On Thursday, July 9, 2015 at 12:55:14 PM UTC-4, wrote:
range. Maybe a slight adjustment for seasonal cold water temperature.

Because the safety regulations are the reason that valve even exists. Without the safety regulations, I wouldn't have to adjust or replace anything, I'd just turn the hot valve a little less or more to get the temperature of water I need.

As for the crappy water heater - yeah, maybe it is not the best, but if I had two-handle faucets, it would be totally fine.

Apparently two handle, old style shower valves are still sold:

http://www.homedepot.com/p/KOHLER-Re...A-BN/100094257

So, I'm not sure it's some safety regulation is the reason you
have the more popular single handle type. The single handle ones
are the type most people prefer and are going into most new homes,
etc. But it looks like you can have what you want. How feasible
it is to change it, IDK, that depends, but it's not trivial that's
for sure.

I agree you have a valuable point though, and one that you probably
wouldn't realize until it's too late. Which is if you have a tankless
that can't maintain a constant temperature across the flow rate
when using the shower and/or shower plus other draws at the same time,
then you will have the problem you describe. If that is happening
with just the shower, it definitely sounds like the tankless is under
sized. What make/model is it? Also something that should help
would be to reduce the flow rate at the shower head, assuming it's
not already limited flow.