Jan Philips wrote in
:

At my parent's house, one shower has a handle that goes from hot to
cold. You have to put the handle at one certain position to get any
hot water, and then it isn't very much. If you move the handle just a
little bit in either direction, the water is cold. (The sink in the
same bathroom gets plenty of hot water.)

My mother asked the plumber who installed it and he said that the
reason is that it is a low-flow shower head.

This is a case of a trades person who assumes the customer is clueless on
the subject. In fact, the tradesperson is clueless or, knows they ****ed
something up (or both) and are fixing it with BS.

There is nothing wrong with the customer being clueless on a subject...as
long as they are smart enough to hire someone who is proficient. You mom
may have been clueless, was smart enough to recognize it but unfortunately
not lucky enough to get a competent plumber.

I think that the valve
is either installed incorrectly or messed up.

What do you think it is likely to be?