First four numerical digits in the Rheem Serial Number

1) Rheem 81V40 electric water heater ... manufactured pre 2009 ... posted serial number does not match manufacturer
http://waterheatertimer.org/pdf/Find-Age-of-water-heater.pdf
http://waterheatertimer.org/pdf/Rheem-Ruud-2009-water-heater-parts-by-model-number.pdf page 5

2) Possible date of manufacture 11-1994

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

the age of all water heaters is listed under the serial #, not model #. you will have to call a plumbing supply house that deals in your brand of waterheater, a.o. smith, ruud, bradfordwhite,etc.

Check that serial number.
Rheem is first two digits= month and next two digits= year, which means 1990.
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

I need some upvotes for the free, hard work being done for folks.

1) Rheem 71-30 is 2-element electric water heater
Reset button is located behind upper panel located on side of tank.

2) Rheem no longer shows that exact model number so your tank is older.
To find age of water heater look at 2nd and 3rd digits of serial number, for example 06 is 2006

3) To download sample manual:
I recommend you read entire manual so it will save you money
http://www.rheem.com/product.aspx?id=91C402E4-D88A-4E32-BB5E-6D1A8D41FC37

4) This water heater is fully repairable by homeowner using generic parts from hardware store
To troubleshoot water heater: Requires $5 multimeter from Home center and about 45 minutes.
Repairs take from 40 minutes to 1 afternoon, depending on skill and problem.
Repairs cost $30-70
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.

The sound is from sediment.
Sound signals need to clean out tank:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Electric water heater can be restored to full efficiency after cleaning:
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html
http://waterheatertimer.org/Water-heater-will-not-drain.html

To find age of Rheem, look at 2nd and 3rd digits on serial number, for example 06 is 2006.
Rheem normally uses 4500Watt bolt-in type element, available at home center or hardware.

You can find age of water heater from serial number located on label on side of tank.
Label also shows tank size, and tank wattage and element wattage

http://waterheatertimer.org/Figure-Volts-Amps-Watts-for-water-heater.html
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

Serial number decoder:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf
For example Rheem serial number 0790...
First 2 digits are month 07 July
Next two digits are year 1990

Rheem site does not show 81v40d
http://www.rheem.com/products/tank_water_heaters/electric_water_heaters/

Serial number should be on label on side of tank.
Labels contain information like model number, tank capacity and BTUs
First 4 digits of serial number are date of manufacture: for example 0604 is June 2004
Take photo of label for easy reading and for your records

http://www.rheem.com/products/tank_water_heaters/gas_water_heaters/

Label for Craftmaster Gas 30 gallon:

You have electric water heater.
Electric water heaters are fully repairable by homeowner, excluding hybrid type.

Rheem electric water heaters:
http://www.rheem.com/products/tank_water_heaters/electric_water_heaters/
Rheem fury series manuals and models
http://www.rheem.com/product.aspx?id=09DF2BDD-5E11-4D32-B574-84ACFB8A4619

Age of Rheem water heater is shown first 4 digits of serial number located on side of tank: for example 0705 means July 2005.
Serial number also shows model number & wattage of elements in case you need to buy new generic 4-bolt type 4500 Watt element from hardware store.

A) If your water heater receives very cold incoming water, it may not heat fast enough to keep tank warm.
Add a tempering tank.
http://waterheatertimer.org/Tempering-tank.html
Add insulation over entire water heater except TP valve. Insulate pipes completely.
Read more information:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

B) 40 or 50 gallon water heater is 240Volts and tank will have 2 access panels on front.
Rheem ships all water heaters with 4500 Watt elements.

Narrow down the suspects:
C) Water heater has some hot water.
This tells us upper element is working.
And that water heater electricity is working.
And that reset button is not tripped.
And that heater is not experiencing run-away overheating event.

That gives us 3 solid suspects.
1) 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:
This will restore water heater to full 99% efficiency.
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
If you want to test thermostats and all parts before replacing anything:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

3) Burned or shorted wire. Turn off power. Open covers on side of tank. Remove insulation. Inspect parts for signs of high heat and burning. Especially 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.

Rheem 4500 watt element

http://waterheating.rheem.com/dotcomWeb/pageflows/warrantyverification/WarrantyVerificationController.jpf
Just click on the above link and enter your serial number. It will give you the information you are looking for.