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We observed a really weird behavior this morning.

While showering, if any sink faucet is turned on anywhere in the house, the shower continues without any problem. However, when that sink is turned off, water flow in the shower drops by over 90% to a trickle. Turning off the shower and back on restores water flow to normal.

We've never had this problem with the shower before. (The shower plumbing was all completely replaced 6 months ago as a part of a remodel)

Is there any explanation for this?

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Is there a pressure- or temperature-balancing valve on the shower? If so, it could be operating incorrectly when the pressure changes by opening/closing the other taps. This could happen from build up of minerals or something clogging the value, or the valve could be broken in some way (eg broken seal or o-ring). If you can provide more info about this we can probably help more. –  gregmac Jul 26 '13 at 17:01

1 Answer 1

up vote 3 down vote accepted

The part of the mixing valve that keeps the temperature/pressure in check has failed or clogged. So when you reduce the water flow to the show by opening a sink tap, the valve reacts to keep the temperature/pressure constant. Then when the tap is closed, the valve again attempts to react, but since the valve is broken, it does not react correctly.

The good news is this is easy to fix. Take the handle and the trim cover off. If your shower valve was installed correctly, you should be able to turn off the water to the valve by turning the two screws to either side of the valve. If not, you'll need to turn off the water to the whole house.

Removing the valve (sometimes called a cartridge) varies. Some have a threaded cap nut on the front that you just twist off. Others have a circular retaining clip around the outside of the valve assembly or inside the front of the valve. Once you've freed the cartridge, take it to a plumbing supply store and they can give you a replacement along with any additional steps you need to follow, such as lubricating or replacing O-rings.

Assemble in reverse, but put a fat line of plumber's putty behind the trim cover along the inside edge (leave a 2 inch gap long the bottom).

Here's a good set of instructions with pictures: http://www.homedepot.com/webapp/catalog/servlet/ContentView?pn=KH_PG_BA_Repairing_Cartridge_Tub_Shower_Faucet&storeId=10051&langId=-1&catalogId=10053

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I agree but you might want to check the filters on the mixing valve first. Newer - past 15 years - mixing valves often have water filters (I don't understand the purpose of this because it causes fails at the valve when I would rather my head fail). You will see a small filter for the hot and cold. You should be able to remove these and clean them well (soak in vinegar). Often - not always - blockage in the filters causes valve malfunctions. –  DMoore Jul 26 '13 at 17:15
    
Also some pressure balancing valves are separate from the main mixing valve. Sometimes there is a second threaded fitting that contains the pressure balancing portion. Though I would recommend replacing, sometimes you can clean the pressure balancing piston to restore normal operation. –  mfarver Jul 27 '13 at 1:15
    
Unfortunately, the Home Depot link appears to be down (tried for three weeks) but I think you've gotten me started in the right direction. –  EricLaw Aug 9 '13 at 19:01
    
Replacing the cartridge did indeed resolve the problem; it shouldn't have failed so quickly but water in Austin is very hard and reviews of the faucet in question suggest that they have poor quality cartridges. Thanks! –  EricLaw Jan 23 at 18:32

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