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Review Rheem Marathon Water Heater/
Non-metallic residential water heater


Marathon has 5 choices:

1) Ordinary 208-240V residential heater/ no warranty for commercial installation
30-40-50 gallon heaters for residential
85-100 gallon demand response heaters
2020 Marathon-specification-sheet.pdf
2020-Marathon-residential water heater manual.pdf
Marathon-parts-sheet-2011.pdf

2) 120V point of use heater
15-20 gallon heaters for residential
Marathon-point-of-use-spec-sheet.pdf
Marathon-parts-sheet-2011.pdf

3) Light duty commercial 'non-balanced' 3-phase heater/
75-85-105 gallon 3-phase / for fairly high demand at higher efficiency
or can be rewired for single-phase
Rheem-light-duty-3-phase-Marathon-spec-sheet.pdf
Rheem-light-duty-3-phase-Marathon-manual with wiring options.pdf
Rheem-light-duty-3-phase-Marathon-parts-list.pdf
Non-balanced means that 3 hot conductors from 3-phase are used to power 2 elements.
Balanced 3-phase heaters are for higher-demand and have more elements, for example 3, 6, 9, 12 etc elements inside tank, so each conductor is delivering equal or balanced power. Not available in Marathon series.

4) Heavy duty 240V commercial heater
75-85-105 gallon single phase
2020-marathon-heavy-duty-commercial-specification-sheet.pdf
Marathon-light-duty-commercial-water-heater manual and warranty.pdf
Marathon-heavy-duty-commercial-water-heater-parts.pdf

5) Thermal Storage tank for solar and geothermal
50-85-100 gallon storage tank
2020 Marathon-storage-specification-sheet.pdf
2020-Marathon-thermal-Storage-tank-manual with warranty.pdf
Commercial parts-sheet-marathon-storage.pdf

marathon water heater1) Ordinary 208-240 Volt residential water heater
 The 30-40-50 gallon Marathon heater is ordinary two-element / residential/ 240 volt electric / non-simultaneous water heater, except with internal polybutene tank instead of steel tank. Requires 30 amp breaker and 10 gauge wire.
Just as a steel tank, Marathon can be installed anywhere inside home as long as temperatures are above freezing, needs no ventilation or air supply, does not need to be spaced away from wall or other belongings, can be exposed to dust and dirt, and trace airborne chemicals.
Unlike steel tank, Marathon can be exposed to dampness and moisture, and chemicals that would corrode steel tanks. Marathon has non-generic elements, and other parts are proprietary to Marathon. Thermostats are ordinary generic 90-150° thermostats, same as steel tank.
The larger size 85-100 gallon residential Marathons must be enrolled in a local utility demand response program where utility controls lower element. Note that all 80 gallon residential electric water heaters must be enrolled in demand response except the 80 gallon heat pump that requires 10x7 room, cannot be exposed to moisture, dirt, or corrosive chemicals, and requires ample air supply. Best hybrid heat pump

Sidestep the demand response requirement for residential sizes over 50 gallon by buying the 75 85 105 gallon commercial grade Marathon 240 volt heavy duty heater. Commercial-specification-sheet
Or install two 40 gallon or two 50 gallon heaters for higher first hour delivery, faster recovery, and more control (turn off 1 heater when not needed).
Buy Marathon tank
Marathon tanks
50 gallon
30 gallon
Resources:
2020-marathon-water-heater-manual.pdf
2020 Marathon-specification-sheet.pdf
Marathon-parts-sheet-2011.pdf

Marathon under-counter water heaterLarger image
2) 120Volt Residential point of use water heater
The 15-20 gallon tank is same as larger Marathon, except with 1 non-generic element and 1 generic 90-150° thermostat.
Available in 120-240 volt.

