Rheem 81V-400 water heater/VERY SLOW HOT WATER !

1) I bet the water heater has hot water, but hot does not arrive at tap because of bad faucet cartridge or bad recirculation system check valve
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#crossover

2) When cold water arrives at tap, go check if water heater has hot water. Then do the crossover test. Then add comment and say what you discover

3) Electric water heaters can be repaired by homeowner using generic off-the-shelf parts from local hardware store.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If the drip is coming out of drain valve: Put a hose cap over end of drain valve.
If water is seeping out of cracked drain vale, then replace it.
If the leak is seeping around previously sealed threads, then tank may be rusted beyond repair.
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html
http://waterheatertimer.org/Leaking-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Does same thing happen at all faucets?
Are you getting error code?
You might have crossover problem with defective faucet cartridge.
Copy following links for troubleshoot resources:
http://waterheatertimer.org/pdf/No-Error-Code.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Press the reset button located on upper thermostat
Following link has troubleshoot for electric water heater
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I can offer resources:
1) Open following links for possible help.
http://waterheatertimer.org/pdf/No-Error-Code.pdf
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

2) As a note: When the motherboard on a computer or phone needs replacement, most people figure the device is about used up. The same is true for tankless.
Despite promotions claiming longer life, tankless warranty is not longer than warranty for typical tank-type water heater.

3) Fix most tankless problems yourself, same day, for $300-400
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Take a moment and rate answer for accuracy:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Ordinary tank-type electric water heaters are fully repairable by homeowner.

A) Basic info on Rheem
Rheem electric water heater website:
http://www.rheem.com/products/tank_water_heaters/electric_water_heaters/
Find age of Rheem: look 2nd and 3rd digit of serial number, for example 06 is 2006

B) Water heater has some hot water.
That says certain things: upper element is working, water heater electricity is working, reset button is not tripped, and heater is not experiencing run-away overheating event.
http://waterheatertimer.org/How-it-works.html

C) That leaves 4 suspects that cause lower amount of hot water.

1) TP valve is running hot water out of tank -or- leaking faucet is running hot water out of tank.
Check for leaks
http://waterheatertimer.org/How-to-replace-TP-valve.html

2) Lower element is burned out.
Test lower element using inexpensive multi-meter from Home Depot.
http://waterheatertimer.org/How-to-test-water-heater-element.html
Replace element with same wattage shown on label, and clean out sediment:
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

2) Upper or lower thermostat is bad.
Replace both thermostats for about $25.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

3) Burned or shorted wire located between upper and lower sections of tank.
Turn off power. Open covers on side of tank. Remove insulation. Inspect parts for signs of high heat and burning. Especially check lower element. Tighten any loose wires. Replace part that shows signs of burning.
Put insulation and cover back over thermostats so cooler outside temperature doesn't cause thermostat to misread tank temperature.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Is it gas or electric. If electic you need to replace the cal rods -there are pobably 2-but this is for someone who knows how to work with 220v.If gas or electric draining thetank may help if the sediment has never been removed- The temp send=sor for each may also need to be adjusted or replaced

Not uncommon for calcium etc to build up in water heater, from water pipes, and water itself. (hard water) Simply removing aerator and screen etc and cleaning it will usually take care of it, but if too dirty, soaking in C.L.R. will help, or for a few bucks just replace it from hardware store. Just make sure you take old one with you to match it up, as there are many that look the same but aren't. You may also help the situation by shutting off power to water heater, allow it to cool, and drain it out, which should remove some of the calcium laying in bottom of water heater. I've had some with such a build up over the years, that I removed the element to look inside and it was right up to the element. Used a shop vac, with piece of 1/2" plastic line taped to it and sucked out as much as I could. Took about 1/2 hr, but got most of it out of there.

my 50 gal heater puts out only 20 gal of hot water

Tankless Water heater tempature is regulated by the rate(amount) of water flowing through the device. To get warmer water for the hot side, slow down the elbo stop leading into the device. Since it is a shower,and you did not say one handle or two.You will have to slow the rate for the cold as well , if one, unless it is an ajustable valve. The drawbacks to the tankless heaters.