I have 30 gallon electric water heater in side attic. It feeds a two handle American Standard shower with diverter spout and one two handle sink and one cartridge single level sink. It started putting out mostly cold maybe slight warm water. Went to drain and flush tank but now it gets confusing:

1- Cold water crossing into hot lines: Turned supply valve to tank off and opened pressure relief valve to drain but kept filling. It has a turn off valve on hot supply line as well then turned it off and tank started draining. Opened hot line valve again and tank started filling high pressure not just a trickle. Obvious that cold water is flowing back into tank.

2- Suspect shower connection crossover: Turned both in an out supply valves on tank to closed. Tested and turned on the hot water side on all fixtures and all still flowed full force but obviously cold water. There is cold crossover into entire hot side of line. Turned the individual supply lines off to each sink and cold water still poured out of hot side of shower. Had not used shower in a while and noticed a fair amount of copper corroded particles come out of hot shower side consistent with some obvious breakdown of something.

3- There is no hot side cross over into cold side: With tank valves both open if hot side opened on a fixture the line out of the tank is very hot consistent with flow but only slightly warm at fixtures consistent with dilution cold to hot cross over. When just a cold fixture is on, the line out of tank remains cool indicting no hot to cold cross over.

I have to think some connection in shower is the problem but having trouble figuring it out given the shower is a two handle separate supply. My assumption is with 2 handles the water stays totally separate unless the handles are turned on. However there is still the crossover cold to hot in both sinks even with shower handles closed.

I got a very corroded spout off with a lot debris. I likely will need to start removing shower tile but any suggestions before I start chipping tile appreciated.

TWO HANDLE faucets CANNOT cause the problem. It is either the single handle faucet, or something else.

My thoughts as well. If it was the single lever sink, won't closing the supply lines below that sink eliminate it as an issue if other fixtures still had the crossover with that single lever shut off?

IF it were at that faucet, yes, especially if neither faucet would work, but that would mean it is somewhere else, and that is usually something YOU have done to create the cross connection., like connect two faucets together, then put a valve on the outlet.

No work has been done on the lines for years and until recently it worked fine. We did do a major renovation of that area 15 years ago and the only thing I can think of was that something was done then maybe using valves/plugs to isolate that line and the isolation device has corroded away.

I still don't understand how the crossover is only from the cold to hot lines but none from hot to cold. If there was a direct plumbing crossover I would expect either direction. I read some other posts and it was mentioned that an anti-scald valve connects the hot and cold lines before the shower valves some even having a swing flap to prevent hot water entering the cold lines. This might explain the one way flow. Such valves are supposed to be installed with access for maintenance but likely plumber just covered it behind tile years ago with renovation. Unless I get some other suggestions I likely will have to start removing tile YUK!!

Just because the hot water isn't lasting doesn't indicate crossover. If the heater is in the attic, I am willing to bed that the tank hasn't been drained, and that the bottom element is burned out, or the upper thermostat is bad.

Which way the water crosses over depends on the pipe sizes, the pipe routes as installed, etc. and since the hot usually has more flow restrictions, the cold USUALLY flows into the hot. It can happen the other way, but it is much less common. IF the upper element were burned out, you would not have any hot water.