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Marathon water heater elements/
Elements are weakest point of this water heater
Marathon has exact same energy factor as regular steel metal tank.... there is not a greater insulation valve with Marathon:
Read review Marathon water heater
Remove sediment from marathon

Larger 1-7/8" element gives larger opening for cleaning out sediment.
Use element wrench SP610070, or buy 1-7/8" socket at local auto parts.

The SP610070 is ordinary cheapo element wrench, while auto parts socket will grab elements better

Buy:
1-7/8" socket
Elements burn out frequently?
Use lower wattage ... 3800 volt.
Use low density (foldback) elements.
Typical shower bath is 104F.  Turn down thermostats a bit... say 118F. Higher thermostat percolates more chemicals out of water, and the sediments affect water heater element.
Lower thermostat will reduce first hour deliver, but tank can be rewired for simultaneous to increase delivery: Calculate first hour   How to rewire for simultaneous operation

Hard water, acidic water, and other water conditions contribute to the issue of burn-out.
Turn up the water softener, have water tested, install good filter on incoming water.

Element advisory: Marathon elements burn out more frequently in some cases. Homes with hard water should install 3800 watt element to reduce sediment, and to reduce problem with elements burning out frequently.  Rheem Marathon parts list

Removal of sediment is easier with Marathon water heater, because element opening is larger.
Full efficiency is restored to all tank-type electric water heaters when sediment removed. Unlike gas water heater,  remaining sediment will not affect efficiency on electric since the element sits several inches above tank bottom.
Remove Marathon elements using 1-7/8" socket

All gas and electric water heaters that are exposed to hard water will form sediment deposits. Calcium-based sediments are not harmful, but must be removed periodically. All water heaters should be flushed every 6 months to prevent problems.  Read about sediment and flushing
Rheem marathon element wrenchRheem marathon element wrench
Buy:
1-7/8" water heater element wrench/ Use screwdriver for handle
Professional 1-7/8" sockets

Buy socket:
1-7/8" socket with 3/4" drive
1-7/8 sockets at Amazon
24" breaker bar with 3/4" drive
Rheem marathon element


Upper element has a fuse to help prevent dry fire, but Marathon elements are interchangeable as long as length of element does not conflict with tank diameter. 
Use 1-7/8 element wrench
1.5" UNF straight fine thread not 1" NPSM straight// 12-14 threads per inch vrs 11.5
Read parts .pdf before buying
Buy:
SP610160/ 4500 watt copper/ Best value
SP213670/ 4500 watt

SP210197/ 3800 watt
SP210201/ 3000 watt
SP210198/ 3800 watt
SP213480/ 3000 watt
SP213540/ 2000 watt
SP610130/ 3800 watt
SP213820/ 3800 watt
SP610140/ 5500 watt
SP610150/ 3000 watt

Resources
Marathon parts sheet/ pdf
Regular water heater elements/ identify and replace
Why does marathon water heater have fused upper elements? 
Question: Do I really need fused upper element if I am smart enough to never dry fire the upper element?  
Seems to me its just another part to fail.
Dry fire
Answer 1:
Fused upper element is NOT required for tank to operate normally.
Fuse is not ordinary fuse that you can reset... it is a resistor that is added inside head of element.
Upper element dry fires.. but lower element rarely does.

What is dry fire?
Element must be covered with water when power is applied, or upper element will burn out.
Why only the upper element dry fires? Because 30 amp residential water heaters are non-simultaneous, which means upper element receives power first... so it is first to burn out. With non-simultaneous water heater, the lower part of tank does not receive power until upper element heats first... but if upper element dry fired, the top of tank never gets heated, so lower element never receives power... and tank is cold.
Resource:
How electric water heaters work

