Water will not turn off

You can remove the center (Diverter valve) without turning water off, Will need deep well socket. Check the o rings and or washers and replace or take valve to Home depot to match with new. If you can't find one there go to local plumbing supply or ace. I turn hot then cold on and off real Quick while the valve is out just to clear anything that might be in lines. stand to the side or you will get wet! You might want to get new hot and cold valves while your at it just make sure to turn off water at meter before you remove them.

You need to replace the mixing valve. It's not terribly hard to do, if you have some tools and, if the valve is not stuck in the fixture. Here's a video on how to replace it. Take note of how the tabs on the valve are positioned so you can put it in the same way it came out. If cold and hot water are reversed, you need to take valve back out and rotate 180 deg. and put back in:
http://www.youtube.com/watch?v=J-SNBNijfQc
Dave
This is assuming you have hot water elsewhere in the house and not ot the shower. Check to make sure you have hot water at other faucets.

Make sure the hot water is turned back on. Pressure balancing valves won't work if both cold and hot water is not on. Pull shower head off and test to see how it flows out of the shower arm. If good, clear out shower head. Whenever water is turned off for a time, and then turned back on, restricting shower heads and aerators usually get clogged. In the future, always open up a tub spout before turning water back on. This releases the air and other debris most effectively before testing other fixtures in the house.

To talk about your problem forces me to assume a few things.....If you allow the water to flow without cartridges and handles, the water must be flowing straight through the cartridge channels and out into the tub. It is not flowing through your tub spout or shower head when you test the water this way. If I'm correct, then the problem is isolated to the water mix operation. That means coming out of your spout or shower head. So now I have to ask a question. When you operate as normal (cartridges and handles installed) does your problem show up only on your shower head or does it also show up when operating out of the tub spout? If the problem is isolated to the shower head, try a head replacement. It may be partially plugged. If it's head or spout there may be something in the mixing pipe that only shifts around as pressure increases. I hope that is not your problem. It would require pulling those pipes out for inspection and most likely replacement. Nimsgame

Hot on left.
Cold on right.
Some single handle faucets have bleed-over where both hot and cold enter mixing valve each time handle is turned on, even when faucet calls for just cold or just hot.
Otherwise maybe a do-it-yourself plumber got pipes backwards... or maybe the tub used to be on other side of wall, and tub was moved during remodeling but pipes were not changed.

To fix problem:
First of all, turn off shower faucet. Turn on Hot at sink and put ear against shower faucet listening for sound of water flowing through the mixing valve.
If mixing valve is working fine, you need to open wall and investigate more.
For example if you have access door, reach in and feel which pipe is hot.
Pipes might need to be reversed if problem comes up in home inspection, otherwise the problem will not harm anything.

If I understand correctly, you have single handled shower faucet.
Normally there is a wait for hot water to arrive at faucet.
But now hot water is already at shower faucet.

Single handled faucets have a mixing area inside valve where hot and cold come together before going into sink.
This mixing valve can have crossover where the hot and cold can pass through to each other even when faucet handle is off. When single handle faucets get old, this can happen.
How it happens:
For example another faucet is turned on inside house, and pressure differential between hot and cold causes hot water at shower handle to pass through mixing area and into cold water line.
So if another faucet has been run, then that lets hot flow to faulty single handled shower faucet.
To confirm this is the problem: turn on other faucet and put ear against shower faucet to listen for moving water.
This condition may also signal there is a water leak.
The problem is not harmful, but will draw cold water into water heater each time, and is energy wasteful.
Replace faucet or have plumber repair problem.

As you look at your valve on the right you will see a large screw fitting.inside this fitting is a temp/ pressure balance spool. it has failed for some reason and needs to be replaced. On both sides of the valve are 2 smaller screws. these are the water shut off valves. turn off the water and remove the larger screw fitting. remove spool and replace. Hope this helps.

these valves can be adjusted by removing the handle and adjusting the white gear that regulates the hot and cold that come out, try going more to one way if it gets colder then go the other way.

Here is a quick check to see if the cartridge may be at fault allowing the cold and hot to mix. Turn off the hot water supply, with hot water supply off turn faucet on. Turn to the hot and cold sides, If you have only cold water then your faucet cartridge is at fault or the cartridge housing is worn. Shut all supply valves and remove cartridge and inspect housing for signs of wear. If wear is visible a new shower valve is needes. If none visible replace cartridge. I have included a link of a PDF file an exploded parts list. I hope this helps in solving your problem. Please rate if the answer helps in any way. Good Luck and have a great week. Jeff

Yes shut of main water, then remove handle, then take off bonnet, that holds stem in valve. You will have to take stem with you to hardware store to match up. Put it back together in reverse, turn main water back on. Now you tell everyone your a plumber and get much respect.