Bosch water heater

Don't know how old your hot water heater is. They don't last forever. If the tank starts to leak inside the cone it will cause the burner to do this because the water droplets will boil and splatter. Only other thing to cause this is with a cold start up when the tank is full of cold water during high humidity.

possible faulty pressure switch connected to the water line and that is failing to turn on the main gas tap solenoid
best have it checked by an accredited gas fitter or service center for the make

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

Gas water heater problem based on weather could imply atmospheric interference, which might indicate venting problem. Check vent specs for that model. Make sure roof or side wall vent are installed correctly, and lenghts do not exceed maximum.
Problem might also be associated with elevation setting, since water heaters are sold to match expected conditions at that elevation.
If rain is associated with cooler temperatures, and more gas appliances run at same time, then unit might be starved for gas or home does not have enough ventilation.
Problem could also indicate that tankless unit needs to catch up on yearly maintenance, including clean burner and combustion parts, check loose wiring, test ignition system, clean air intake, bleed gas line, check gas pressure and check for correct vent, etc.
Before calling service technician, open following link for Bosch troubleshooting resources including Bosch contact, typical manuals, etc.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

That is normal. The flame stays the same but will burn longer making the water hotter.

there is a small dia copper pipe which has a thermal coupling on it the end of it sits in the flame of the pilot light,
if it fails which it does after about 25-15 months,it shuts down the gas burner

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

Thermo coupler may be defective. it controls the main gas valve by closing main if pilot is out. make sure thermo coupler bulb is positioned over and near tip of pilot flame, but not in the flame. Thermo couplers are inexpensive and easy to change out.

the diaphram is the pushing force for the gas valve to open, if you have a split or worn diaphram it will ned to be replaced.

ok the way it works man is the box aka power vent the gas valve wont open until the power vent is open u can manualy open the powervent witch will open the gas valve to turn on the piolt light and in about 30 sec the main burner will kick on now if the powervent is opening the the pilot light isnt lighting then u need to replace the thurmole cuple witch sences the pilot light is on if it doesnt see the pilot the it shuts the gas valve off so it doesnt fill ur house with gas