On Sat, 28 Jan 2012 20:49:27 -0500, Jan Philips
wrote:

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

What do you think it is likely to be?

Is it a temperature controlled anti-scald valve? If so, it has to be
adjusted to allow more hot to flow in the mix. Since it goes cold
again after the hot spot, it may be an adjustment or it could be a
defective cartridge.

First step it to get the model and see if there are instructions on
line. If they does not help, call the customer service of the
manufacturer. They usually offer good service and good warrant
coverage.

I don't see how a low flow head would affect temperature. Get a new
plumber next time.