Same answer different words:
Marathon upper elements are fused to help prevent dry fire. Not 100% prevention... just resistance to dry fire.
Elements will instantly burn out if they are not fully immersed in water.
Upper element will dry fire, but usually not the lower element.
Reason: Marathon is ordinary non simultaneous water heater: Upper element is ON, lower element is ON, or both elements are OFF
When tank is empty, the upper thermostat is cold.
Thermostats are mechanical bi-metal switches. [They require no electricity. They work with any AC voltage.]
When upper thermostat is cold, the bi-metal switch is positioned to send power to upper element.
When tank is cold, no power will flow to lower part of tank, until upper thermostat reaches set point.
When power is applied to cold tank, power flows to upper element. If tank is not full of water, then upper element dry fires. Lower element is unaffected.
When upper element is not working, then tank will not heat any water because lower element will never receives power unless upper part of tank is heated to set point.
If upper element is burned out with dry fire, the water heater heats no water.
Marathon elements are expensive by comparison to ordinary elements.
If you dry fire ordinary element, the replacement cost is $10-20 from local hardware store.
Marathon elements are not available at local hardware.
Resources:
How to wire thermostats
How thermostats work
Customer says:
I think I will get the marathon titanium element for the upper because of the way I use my water heater as a solar energy battery,  the upper does come on a lot.  Much more so than in a standard instantaneous grid install.
Better value?
SP610160/ 4500 watt copper/ non-fused/ Best value
All brands of thermostats are interchangeable

$48
for 2 low density 240 Volt 4500 watt elements/  + Upper and lower thermostat


Buy:LOW density kit:
Apcom plumbers pack Amazon
110-160
Resource
How to replace elements
How to replace thermostats

$35 for 2 high density 240 volt 4500 watt elements + Upper and lower thermostat

Apcom thermostats
Buy:HIGH density kit:
Apcom plumbers pack Amazon

Resource
How to replace elements
How to replace thermostats
110-160
New water heaters have high density elements. Low density elements might last longer
$45 for 2 low density 240 Volt 4500 watt elements/+ Upper and lower thermostat

Therm-o-disc thermostats
Buy:LOW density kit:
Therm-O-disc low density plumbers pack
90-150
Resource
How to replace elements
How to replace thermostats
$45 for 2 high density 240 Volt 4500 watt elements/ + Upper and lower thermostat

Therm-o-disc thermostats
Buy:HIGH density kit:
Therm-O-disc high density plumbers pack
Therm-O-disc high density
Therm-o-disc high density / copper element
90-150
Resource
How to replace elements
How to replace thermostats
Marathon Upper thermostat
Upper thermostat
Larger image
Marathon Lower thermostat
Lower thermostat
Larger image
Marathon is ordinary non-simultaneous residential water heater... except with limited lifetime tank warranty for standard duty/ and 10 year warranty for heavy-duty
Review Marathon water heater
Marathon Upper thermostat
Rheem SP310230
Rheem SP210245 30 gallon and larger

Marathon Lower thermostat
Rheem SP310020
Rheem SP210239 30 gallon and larger

Resource
Marathon parts sheet/ pdf
Marathon thermostats High temperature thermostats: Rheem heavy-duty marathon is rated for 170 ℉/   10 year tank warranty
These are generic commercial grade thermostats
Buy: Rheem Upper thermostat:
Rheem SP210246 High temperature

Buy: Rheem lower thermostat
High temperature lower thermostat

Marathon heavy-duty comes with 10 year tank and 5 year parts warranty
Upper and lower thermostats
thermostat temperature range
Buy residential upper and lower thermostats/ rated 150 ℉/ fits any Marathon 120-240 Volt water heater
Thermostats are interchangeable/
Each works with Marathon water heater
Buy: 90-150 degree F for residential tank:
Water heater thermostats at Amazon
Camco thermostats at Amazon
More thermostats
89T short/ dual-element thermostat at Amazon
Example Camco dual-element thermostat at Amazon
Therm-o-disc thermostat protectors
Thermostat protectors fit any therm-o-disc thermostat
Protectors cover the electric terminals.
When you adjust temperature, or push reset button, covers with protect you from electric shock

Buy:
SP310040 upper protector
SP310010 lower protector
Support the economy:
Buy water heaters from my associate links:

50 gallon Gas water heaters at Amazon
40 gallon gas water heaters at Amazon
50 gallon Electric water heaters at Amazon
40 gallon electric water heaters at Amazon
Hybrid / heat pump/ water heaters at Amazon

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