re: No hot water with a tankless water heater outside.

crack the relief valve - if the unit kicks on then you have a line frozen going FROM the tank to your faucet fixtures. Also can open the cold and hot secondary valves and see if there is water coming out of them. If not, you have frozen line going TO the tank.

I think this is correct. I did both, and when relief valve was opened it kicked on my heater so I knew the heater was working but there was just no flow for it to know to turn itself on.

End of the day it just let me know that the heater was working...still had to wait for lines to thaw out, which they did about 2 hours ago.