I have a 2nd home in West Virginia mountains used on weekends. It gets quite cold there and when we leave, I turn off the water and open the faucets (hot and cold) in the basement to drain the water lines in case power goes out so the pipes don't burst. I leave the heat set at 50 degrees. I have a Rinnai RL75i tankless water heater. Should I do anything to the water heater if I'm draining the pipes? So far I've just left it on and done nothing to it, but it just dawned on me that this may not be a good idea.
With no water in it there is no flow to trigger an ignition cycle, but it does chew up some standby power to no good end just idling there. Killing the power to it doesn't hurt, and precludes any bizarre condition that would make it dry-fire (like a lighting-hit scrambling the control logic in an optimally bad way, which is by definition rarer than being hit by lightning. :-) )
FWIW: Whenever I leave my place for more than a few days I kill the power to the tankless unless there sub-20F temperatures forecast (since my tankless is also my space-heating boiler.)
If you have drain valves at the bottom of the tankless water heater, opening them up to make sure there is no water in the unit would be a good idea. From my experience a frozen tankless is no fun, (and expensive)
I know the post is a little old but I have the same unit installed in my house.
You should unplug the unit, shut off the gas and turn off the water supply line. There should be hot and cold drain valves installed, this is used for the yearly maintenance. Open both the hot and cold drain valves. Rinnai also recommends removing the filter assembly on the cold water in. They also recommend covering the vent termination to prevent animals and bugs from getting in. This one I personally question.