I have a noritz america nr71-sv tankless gas water heater no hot water upstairs shower and sink works fine in kitchen and downstairs shower and sink what is the problem?

How long have you had it? How old is the plumbing?
Did you open the second floor hot water faucet only, all the way?
Did you feel the hot water pipe in the cellar as far as you can?
Maybe the minimum flow requirement isn't being met upstairs, due to deposits in the pipes.

I'm not a plumber but thought I'd mention this since all our plumbers seem to be elsewhere today.

Hi Smonson

You say there is no hot water in the upstairs shower, but what about the sink upstairs... do you hot water at the upstairs sink?

Back to you...

Mark