I have a PowerStar model #44-557701 tankless hot water heater. When you turn on the hot water it starts to heat and then it just shuts off. My dad thinks it might have something to do with the flow sensor. Could this be the source of my problem? It is a year old now.

When deliming (decalcifying) you should do it more than once a year. It depends on the calcium content of your water source if you can stretch it to a year.

Mine is an AE125 electricalIy powered one that I do once every 3-4 months, it's a bit of a pain in the backside but if you don't & you have high calcium content in your water supply, the heater coil will become encased in calcium causing the coil to become overheated due to electrical demand overload, this overheating will cause a ground short from the wire inside the copper coil to the copper coil itself inside the canister & blow a hole in the coil thus causing the electrical conducting wire to come into contact with the water inside the canister (ground short).

You know the heater coil has failed when the circuit breaker on the top of the canister keeps tripping every few minutes after resetting. If yours is the dual canister type heater, the problem will only arise with the leftside canister, the rightside canister never gets hot enough to heat water above the boiling point to create calcium deposition on the coil.

I saw a schematic for pumping a deliming solution into the canisters, don't do this, you have no way of knowing if the calcium has been completely dissolved from the coil. This must be visually inspected because decalcifying a badly encrusted coil can take a full day's worth of time & you need to be able to visualize the process. You must remove the coil from the canister, doing this is another story all in itself, I've done it many times.

Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual.

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