If you haven't recently replaced your shower faucet, go take a look. If you have a single-handle set up, chances are it's a Moentrol valve - the kind where you pull it toward you to activate the water and then twist it left or right for hot or cold.
Or, maybe you have a single-handle where you turn it up from the "off" position and stop it when you get to the temperature you want. In that case, you might have a Moen PosiTemp valve in your shower.
In any event, both valves are marvels of engineering that have stood the test of the time and remain the most popular choice for most homeowners.
First, let's look at the similarities of the two valves. Both offer a pressure-balanced system that maintains water temperature within two degrees. Built-in temperature limit stops which all you to control how far the handle rotates, therefore, controlling the range of water temperature. Available in a variety of connections from common 1/2" CC/Sweat, 1/2" IPS threaded, 1/2 PEX or CPVC. Meets pressure-balancing safety codes.
Now we can look at the differences between the two valves. The Moentrol has Temporature Memory. When shutting-off the shower faucet, the handle stays in the exact temperature position you set during use. The next time you turn on the faucet, within a few seconds, the water temperature reaches your last temperature setting. The PosiTemp doesn't have Temperature Memory. The handle goes from one side to the other always going from cold to hot from the off position. The Moentrol Valve has volume control which controls the water flow rate so you get the amount of water you want at the temperature you desire.
The Moentrol valve operates by pulling the knob out to control flow. Once the knob is pulled out to turn the water on, it is then rotated to select a temperature. Left for hot, right for cold, or a mixture of both in the middle. Once the faucet is done being used, you can turn it off by pushing the knob straight in. The PosiTemp valve does not pull out. To turn on the water flow the handle is rotated counterclockwise. You turn the handle counterclockwise until water dispences. The water starts at the coldest setting. To change to a higher temperature you keep turning the knob counterclockwise. All the way counterclockwise would be the hottest temperature setting. To turn the faucet off turn it all the way back clockwise to the off position.
The Moentrol valve is more expensive then the PosiTemp for the added feature of temperature control where you can set the temperature so it is always where you like it. Check out the Moentrol valve and PosiTemp valve by Moen Faucets.