Is it possible that this is a pressure balancing type of shower valve? Perhaps the pressure balancing feature has gone wonky and is changing the hot/cold ratio while you shower.

When you switch between the tub and the shower, you are also switching between "no restriction" to the water entering the tub... to a certain amount of restriction caused by the shower head. Therefore, there is a difference in pressure between the two and the PB valve may be reacting to that in an improper manner.

Just a guess but it's not out of the realm of possibility.

As for the long delay in getting hot water, your concrete slab is a huge mass that will influence the temp of the water passing through the pipes that are in contact with it. The colder it gets outside, the colder the ground and the slab is. The hot water line is essentially passing through a refrigerator in order to reach your shower. When you don't draw any hot water, the water in the pipe cools to the same temp as the ground.

Stored hot water has to push all that cold water out of the line first and even then, the ground will still influence the temp of the water reaching your showerhead.

not sure if there is a pressure balancing feature. this is the original setup from when the house was built in '78-'79. the cold valve is OFF. running pure hot water.

i understand the slab and ground cooling it. that is one of the main reasons the heater is set to 150, so we at least get some hot to the back. with it at 120, the water never even gets lukewarm. i am debating on an autocirc to solve the delay.

i guess worst case would be to put a second heater in the master jsut to help out and act as a booster. it would only have to generate a 20-30 degree rise until the main heater runs empty, so a small one may be my only option.

you may have a leak from the cold water pipes into the hot water line. This can happen thru the valve in your other shower, particularly if it has a separate volume control and temp control. thru a tempering connection to one of your toilets. or even thru some single handle sink faucets.
check by shutting off one of the supply lines to each single handle sink faucet. on your other shower faucet turn the temp handle all the way to cold. for tempering connections to the toilets, if it has a shut off, close it. if not disconnect the line and cap it.

The most likely suspects are the shower faucet and tempering connections, Sink faucet cross leaks are generaly small and don't usualy cause problems.

To fix these problems once you identify the source, add a check valve to the hot water line at the offending fixture.

there are no single handle faucets of any kind in the house. every faucet, whether tub or sink, has seperate valves for hot and cold. that is what is making this strange to me. no way for the water to mix, if the cold supply to that fixture is off! plus this is the only place in the house it happens.

no tempering valves/connections anywhere either.

Strange symptoms, to be sure. I would check all the faucet washers to see if there is a loose one.

Here is my guess -- and it is just a guess:

I believe that the heat from the water is being lost to the surrounding slab and air. When you are using the shower, the water flow is low enough that much of the heat is lost before it comes out. When you use the tub, the water flows faster and it doesn't have as long to cool off. I am guessing that the master bath is far from the water heater.

The solution would be to insulate the hot water line wherever possible. Another option would be to install a small water heater near the master bath.

If you want to be scientific, use a thermometer to check the temp of the pipe at the water heater outlet and at the hot water supply to the shower valve when the shower is on. The difference will be your temperature drop.

that is what i am beginning to think as well. can't insulte the pipes, since they are under the slab. i am going to try insulating them where they come out of the slab.

i also am going to replace the shower head to see how that goes.

i don't think it is a leaking washer, since the water shuts off completely when you turn it off.

If it is just temperature drop, a higher flow shower head could help with the symptoms. Of course I hate to tell you to fix your problem that way -- it wastes water and energy.

jake, you hit the nail on the head. the pipes are colling off under the slab, the water flow thru the shower head is not enough to overcome this. due to the layout of the house, the pipes run a minimum of 120 ft under the concrete, and that is if they take the most direct route from heater to tub. if they go to the other bath first, and i know they do since it has almost instant hot water, it is probably closer to 150 ft of travel in 3/4" copper lines. i know the lines are not insulated under the slab, that is not required here even though should be. but then again with no building codes, nothing is required!

i did not get around to replacing the shower head this weekend, but i did run an experiment. i turned on the shower as usual, and sure enough after 2 minutes of being hot, the water went luke warm at best. i then turned the diverter slightly so that maybe a half gallon a minute flow was going into the tub. within second the water was HOT! my wife said that was the first shower this winter that she got to shave her legs in hot water.

time for a new shower head, hopefully one that will provide enough flow to stop teh water cooling in the pipes.