laundry sink) of 2.25 GPM the flame bed of the Unit should be 1"+ high and uniformly Blue.
I assume at this time you either have a Maint problem or you could be overflowing the heater's Winter capacity. This unit @ 70F rise only makes about 2.2 GPM. It's strictly a single use faucet heater at 117,000 BTU.
When you do the Maint look to see if there is corrosion between the GV pushrod and the water valve pushrod. Notice there is a huge mis- match in the 2 rods sizes.These units occasionally (more the 125B) corrode there and the GV will seize on and boil the heater dry and open a seam so you really want to maintain it to Bosch Specs and Schedule.
Note: This is a gas appliance & requires skill and knowledge to service. While Bosch may say you can DIY, I'd readily advise a Bosch approved Service Tech or Plumber do any rebuild. The Digital Manometer alone is a $100-150.00 item and it's the ONLY way to get accurate gas pressure readings.

Copy following link for Bosch troubleshooting resources.
Bosch posts service bulletins.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Install tank-type gas water heater for total $350 and save the whole expensive mess of tankless.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

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.

Hello Larry,

Unfortunately nothing can be done about the minimum flow for activation.

However, your heater should have a flow control dial that you can turn all the way clockwise which will reduce your activation rate to about 0.5 gal. but that's the absolute lowest.

If the reason is to conserve water than that's about all you can do.

However, if its because the water is too hot and your mixing in cold which is increasing the volume of water you are using, then you should have a burner or flame control, if its a dial turn clockwise for less flame on the burner or the heater may have a slide control which sliding more to the left will lower the burner flame.

Don't forget water valve maintenance in your manual, follow instructions for a water valve rebuild to restore performance to the heater.

That's about it.

Turn of the hot water valve and turn on a hot water tap in the house. If You have water flow, then you have a problem with a faucet (usually a shower valve). Go to the shower valve and see if you hear water running. If so that is likely the problem. Otherwise there is not enough gas to fire the unit that far and it will fail out. Check the manual most people install these units wrong. If you install it to code then you did it right. If you install it to the manufacturer's specs, you installed it to code and it is right. Then the only adjustment is with a Calibrated CO2 analyzer (instructions in the manual).

Often, this is the result of a restriction of exhaust gases. This type of hot water heater must be adequately vented, (per installation specs). I found this out after taking a number of cold showers. :^( Two years later, and still counting, no more problems. :^)

I'd initially check for a cross connection (cold water leaking into hot water system). This can be done by shutting off the cold water feed to the tankless unit, then opening up hot water fixtures around the home. You should see the water come to a complete stop; if you see a steady stream of water, then you have a cross connection. The most common place for a cross connection is at single handled showers. The valve cartridge starts to fail over time.

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.

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.