tankless in freezing climate

[FrankInWI]

Well, we're finally talking about a toilet and shower, so we need hot water.  I don't want to run a vent pipe up two stories and have it go through my roof (and worry about leaks). I want a direct vent water heater.  They cost so much I might just want to invest in tankless, most already including direct vent.  There will be weeks at a time where the place is un-occupied, and un-heated.  Is there a way to drain these units of water so that it doesn't freeze up while we are away? 

[ScottA]

Blow the pipes and the W/H out with compressed air.

[MountainDon]

Some of the Rinnai exterior tankless heaters have a drain down system built in (or optional?) that drains the water automatically when either gas supply or electrical power is interrupted.

I've heard, but can not confirm, that some brands are more difficult to blow dry than others. I'd check with the manufacturer to see what they have to say about their product in freezing weather. I know that exterior tankless heaters usually have small electric heat devices to protect the units in freezing weather. Only good if you have power tho.