I have no hot water.

blah blah blah

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Has nothing to do with the expansion tank, the expansion tank whole enough air so that when the heater heats the water it does not build up pressure. You know from school that you can compress ait. you CAN NOT compress water. When you haet water it expands until it turns to steam. So lets rule that out.

First look at all the plumbing in the house where there is a cross conection. This would be like a shower valve that has a personal shower head on it. and you turn of the water using the personal shower handle and not the valve at the wall. This then would let cold water to go into the hot water side piping.

Most people do that with out knowing it at the laundry sink. they put a hose on to was the dog and forget to turn off the faucet and then BANG cold water in the hot side.

Go around the house and turn off all the faucets and valves by using the handles. no add on things.

Now last could be the shower valve itself. . Do this happen in just one shower or all of them???

If its just the one and you spend the day doing the walk around, even check the bidet if you have one

its the valve. you need repair to that.

Best of luck in your hunt Tom

If this is an electric water heater, you should see a little access panel on the side. This contains the thermostat and, most importantly, the circuit breaker. It'll be a little red button you push in. If it pushes in, you've found your problem and you then enjoy a nice, hot shower!

I hope this helps! :)

make sure check both elements with a voltmeter, when one element quits working you don't get enough hot water, check for power to both elements too..

Hot Water Hot to Cold And back. I had that problem for 2 years. I had a very large shower head from the time I installed the tank-less hot water heater and the water temp would change from one extreme to another. Over time my shower head had stopped working as well as it did so I bought a smaller shower head and installed it and amazingly my water temperature quit changing temperature. I now turn my water to what temp I want and it stays there. It's been working correctly for 6 months now ... until yesterday and it quit working completely which is the reason I'm on here. lol

You can eliminate the shower control knob possibility by turning on your hot water anywhere else in the house. If you get hot water in other places, then it could be the knob causing it. A substantial lime buildup in the tank can also cause poor heat transfer from the elements to the water.

I think some one mis informed you, the normal temp on any water heater from the factory is 120, it should not go above that for reasons of sevear burns, as far as it't smart part it's just a name, you will lose a couple of degrees just comming torm the tank to the faucets, the water heater will allways keep the temp in the tank at what ever it is set at, it doesn't have the brains or computer to know what time of day it is but when the cold water comes in to the tank it comes thru a dip tube that goes to aprox 1 inch from the bottom of the tank, if you take a long shower and do not have a energy efficant shower head you can use up to 5 gals, per min of hot water where an energy efficant one only uses 2.5 or less and still has the same pressure at the shower head, the temp in your water heater tries to stay at the setting it is at depending on how much is used at one time meaning washing machine, filling tub or dishwasher at the same time, it lets more cold in as the hot goes out and as you know it can only heat up so much at a time and the cold water is just going across the bottom of the tank, so the name is just that no diffrent then any other water heater unless you went to a tankless water heater.

Do you share your hot water with anyone? Is anyone washing dishes, laundry, etc before you shower? How old is your hot water heater? If it has filled with sediment, your hot water is limited. Try turning up the heat control valve on the hot water heater, only go up one notch today, then if not enough another the next day. Be cautious of scaulding hot water after turning it up.

Something you can do, following your fathers logic,   is to attach a hose to the drain of the tank and run water from the bottom of the tank into a drain untill all the sediment is removed (Water runs clean). This is normally done once a year as preventive maintenance  on all tank type water heaters. The logic sounds good to me.