Buy element
20 gallon Marathon tank
15 gallon

Parts
2000 watt Element
Vacuum relief valve

Buy thermostat for 120 volt 1-element:
120-208-240V-277-480 volt/ 90-150°/ ECO 170°
Rheem UV11700
Rheem SP11700
Therm-o-disc 08013
Thermostat for 240 volt 1-element:
120-208-240V-277-480 volt/ 90-150°/ ECO 170°
Rheem UV11698
Rheem SP11699
Camco 8143

Resources:
Marathon-point-of-use-spec-sheet.pdf
Marathon-point-of-use-manual.pdf
Marathon Point-of-use sell sheet with specs
How to install under-counter water heater
 
3) Marathon Eclipse Light duty commercial 'non-balanced' 3-phase heater
208-240-277-415-480 Volt.
75-86-105 gallon.  Default 3-phase. Can be re-wired for single-phase.
Default simultaneous wiring. Can be re-wired for non-simultaneous.

The 75 85 and 105 gallon dual-element Marathon light duty commercial 3-phase water heater with temperature setting up to 170° (vs 150° for residential models) can be wired for single phase or used as 'non-balanced' 3-phase for 240, 208, 277, 415, or 480 volt.

Customer orders heater to specification so that Voltage of elements matches voltage supply.
Higher voltages deliver more heating power in less time and at less cost, but voltages higher than 240v are not DIY and should only be installed and maintained by experienced electrician.
Overall, higher voltages, simultaneous wiring, large tank, and commercial 170°F thermostat combine to give large amount of hot water, and halve the time it takes to fully re-heat tank.

Default wiring is simultaneous to maximize amount of hot water. Wiring can be specified when ordering or changed by customer to non-simultaneous (same as residential models).
Simultaneous
means both elements work independently and can both be turned ON at same time.
Non-simultaneous
is standard for residential heaters and means upper element is on, lower element is on, or both are off, but never are both elements on at same time.
10 year warranty.
Buy high temperature upper thermostat:
Minimum temperature 110° Maximum 170° High Limit trips at 190°/ 208-240-277-480 Volt
Thermostat differential 17-27°F
Marathon water heater thermostatRheem SP8293 / 170°
Camco 08304 / 180°
Buy lower:
Rheem SP8295 / 170°
Camco 08314 / 180°

Resources:
Rheem-light-duty-3-phase-Marathon-spec-sheet.pdf
How to install meter on electric water heater
Change water temperature using timer
Control water temperature using Z-wave
Rewire non-balanced 3-phase heater to single phase

4) Heavy duty 240V commercial heater
Need more hot water?
The 75 85 105 gallon 240 volt dual-element Marathon heavy duty single-phase commercial water heater can be installed DIY in home or business. Requires 30 amp breaker and 10 gauge wire.
Specify model number when searching: MHD75245, MHD85245, MHD105245
-80 gallon hybrid heat pump costs less to operate but will never be as durable and effort-free as Marathon. Best hybrid heat pump

Two 50 gallon residential heaters of any type will deliver 114 gallons first hour and reheat at 21 gallon per hour, while the 105 gallon commercial Marathon will deliver 107 gallon and reheat at slower 18 gallons per hour.  Two water heaters


First hour delivery is amount of usable hot available from one continuous draw. Actual amount is typically lower than shows in spec sheet.
Install AO Smith 55 gallon with 5500 watt elements ... or 80 gal Hybrid ... or Marathon lifetime limited warranty 75 85 105 gallon 240 volt dual-element heavy duty commercial grade heater.
Resources:
Marathon commercial-specification-sheet.pdf
Energy Smart spec sheet
AO Smith-heat-pump-spec sheet for 10 year

5) Storage tank
The 50, 85 and 105 gallon Marathon thermal storage tanks have no elements, or have 1 upper element for back-up, depending on model.
These tanks are made to store hot water from solar, geothermal, indirect (boiler or condensing heater etc), or from other sources. Maximum operating temperature of water in tank = 180° F.
Lifetime limited warranty on tank and 6-years on parts for residential installation.

Warranty void or diminished if water temperature exceeds 180°F
TP valve will release hot water at 210°.

