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.
Unplug microswitch on bottom of water heater. can find in owners manual diagram.
I have the exact same heater and the same problem - a 4 yr old Bosch Aquastar 125HX no hot water. It has been performing fairly well until about 2 months ago. It appears that the heater starts out normally, and after a few seconds the flames go down and only warm water is produced. Calling Bosch tech support, I got a technician who kept only talking about the water valve rebuild. So I ordered the kit, replaced perfectly good parts with the new ones from the kit, and the heater behaves exactly the same as before.
I had same problem. I rebuilt valve had good pressures. Found the problem. It's the flame sensor detector rod . It was not listed in my factory parts list but it's in the diagram. Right next to the piezo electrode igniter. They don't sell it by itself so you need to buy the piezo electrode and flame detector rod for about $28 delivered to your door.
I tried cleaning the flame detector rod got it real shiny but it didn't work well enough. I put a new one in .it will last about 4 years. I just bought 2 pairs today for $46 ... so that should last 10 years I hope
It takes about 15 minutes to install and no water lines are involved
It takes about 15minutes to install and no water lines are involved
Robert Pydynkowski, You have solved our problem of 12 years, and now we can turn off the hot water and turn it back on immediately! We were having to wait 4 or 5 minutes to have the heater re-light, even though the gas was flowing and the spark was sparking. I'm amazed that Bosch doesn't let their customers know in the user's manual that the flame detector needs to be replaced periodically. I had tried replacing all of the expensive diaphrams and valves to no avail and was ready to buy a different unit until I saw your post. Bosch shoots themselves in the foot by not catering to DIY homeowners. I can't even find the service bulletins on the internet anymore
Link to House Needs product: https://www.houseneeds.com/repair-parts/...
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