According to Adrian Mariano :
I'm trying to decide between thermostatic and pressure balaced
temperature controls for my shower faucet.

I hunted around through previous postings and got the impression that
thermostatic valves have a slower response than pressure balancing
valves. I was told by one salesperson that the response was "1
second" which seems kind of slow to me (though he said it like it was
fast).

I suspect he just drew the "1 second" out of the air thinking that was
sufficiently "fast" to impress you that it was "fast" ;-)

That being said, thermostatic devices by their very nature will
have a "thermal inertia" lag of some amount. The bits in
a pressure balance valve can usually physically move faster than it
takes for the thermal sensor to even notice the temperature change.

Another question I had was that if the thermostatic valve is
supposed to produce a fixed temperature output, what does it do when
the target temperature is impossible to obtain with the input water
supply? For example, if I demand 110 degree water but my hot water
pipe is full of 65 degree water, what happens? I was told by another
salesperson that the thermostatic valve merely sets a maximum possible
temperature. But if that's true, then when someone turns on the hot
nearby, or if the washing machine gets turned on, then it seems like
I'd get hit by a sudden blast of cold water. Is this true?

If the valve can't make it, it can't make it ;-)

Some of the thermostatic units may have a "shut the water completely
off if I can't make the setpoint" feature. You'll have to read the
instructions carefully or call the manufacturer to find out for sure.
Most major valve manufacturers (eg: Moen) have support lines just for
questions like this.

Pressure-balance valves can't have such a feature because they don't
know what the water temperature is.

I got the impression that pressure balance acts faster than
thermostatic. And as long as the temperatures of hot and cold remain
the same, it should give the same output temperature no matter what
happens around the house. Is this better than the thermostatic? Can
anybody point me to information on how the pressure balance valves
actually work?

Pressure balance valves are also sometimes called "proportioning valves",
which better describes what they're doing. They're trying to maintain
_exactly_ the same ratio of flow volume between the cold and
hot water supplies, no matter what the difference in pressure is.

As long as the hot water temp and cold water temp stays constant, then,
the pressure balance valve delivers a constant temperature.

Which means, amongst other things, if the hot water hasn't gotten to your
shower head yet, it's perfectly happy to give you a cold shower _until_
the hot water catches up.

But once it does, they work surprisingly (to me at least) well at keeping
the temperature constant.

Prior to installing the control valve (we're on a well), just cracking a
valve open a bit would get a shriek from the shower occupant.

After I installed it, the shower temperature stays perfectly constant, even
with several other taps (including an in-ground sprinkler system pulling
8-10GPM) going full blast at the same time, even tho the water pressure
at the shower is greatly reduced...

I'm a believer in pressure-balance.

[Which is sort of unfortunate, because now we have a teenager that's
difficult to get out of the shower ... Instead of cracking open
a nearby hot valve to get him out, I have to go down stairs and shut off
_all_ the water, and he whines about being still covered with soap ;-)]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.