Shower - valve intermittently not mixing cold/hot

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J Wa

New Member
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Los Angeles, CA
Hello, First time homeowner here in need of some help. Our home was a down to the studs renovation two years ago so everything is new. The master shower has a pressure balance Phlyrich valve that's recently begun to only deliver hot or cold water. This happens intermittently and not every time, but it is starting to be more frequent. If i turn the knob counter-clockwise to start the water, then it just stays cold. If the knob is all the way then it will change to hot. If i turn it back slowly, there will be like a click sound and it will turn cold again -- no mixing.

The problem began after we had a water softener installed last week (a recommended vendor from this forum - thanks for that. they've been great!), but I suspect that's coincidence and not the cause.

Looking back, I feel like the knob was difficult to turn when it was first installed but then got easier over time. That's probably unrelated too, but I'm not a plumber.

Can anyone help me out? Should i take off the knob and see if it just needs a cleaning or something?
Thanks in advance for any advice or assistance!


Retired Defense Industry Engineer xxx
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New England
Today's shower valves must have anti-scald technology in them. The most common type uses what's called a pressure balance valve. This is a sliding spool valve that when the pressure on both sides is equal, lets the valve control the hot-cold balance you set. If the pressure drops on one side, that forces the spool valve to one side, that in theory at least, tries to keep the hot-cold balance the same, but the volume may decrease so you don't get scalded when say someone flushes a toilet.

It sounds like the 'click' you hear may be that spool valve moving.

Does the volume change when this happens?

Is the water softener plumbed to only go to the water heater, or does it feed both hot and cold (the more normal method)? If it only feeds one, the restriction may affect the working pressure, and mess with the pressure-balance spool valve.

Another thing that could be causing this is some debris in the supply line that occasionally, flows in and stops the flow on one side. The shape and size of that debris may cause it to work properly some times, and block the flow other times. That would also cause a pressure change, that would trigger the spool valve to function because of the restriction lowering the pressure.
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