We have a rheem electric hot water system which is suddenly not producing hot water. We have checked the electricity supply, the valve and both these were OK. We are at a loss to explain why this is happening

check your water pressure and then make sure it is not to high or all the problems you have had were not the answer. it is a pressure /temperature valve not just . temp.

Add an extrol expansion tank to the piping. You can get one for about thirty dollars from the hardware store.

What type heater:

1) Electric: press reset button located under front cover, check circuit breaker:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

2) Gas: clean air intake, call for service:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

3) Tankless. Throw it away and install tank-type.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html

4) Add a comment with more information about your heater.

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

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

Rheem 50 gallon Pro52-2 has EF .95, which makes it ordinary 240Volt 4500 watt non-simultaneous electric water heater that produces about 21 gallons hot water per hour, with 67 gallon first hour delivery.
Recommend 80 gallon per hour first hour for oversized bathtub.
http://bostonheatingsupply.com/Rheem/Rheem%20Residential%20Electric/Professional/Rheem_ResidentialElectric_Professional%20Spec%20Sheet.pdf

Maintain electric water heater and it will last indefinitely.
http://waterheatertimer.org/How-to-maintain-water-heater.html
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

Anode rod is located on top in separate port.
Anode keeps tank from rusting.

Year of Rheem water heater manufacture shows 3rd and 4th digit of serial number.

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

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

A) Rheem electric water heater website:
http://www.rheem.com/products/tank_water_heaters/electric_water_heaters/
Find age of Rheem: look 3rd and 4th 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.

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.

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.

Gas unit? if so check pilot --will go from there-if electric check breaker in electric panel

If you are reffering to the drain valve at the bottom of the hot water tank. It may be leaking due to the O ring being cracked or worn out. You can either drain the tank and replace the valve or you can buy a hosebib brass cap and thread it on the existing valve. There is no cause for alarm when doing this as this valve is just to drain the tank when replacing your hot water heater

you have a couple things that may cause this. you may have a bad element a bad thermastat a tripped eco (emergency cut off ) or problem with electric suppling heater.if you have no electric backround or training you should call a pro