Water heater heats up fine until it seems the tank reaches a certain temperature. The, regardless of if i am using gas or electric,I get a DSI fault and it won't heat again. If we use some hot water, and then turn either switch off, then back on, it will reheat for a while, again,until it reaches a point where it seems it should shut off,then it faults again. No problems with initial heating or ignition of the heater. Only maintaing temperature or reheating after use without re-setting switches.

Gas or electric?
Electric water heater ... then test for crossover, check for water leak, replace thermostats, test elements
Gas water heater ... test for crossover, check for water leak
http://waterheatertimer.org/Crossover.html

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/How-to-test-water-heater-element.html

water leak: turn off all taps and put ear against pipe and listen for water leak... check if water meter advances when all water taps are off.

Gene

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

If they are on outside of tank , I'm thinking your new tank has motor-aid feature as well. That is used to circulate coolant through from engine of motorhome while driving and it keeps water heated. That feature usually adds to price of tank. Hope you didn't pay for some feature you"ll never use.

I don't know how you can drill into aluminum tank and install element.
That seems a good way to ruin the heater.
Why not just buy small 'under counter' 1500 watt electric water heater at local box store and install in-line before the water reaches atwood gas heater.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Otherwise I do not have a solution.
Don't blow yourself up by messing with gas appliance.

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

Electric water heaters never power BOTH the upper and lower elements at the same time.

If the water in the tank is cold, either the top OR bottom element will be powered through the upper thermostat (t-stat), never both at the same time. The upper t-stat will send power to the upper element until the temperature of the water in the top of the tank reaches the set point of the t-stat say 140 degrees. When the water temperature in the top of the tank reaches 140 degrees, the t-stat is "satisfied" and disconnects the power from the top element and sends it to the lower t-stat.

IF the temperature of the water in the lower half of the tank is lower than the set point of the lower t-stat, the t-stat closes to send power to the lower element. The lower element REMAINS powered by the t-stat until EITHER the water in the lower part of the tank reaches the set point temperature of the lower t-stat again, say 140 degrees; OR the temperature of the water in the upper part of the tank drops, at which point the upper t-stat "calls" and switches the power away from the lower t-stat and sends it to the upper heating element instead. When the top t-stat is satisfied, it switches power back to the lower t-stat. The process will continue back and forth for as long as needed to bring the temperature of the water in both halves of the tank to 140 degrees.

You must wait until the top of the tank is heated - before the lower element will be powered to heat the water.

I hope this helps you understand how the switching happens in these water heaters. Good luck! Please rate my reply. Thank you.

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.

If the water seems hotter than normal, then a faulty thermostat would be a good bet.
If water doesn't seem any hotter than normal, then check incoming water pressure.
usually 2 things cause that, too much pressure coming in, or thermostat faulty and not cutting out when normal temperature reached, thus higher temperature, and pressure. If both them check out, replace valve.

The two switches control the water heater either in the gas or electric mode. The switch with the lights would be for the gas operation, and the switch without lights would be for the electric mode. Prior to operating either switch, make sure the water tank is full of water. Turn your hot water tap on, if water comes out, tank is full. Connect trailer to 110 Volt source, turn circuit breakers on, Place switch without lights to on position. If water does not heat in 45 minutes, Install new heat proble. The gas mode should be repaired by someone that is familiar with that system. Atwood can provide you with a local service center.

it uses resistive heating elements that are encased in an electrical insulation and then covered with metal, like the cooking elements in a stove, or on a cooktop range. when electricity flows through them they heat up and get hot (like a light bulb filament wire glows) When the water reaches a certain temperature set on the thermostat, it opens the electricity,turns it off to the element until the water cools down past whatever temperature it is set at, and then it turns back on. That is why if too many people take a shower you will run out of hot water because cold water is always flowing through the water heater and when all the hot water is used, you have to wait for the heating elements to heat the water back up. To make sure it heats up quickly make sure all the HOT water faucets are off so that water isn't flowing through it and is just sitting there heating up. All work on the same principle.

sounds like the electrical thermostat is defective and is being over ridden by the overheat safety stat.

An on-demand, or tankless, water heater is a system for heating water as it passes through the pipes, usually very near the point of use. Traditional tank water heaters bring the water to approximately 140 degrees Fahrenheit (60 C), in order to ensure the water supply lasts long after the heater turns off. Since this is obviously far too hot to be pumped out of a shower of faucet, cold water is added to the mix in the pipes. Coupled with the loss of thermal energy over time, the average tank heater has an efficiency of approximately 80%. On-demand water heaters have a number of advantages over their tank-using counterparts. While tanks often degrade over time and begin leaking, the apparatus used in an on-demand water heater is much less susceptible to the burdens of age, and will often outlive the house they're installed in. The amount of energy used is a fraction of that used in a tank water heater, due to increased efficiency and a much smaller window during which a heating source is in use. While tank hot water heaters can, and often do, run out of hot water after a number of showers, on-demand heaters provide a never-ending supply of hot water, ensuring that warm water is always available when needed. A tankless water heater can cost between $500 and $1500, and they most commonly use either electric or natural gas energy to heat the water. Most mid-sized households report an energy savings of approximately 25-45% for a gas or propane heater, and up to 50% with an electric heater, over using traditional tank heaters. Water is usually set to a maximum temperature of 120 degrees Fahrenheit (49 C), though many heaters allow for custom setting of the maximum temperature via remote control. Outflow rates range from 8-14 gallons (30-53 litres) per minute. Installation for most tankless water heaters is incredibly easy, involving simple plumbing. In the case of electric heaters, all that is required for a fuel source is to plug the heater into an outlet and begin heating your water. Some modern on-demand water heaters can incorporate a solar preheating system. This system uses a traditional solar heating system and tank to bring the water nearer to the desired temperature, saving substantial energy use in the final on-demand heating. A solar preheating system costs between $500 and $800.