I have read most of the posts and responses for this similar problem and have completed all the recommendations below, but still only have luke warm water in shower only.
Problem description:
Over the past few months the 2nd floor shower has been getting progressively less warm. Now that cold weather is here the luke warm water is no longer something I can ignore. The sink only a few feet away has great hot water as does every other faucet in the house.
Attempted fixes thus far:
1)Flushed (not drain) hot water tank, saw no discoloration or debris. (fyi, tank was replaced 2 years ago)
2)Removed and cleaned aerator at all faucets. They had little to no debris.
3)Removed shower cartridge and turned on main water to house allowing both hot and cold water to 'blast' out directly from pipe. It is a single handle simmons temptrol style so both hot and cold water come out same pipe. Noticed that cold blasted out, but hot was maybe only 20% of pressure that cold was. It was however very hot.
4)Removed shower head and allowed water to blast out of pipe for several minutes.
5)Installed new cartridge in shower handle.
None of this has provided any improvement at all.

I was wondering if there is a vinegar mix / liquid plumber / CLR or other solution I can pour down the hot water supply line at the shower which might loosen/dissolve any debris inside the pipe.
Any ideas would be much appreciated as my wife can no longer tolerate the shower temp.

Thanks, Tom

We do not have Simmons faucets in our area but most of the faucets work on the same principle, there is an adjustment that can be made on the faucet somewhere. On some models it is just under the handle and you have to remove the handle to see it. After you have the handle off you should see a partthat might slip off the stem, then you are able to rotate this part a few degrees and replace it on the stem. Doing this lets your handle move further therefore leting more hot water come out of the faucet.

If you search for the Simmons faucet information you might be able to find your faucet and it should have instructions on how to adjust the hot water.

Good luck

Thanks for the idea. There is a type of limiter screw on the temptrol which I have set for full hot water. I don't think it has anything to do with the faucet since the pressure/volume of hot water coming out of the pipe when the entire handle assembly is removed is very low compared to the cold. I'm no plumber, but it seems to me that the hot and cold pressure should be about the same coming out of their pipes.