Bosch Tankless Hot Water Heater 250SX
I would like to title this as "Left on the side of the road... for dead by Bosch Service"
3 years into a 12 year warranty, with a catastrophic failure of the heat exchanger, I'm standing in a pool of water and unlike a conventional hot water heaters, there's no where to turn except back to Bosch. 
2 hours on "hold", I finally get thru to Bosch service and I use the term service loosely.
From the moment I was connected, and told them I had built my own house and installed the hot water heater myself, the service tech was on an immediate mission to void my warranty... regardless ofthe cause... for which he could have cared less.  He started asking every question he could think of to try to "catch me" on anything/everything.
Do you have a manometer?  No? That'll require a $150 visit from a plumber... (There aren't any trained within 100 miles of my house.) thus delaying any hope of getting hot water within a month.
Do you have a water softener... That's an issue.
How long is your exhause pipe... "About 25 feet"  "Well let's get up in the ceiling and measure it exactly.. with one elbowyour warranty is void."  Wait... wait... exactly it's 21.5 ft one elbow... it's within spec...   I was just giving you a ballpark figure... didn't know you were looking for exact figures... (at this point I got an uneasy feeling in the pit of my stomach...They were not trying to help me...
I think the heat exchanger actually froze. We had a snap of 12 degree weather. I had used the Boschair intake flex hose and vented to the outside of the house... exactly as it was described.I had also used the incredibly expensive stainless steel Z vent pipe... also from Bosch....which added an additional $400 to the cost of the unit.
Since I had water leaking from the heat exchanger, you obviously got moisture in it somehow...also the water had dripped into the gas jet directly below the exchanger... (last I checked... water follows gravity)
It didn't matter... this was sufficient for the service guy to exclaim "Your Warranty is Void!"
The only thing I did wrong was to buy a Bosch.
Anybody want a $1000 boat anchor?

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