What's the Coldest temperature your tankless water heater has had to run in?
The coldest temperature my unit has run in thus far is effectively -26C. I say effectively because even though it has been as low as -32C, the dip to that temperature only lasted a few hours through the night. We have had a 2 day stint where the thermometer didn't move much up from -26C.
Clarifications: My unit is installed indoors (basement). However, because these units are directly vented to the outside, some folks are concerned about the coldness of the intake air and how it might affect the performance of the unit (e.g. condensation, freezing of some components, etc.).
as at December 2012.