High operating cost. Element has 1/2 inch hard water build-up, i.e. calcium, lime. I change the heating element every 2 months. Does the unit have a thermocoupler? can't find anything in the manual or the website parts list. Could that cause a spike of the electric bill? Thanks, JR

lime is probably clogging the outlet water lines

You can get a universal heating element at your local home improvement store. Take your old one with you. If you have hard water I would get one for hard water. Another thing you need to check when you pull out your bottom element is to look inside and see if you have lime build up. If you do you need to get it out. I used a shop vac and got a peace of pvc tube and heated one end and made a bend on it so you can touch the bottom and suck out the line. On the other end wrap duct take around the tube till it fits snug inside of the shop vac hose. If you don't get the lime out you will burn your new element out. It has happen to me. I hope this helps.

This is the build up of calcium on the heating element that is now breaking off.if you turn off the heater and water you can drain and replace the element which is a small step to cure this .long term you would need to consider new tank or even go to installing soft water unit as the problem shortens the life of all devices like taps shower heads washing machines dishwashers within you system.

Before calling service technician, open following link for Rheem tankless resources including error codes and possible manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Read your manual for troubleshooting, advisories and required maintenance.
Tankless are maintenance heavy.
1L means tankless heat exchanger has lime build-up.
De-lime tankless unit.
U-tube has videos for de-liming tankless, but there is more to consider:

De-liming is yearly maintenance requirement.
Tankless are prone to lime build-up
Regular de-liming will not restore full efficiency to tankless unit.
Tankless cannot be exposed to water with more than 11 grains hardness.
Temperatures over 135 can cause minerals in water to harden on metal surfaces of any water heater.

Also clean Tankless water filter each month, and clean and maintain burner and combustion chamber yearly.
The cost of maintenance and cost of expected repairs and cost of parts plus cost of original installation will never pay back the $1-8 monthly savings on gas bill. All costs are energy.
Make sure there is qualified service technician in area before buying tankless.

Upvote the help.
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.

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.

It may be one of 2 elements not working, a thermostat problem, scale build up in tank, to name a few possibilities. You'll need to shut off power to water heater before doing anything to it, and perform a few tests, and check a few things to determine which one is the cause. After power is off, you remove cover(s) on side of tank and peel back the insulation to get access to element(s). Keep in mind that should you need to change one of the elements, or both (assuming it has 2 otherwise you wouldn't have any hot water at all if only 1 element and not working.) This is where it would be good to have a voltage meter ( or multimeter ) and know how to use it. Is that possible? Before you reply, please shut off power and remove cover(s) to be sure whether single or dual element. If it's single, it may just be thermostat adjustment, or faulty, or scale build up around element. Keep in mind that water to tank will need to be shut off and tank drained down below element before removing/replacing. Get back to me with info on single or dual element. Thanks

http://www.rheem.com/product.aspx?id=09DF2BDD-5E11-4D32-B574-84ACFB8A4619
http://globalimageserver.com/fetchDocument.aspx?id=052c9e32-ce37-48d7-8990-9e430f23fef3

82V80-2 Energy factor is .86 according to water heater industry self-regulated standards.

Rheem pdf says these water heaters are wired for non-simultaneous operation.
Except special order water heaters.
Non-simultaneous operation uses single 240V circuit as described below.
If your water heater is wired for simultaneous operation, and each element is on a separate circuit, that makes your water heater special order. I would guess the reason is for higher first-hour delivery, which implies rapid heating and would likely affect energy factor.
You might want to get serial number off model and call Rheem for specs on that special order.

Your water heater energy factor may be the same because 'energy efficiency is based on the amount of hot water produced per unit of fuel consumed over a typical day.' This means an aircraft carrier can be considered highly efficient despite overall cost. And naturally the guys making the aircraft carrier are also doing the rating.
http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13000

Non simultaneous operation means the upper thermostat controls heating functions.
240V goes to upper thermostat first.
When water inside tank is cold, thermostat reads temperature through tank wall.
Upper thermostat turns on upper element until top 2/3 of tank reaches thermostat set point.
Once set point is reached, upper thermostat turns off upper element and sends power to lower thermostat and lower element.
Lower thermostat reads temperature through tank wall, and turns on lower element until bottom 1/3 of tank reaches thermostat set point.
As water cools inside tank, lower thermostat is first to respond since hot water rises.
Lower thermostat turns on lower element until tank again reaches set point.
When hot water faucet is turned on, hot water exits top of tank.
At same time, cold water enters bottom of tank through the dip tube.
The heating cycle repeats.
At all times, the electricity flows through upper thermostat. And upper thermostat is powered by one 240V circuit.

With simultaneous operation, the upper and lower thermostats work in same manner.
Except thermostats are wired separately.
Lower thermostat does not wait for upper part of tank to be heated first.
Lower thermostat turns on whenever lower part of tank cools.
The advantage is more hot water available rapidly when demand is high. This is called first hour recovery.

One method for reducing electric consumption is to set lower thermostat so it only turns on during certain times of day. For example Whirlpool Energy Smart operates in this manner and shaves a few bucks off the bill each month. This says that simultaneous operation is not an energy saver, unless that circuit controlling lower thermostat is set on a timer.

'Rheem 81VP15S point of use' is a single-element, 15 gallon, 120V electric.
Other 'point-of-use' sizes range from 2-1/2 to 6 gallon to 20 gallon.

Electric water heaters have 2 parts: thermostat and element. One is not functioning correctly. Most likely the element caused by sediment build-up.

How to determine if tank has element or thermostat problem:
Do not turn off water going into tank. Put hose on drain valve. Open drain valve. Does water come out like open spigot? If water does not flow, then heater is likely clogged with sediment.

Solution 1: If water flows out like a spigot, the thermostat is probably bad. Read: How to replace thermostat.

Solution 2: Water does not flow, so element is surrounded by sediment build-up and cannot heat water. You need to clean out tank with shop vac, and put in new element ... plan full day and 3 trips to hardware store. Simply draining tank will not remove sediment. Sediment is chunky and will not come out voluntarily.

Buy element before starting:
-Turn off electricity
-Open cover on side of tank, remove insulation (these have to be put back for proper operation of thermostat)
-The are 2 electrical parts >>the element and the thermostat which is located above element
-Element wattage is printed on element and also shows on label on side of tank
-I think Rheem element is held in place with 4 hex bolts
-Take photo of element to hardware store so you have part on hand before removing old element
-If element is 1-1/2' diameter hex type, you need inexpensive element wrench from hardware store + the element

Drain tank and clean sediment:
-Read: To open drain on water heater so water comes out
-Read: How to clean sediment out of water heater

It is probable a lime build up in the bottom of the water heater tank. You can remove the bottom heating element and clean the lime out, keep in mind this sometimes turns into quite a job though. And you may damage the heating element during removal and have to purchase a new element.

I had a message from the smart energy controller on my Whirlpool Water Heater that my lower element was failing. I turned off the power and hooked a garden hose to the valve on the bottom and opened the valve with a 1/4 inch screw driver.Nothing happened! I thought the valve was plugged with settling hard water particles. The water heater was in a closet and I couldn't just let it run out onto the floor.
I was out in the garage looking around when I spotted the bicycle pump and found a female jet hose end that could work. Using a small piece of plastic tubing to connect the bicycle pump to the hose end with clamps( I had to remove the bicycle pump hose) and the other end of the hose to the water heater about five pumps and I could hear the gurgling of the air inside the water heater. I disconnected the hose end and put the hose outside to drain. Voila!