Lifetime limited warranty on tank and 6-years on parts for residential. Commercial warranty 5 year tank 1 year parts.
Note information in specification sheet:
Resources:
2020 Marathon-storage-specification-sheet.pdf
2020-Marathon-thermal-Storage-tank-manual with warranty.pdf
Commercial parts-sheet-marathon-storage.pdf

Review Marathon 208-240 volt Residential heater
Marathon wiringLarger image
Marathon
208-240v residential heater can replace most ordinary residential electric heaters

Typical, or standard, residential water heaters are 208-240 Volt non-simultaneous, with 4500 watt upper and lower elements, and wired using 30 amp breaker and 10 ga wire.

The only difference is that Marathon residential heaters typically have 4500 watt upper element and 3800 watt lower element but still require 10 gauge wire and 30 amp breaker.
IF desired, Marathon 5500 watt elements can be installed for faster recovery and slightly higher first hour delivery. Parts sheet


Mobile home water heaters are typically 3800 watt and often use 12 gauge wire and 20 amp breaker. Marathon can be ordered as specified with 3000-3800 watt elements to work with 20 amp breaker.

Non-simultaneous operation means both elements are never ON at same time.
Upper element is ON, or lower element is ON, or both elements are OFF.
Check label on side of tank to verify voltage, wattage and non-simultaneous operation.
Resources
How electric water heaters work
How to troubleshoot electric water heaters

Non-simultaneous water heaters, including Marathon, can be rewired for simultaneous
.... but requires rewiring tank with two 30 amp breakers and two 10 gauge wires, each circuit supplying power to either the upper or lower part of tank. Rewiring tank will void product warranty and only give slightly faster re-heating after tank is depleted of hot.
Resource
How to wire thermostats for simultaneous

Resources
For wiring, read page 10-11 manual
See wire sizes
See basic water heater circuit
Buy Marathon elements
Titanium low-density Elements
SP610160/ 4500 watt copper/ Best value
SP213670/ 4500 watt
SP210197/ 3800 watt
SP210201/ 3000 watt
It is NOT necessary to buy the exact replica model number/ Select correct wattage and length
Vacuum relief valve
Resources:
Marathon-parts-list.pdf
Marathon elements

Buy generic upper thermostat for 240 volt 2-element:
120-208-240V-277-480 volt/ 90-150°/ ECO 170°
Thermostat differential 17-27°F
Rheem UV11698 90-150°
Rheem SP11698 90-150°
Aprom / 110-160°

Commercial thermostats ECO 190°
Rheem SP8293 / 170°
Camco 08304 / 180°
Buy Lower thermostat
Rheem UV11695 90-150°
Rheem SP11695
Therm-o-disc 08123 or Camco 8123
Aprom / 110-160°


Commercial thermostats
Rheem SP8295 / 170°
Camco 08314 / 180°
Stick menAdvantages Marathon:
Lightweight by comparison: 30 gallon size weighs 70 lbs. 40 gallon 81 lbs, 50 gallon 91 lbs. Shipping weight, add 5-9 lbs.  Specification-sheet.pdf
1) Tank will not rust 2) Limited lifetime warranty on tank for single-family residential, non-commercial installation, 3) No anode rod means tank life not diminished by water softener. No anode eliminates need for anode inspection, and eliminates odors caused by harmless bacteria reacting with anode rod, 4) Internal polybutene construction means tank will not rust out and cause leak. 6) Larger element openings make sediment removal easier, 7) Can be installed in moist environments such as boat or agricultural installation, with diminished warranty. 8) Marathon wiring is enclosed inside conduit means easy rewire for off-peak operation or to meet other objectives. 9) Brass drain valve/ not plastic. 10) Large 85-100 gallon tanks are available by buying commercial model.
Rheem Marathon
Disadvantages Marathon:
1) Marathon costs more. 2) Marathon elements and vacuum valve are non-generic but can be ordered in advance to avoid repair delays 3) Marathon elements burn out more frequently and are susceptible to hard water. Use softener.
4) If home has hard water, Marathon elements can be changed to 3800 watts for longer life of element, but smaller wattage slows heating and reduces recovery slightly 5) Maximum 6-year parts warranty, which is better than some heaters, worse than others (Rheem hybrid heat pump has 10 year parts warranty).

