How do I turn up the water temperature on a Marathon 85 gallon electric water heater? I don't see any visible temperature control settings.

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

You have marathon water heater?
Thermostat is probably bad. Has gone out of calibration.
Replace both upper and lower thermostats for $25 from local hardware store, takes about 20 minutes, turn off power, take photo of wires before starting. These thermostats can be adjusted all the way down to 90 degrees, and then dial up a bit to reach 110.
Copy following link for instructions how to replace thermostat yourself.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

If problem continues, then replace both elements.
Elements are special order made specifically for marathon.
Requires 1-7/8" socket to remove elements.
Upper 3800 watt, order part SP610130
Lower 3800 watt, order part SP210197
http://hometips.me/Mar/Rheem-Marathon-parts-2.pdf
http://waterheatertimer.org/How-to-replace-water-heater-element.html

More information about Marathon:
http://waterheatertimer.org/Review-Rheem-Marathon-water-heater.html

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

Replace heating elements can be found at amazon, click here and take a look: Amazon com rheem heating element . I hope this was helpful... cheers

Turn power off. Open covers and inspect for burned parts.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker

Test elements. Marathon takes specific element, and does not use generic element from hardware store.
http://waterheatertimer.org/How-to-test-water-heater-element.html

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

Firmly push red ECO reset button located on upper thermostat, or check circuit breaker.
http://waterheatertimer.org/images/No-hot-water-1-275.jpg
Copy following link for general troubleshoot information
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/Review-Rheem-Marathon-water-heater.html

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

Question posted under marathon electric water heater.
Marathon is 240Volt dual element non-simultaneous water heater.
Non-simultaneous means that upper part of tank has to reach temperature before power is sent to lower thermostat and element. So if upper element is bad, then water heater has no hot water.
http://waterheatertimer.org/Replace-anode-rod.html#audibleBW

1) You need to by-pass upper element without by-passing upper thermostat.
2) You should never by-pass upper thermostat because it has ECO-reset protection.
http://waterheatertimer.org/How-water-heater-thermostats-work.html

3) Open following link for image of wiring:
http://waterheatertimer.org/images/Marathon-wiring-700.jpg
4) Take photo of your wiring in case color code is different on your specific model

5) Look at upper thermostat.
Remove yellow wire from terminal 2 on upper thermostat, and cap off temporarily.
Remove red wire from terminal 4 on upper thermostat, and move it to terminal 2 on upper thermostat.
6) For more effective operation, look at lower thermostat.
Remove black wire from lower thermostat terminal 1 and move wire to terminal 2 on lower thermostat.

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You checked ohms, did you check for 240V across lower element when lower element is turned on?

Energy smart electric water heater has a control box on top of unit. Water heater does not have typical thermostats that can be tested and replaced. Temperature sensors located above each element send information to control box through small black and red wires. Temperature sensor sells for about $40, but there is no test listed that shows how to determine when sensors fail.

Potential problems:
-Temperature sensor is not reading correct temp
-Control box not processing correct temperature information
-Selector on control box set to 2, 3 or 4
-Thermostat temp on control box set lower than 120 degrees F
-Sediment build-up starting to surround lower element so water doesn't circulate (see if drain valve runs full-open

It could be that the reset button is faulty and will not reset. they are not very robust and can fail when operated repeatedly. The thermostat could also be open circuit especially if the original element had failed. Try replacing both reset and thermostat controls.

check the water control valve to see if it is fully opened

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.