This is a common problem with Moen shower valves and fairly simple to fix. The pressure balancing cartridge is sticking inside. You need to remove the cartridge and clean it with white vinegar. The problem appears in valves that are not used very often - like in a guest room shower. The impurities in the water leave a varnish-like residue that causes the element to lock up and restrict hot water flow. If you are the original owner Moen will send you a free replacement cartridge but cleaning the one you have will do the trick. Once it's working be sure to run the hot water at least once a week.
Here's how to fix it:
1. Turn off water supply to the house or at the PEX panel if you have one.
2. Remove handle by loosening the set screw at the bottom of the handle and pulling off.
3. Remove escutcheon plate by removing 2 screws on the face.
4. Remove escutcheon tube surrounding the valve by pulling with a needle nose pliers.
5. Using an awl or small screwdriver pry the retaining clip up and remove - DON'T DROP IT behind the tile!!
6. Using a pliers, pull out the cartridge. If it won't budge you can try some WD-40 or PB Blaster and if that doesn't work you'll need to buy a "Moen Cartridge Puller" (about $13 @ Home Depot) and it will come right out.
7. Shake the cartridge. If it does NOT rattle you've found the problem. Soak it in white vinegar for 15 minutes and try shaking it again and repeat till you hear the pressure balance element move back and forth. A shot of PB Blaster also helps.
8. Once the element is moving freely you can reassemble everything by working in reverse - don't forget to reinstall the retaining clip or the cartridge will become a missile! TIP: To prevent dropping the clip tie a string through the hole in the top and tape the other end to the wall. If it slips you can just pull it up. Also, spray a little PB Blaster in the valve body before inserting the cartridge to make it easier to seat it.
Hope this helps!
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