1) Turn on hot valve at the lav in bathroom alone, unit comes on and stays on: hot water as expected
>At the 2 GPM rate determined by the aerator, I assume.
2) Turn on hot water at tub/shower in same bathroom alone (full), unit comes on for 15 to 30 seconds, then shuts down: NO hot water
>If it starts back up after a few seconds it works just like every tankless heater in Germany that I used.
3) Turn on clothes washer with hot or warm setting alone, hot water as expected
>At what GPM (your water meter and a clock will tell you)?
4) Turn on warm setting washer and tub/shower at the same time, unit comes on and stays on providing hot to both until washer stops filling, then shuts down with no hot water to tub.
>It seems to be responding to a decrease in demand. Check your GPM demand [see #3] for each appliance that you have used, but this does seem to contradict #1's response.
1) Plumbing had to be repaired above this bath after old HWH was removed and before new tankless was installed. There is a possibility that hot and cold were crossed at the tub.
>How can you confirm/disprove this?
I wonder if the thing is responding to the rapid change in GPM that you get when the washing machine valve closes.
Also, with a clock and your gas meter you can check that this thing is consuming gas at the design rate.
You could ask Noritz for an e-mail confirming that there is no Noritz Technical Service Bulletin that addresses this problem.
What Noritz says needs to be in writing. If you e-mail them and they respond by calling you, watch out.
There are a lot of analogies between electrical circuits and plumbing 'circuits.' For an electrical problem analogous to your problem I'd use recording voltmeters, recording ammeters and oscilloscopes. I don't know if plumbers have the plumbing equivalent of these instruments.
Ask Noritz what they would use to troubleshoot this problem. As always, the countermove to the "I don't know" answer is, "I respectfully request that you immediately refer this complaint to someone who does know."
A tanked heater dumps a few kw into 50 gals of water and takes a half hour to do it.
These tankless things dump a large amount of power into very few gallons of water and they have to respond instantly. IMO these heaters are treading a thin line between adequate performance, inadequate performance and catastrophe.
Lawsuits make for bad press so the choice is between the first two. Not many people sue over a cold shower or touchy, flaky performance.