thermostatic vs. pressure balance

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). 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?
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?
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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.

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.

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.
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snipped-for-privacy@nortelnetworks.com (Chris Lewis) writes:

Obviously something has to give if there's no hot water.

I'll give this a shot. The notion of being able to call up and actually talk to someone with a clue seems sort of novel; it didn't occur to me that it was a possibility.

I think I have a decent understanding of what pressure balance valves do, just not how they do it.

Hmmm. Maybe I should install a special shutoff on the hot to prepare for this possibility in the future. :)
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On Sat, 21 Feb 2004 03:10:32 GMT, snipped-for-privacy@cox.net (Adrian Mariano) wrote:

Ah, but that won't work. No hot water=no cold water. I found this out after installing my Delta pressure balanced but before installing the water heater. You get just a dribble then thud, it shuts off the cold. There is some adjustment under the single knob for hot/cold ratio. At least on the Delta.
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