I just had a Noritz EZ TR 40 installed in the last month, and liking it so far. I think I originally had the opposite problem that you have, in case it helps somebody else looking on. In the shower, the hot water would only trigger if hot was on full, then it would get scalding hot. Back it off even a hair or two, and within about 2 minutes - all hotness gone.
I remembered seeing when the guy was installing it that the number on the front panel said 135. And I had read 120 is the default.
I also just recently had a new shower valve installed, and he replaced the shower head with a nice low-flow version.
The two combined was the problem - not enough flow on the hot side to trigger it.
I lowered the tankless temp to 120, and then all was good.
I do also wish I got the Noritz EZ TR50, and that was the original plan. But the guy helping me piece together things didn't think it was needed for such a small house with one person and a pooch. So I went with the 40. But the EZ TR50, I believe is actually the second generation version and has a 25 year warranty on the heat exchanger. The EZ TR40, I believe is the gen 1 of the series still, and doesn't have that same 25 year warranty. The tradeoff probably being that the 40 has been in use for longer and is tried and tested for longer I guess. And these things are supposed to last 20-25 years if maintained well. I'm 55. So the math tells me I may not be around to see it's replacement - so I'm happy with what I've got now.
