I12 eccotemp tankless water heater
A quick web search turns up some information. Obviously water heaters mounted outside require special handling. The Rinnai line has two lines of defense against freezing, including ceramic heaters and a mode that will run the combustion periodically to keep the unit warm. Naturally, the water lines still need to be insulated and heated to ensure against freezing.

These protections of course also work for indoor-mounted units. Other brands don't necessarily include these features though. It doesn't look like Eccotemp does.

I haven't bothered to read through their installation manual. However, guidance found on other sites suggest that as long as the air vent is at least 6' long in insulated space, this should be sufficient to prevent freezing from infiltration of outside air. So your 10' run is almost twice as long as you need to accomplish this.

Other things you can do to minimize the risk are:


Make sure that the vent openings on the exterior are facing away from prevailing winds.
Install a backdraft preventer (flapper) on the exhaust, to further reduce the likelihood of air movements through the vent system while the unit's not operating.


Some useful/informative pages I found include:
https://www.mclayservices.com/blog/cold-outside-will-outdoor-tankless-hot-water-heater-still-work/
https://www.waterheaterleakinginfo.com/cold-weather/
https://propane.com/2018/09/17/eight-cold-climate-tips-for-tankless-water-heaters/
https://www.eccotemp.com/blog/tankless-water-heaters-indoor-vs-outdoor/