Unit. Blows 3amps. Fus on start up This happeedcafter power outage

need a meter to test circuit for damage

Depending on the installation location, there may be a fused disconnect near the unit. Check for voltage there. If you have power at the disconnect, open the cover on the unit and check for power at the connection point.

reset circuit breaker fully off the on
power outage can call surge
check fuse on circuit board

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If unit has electronic control valve, then simple power outage surge can but out electronics.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#blower

Gene
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replace it with the same thing but newer

Water inside tank-type water heater can remain hot for some time.
That's one advantage of well-insulated tank-type water heaters.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

Power outage can be related to failure of gas control.
Unplug stuff when power goes out. And then plug back in after power is restored.
Water heaters with electronics are not protected from surge.
Electronics can blow out during and after power outage. For example nearby lightning strike will cause current to run along household wires. With ordinary power outage, once power is restored, the surge can affect all kinds of electronic circuitry such as timers, water heaters, water softener, treadmill etc.

Sounds like you have intellivent gas control valve:
Open following link to identify gas control, and download typical manual, and download intellivent service manual with troubleshooting, error codes, etc.
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html#intelli

Take a moment and vote on answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Power outage will reset Rinnai unit to that temperature. Read manual for more info.

How old is your unit...if less than 5 years old tell Polam/Rheem customer support you are recieving a code 76 error - this is a loss of communication between the remote and the burner control assembly. Part will be covered under MF defect warranty. However, the part is covered....the labor to install is not.
If you are slightly technical you can install the part yourself...customer support will walk you through the process and they give you a step-by-step procedure to follow.

What type of heater is this? I need to know the model name and make and then we can go threw the process of how to do it. Most gas heaters do not have power to them and I only know of a few tankless heaters that do.