Our guest Moen tub/shower is not used very often. (The house is 3 years old.) We were cleaning the tub and when we turned on the water we are only getting a dribble for hot water. When we switch it over to cold it comes on full force. There are no issues with hot water elsewhere in the house. This happens both when the plunger is up for the shower and down for the tub.

Hopefully someone has a quick fix as we have 6 houseguests coming tomorrow!

It sounds like you have problem with your cartridge, shut the water to the house, remove the handle and the cartridge, with the cartridge removed ask some to turn on the water for a few seconds to flush out any debris out from the faucet, now check the cartridge for O rings on cartridge for any damage if it is damaged you will need to replace the cartridge, if no damage clean, lubricate and install, check if it works.

If it would not work you will need a cartridge, and you can get one from Moen for free no charge to you call Moen support at 800-289-6636, good luck.

John