We hav a Siebel Eltron Tempra 29 B tankless water heater. There is power from the breaker box but no power to the third circuit in the water heater. Suggestions?

SOURCE: reset button for tankless water heater

you need to unplug the unit, wait for 30 seconds reset the temp to the lowest temp, then try it, if this does not work it might have done something to the circuit board, are you on a gfi out let it needs to be there is water and ele, and should be on the gfi with a reset on it,,

SOURCE: Where do I find the reset button on the Stiebel

On the Tempra 29 there is a "safety thermal cut-out" on the top of each of the heating elements within the box. I have had to open it up before to push the breaker. They are a small red plastic push button on the top. Carefully press the braker button without touching the electrical wiring inside. I hope that works for you.

SOURCE: Specifications sheet for Stiebel Eltron Tempra 24

hope this is what you need.
http://www.stiebel-eltron-usa.com/techdata_tempranew.html

http://www.stiebel-eltron-usa.com/pdf/install_tempra_tempraplus.pdf

SOURCE: I have a Rheem RTG-74

Does it have a battery in it? check that

SOURCE: The tankless water heater was working this

If the circuit breaker will not reset it could be a few different things. First, circuit breakers do wear out. The bi-metal strip that makes the breaker function does break down metallurgy wise. A new one would have to be installed to fix this problem. The other possibility would be that a wire inside the water heater has grounded out thus not allowing the breaker to reset. A wire might have gotten to hot and the insulation around it might have melted or became brittle and fell off allowing the wire to touch bare metal inside the water heater.