Similarities to other tank-type electric water heaters
1) Same efficiency as regular tank-type electric water heater... .90-.92. Can be maintained to preserve full efficiency by removing sediment and cleaning or replace elements that have build-up of scale, 2) Can be repaired DIY by homeowner. Repair and installation is generally easy. Replacement parts available. Can be maintained to last forever. 3) Maintenance schedule is same as typical electric water heater (flush each 6 months, observe any leakage and repair immediately), except no anode inspection needed. 4) Electric water heaters are simple to install. Marathon installation is same as typical electric water heater, except Marathon is easier to lift. 4) Requires 30 amp breaker and 10 gauge wire. 5) All electric water heaters can be re-wired to meet off-peak objectives. Rewiring is easy with Marathon, but rewiring will void warranty. Change wiring back before making warranty claim. 6) Adding timer to any electric water heater will not void product warranty. Adding water heater blanket will not void warranty. 7) Marathon tank warranty is reduced if water heater is installed anywhere except residential home, but this is true for all brands residential water heaters gas and electric. If you relocate, and take Marathon with you, then warranty is voided. Leaving unit in old home, the new homeowner receives remainder of 15 years warranty based on date of manufacture showing on label on side of tank ..... and, in this case, the tank is not covered by lifetime warranty / requires purchase receipt.
Buy:
Marathon water heaters
Resources:
How to troubleshoot and fix water heaters
Marathon water heater elements
Marathon thermostats
Off-peak wiring
Read about anodes
Water heater element

Marathon upper element has internal fuse' (a resistor) that might prevent dry fire ... dry fire is caused when tank is powered up before the tank is full of water.
In real world, if you fill Marathon completely with water before applying power, then it will not dry fire ... and in that case, all Marathon elements are interchangeable as long as length of element does not conflict with tank diameter. 
Marathon elements are not generic, they're expensive, and frequently burn out

Install 3800 watt element to prevent early failure of elements.
Use water softener when hard water is present. Turn tank temperature down. Flush and clean water heater burned out elementeach 6 months.
It is NOT necessary to buy the exact replica model number/ Select correct wattage and length. Read parts .pdf before buying.
Buy:
Buy Rheem marathon elements from my affiliate links
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:
How to replace elements
Parts sheet
Rheem marathon element wrench
Rheem marathon element wrenchBuy:
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

Resources:
Marathon-parts-list.pdf
Marathon elements
Marathon water heater installation
Larger image
Use flexible copper to connect Marathon to water supply
Make plumbing attachment using 3/4" female nipples. Brass nipples recommended, not galvanized. Flexible copper connections are reliable. Use new flex lines, old flex lines can leak.
Braided steel lines have rubber hose inside that can shed black particles over time. Stainless flex lines can rust in softened water. Insulate pipes completely when finished.
Resource
Marathon installation starts page 4

Do not make solder joints at water heater, or within 12" of water heater. 
Direct heat from torch can damage Marathon tank. Can damage any heater or appliance.
Pre-assemble copper pipe and then thread pre-assembled copper pipe onto tank.
When using soldered joints instead of flexible connections, then plan for at least 12" space above heater.
Buy:
Flexible water heater connections
3/4" brass nipples
Resource
How to solder pipe
Toxicity?
Chemicals
polybutene 
According to mfg, there are no toxic chemicals from using Marathon water heaters, which is business-speak since all chemicals are toxic at some level. Page 1 of Marathon use and care manual has following warning: 'Calif Proposition 65: This product contains chemicals known to State of California to cause cancer, birth defects.'
Manufacturer's stated purpose for disclaimer is avoidance of lawsuit, which means you-the-consumer cannot claim harm since you were warned. On other hand, much information on this page is also from product manual, so Rheem is offering a level of transparency (by force of the people).

Is Marathon polybutene tank cause for alarm? My opinion:  Probably no more cause for alarm than everyday life.
Other parts of heater could be made of Polybutylene, which is widely used for household water pipe and other products, for years. All household products pose risk, including bug-spray to pills to gasoline. The modern world is covered in chemical waste, air pollutants CO2, NO, CO, O3, CHO, CH4, smoke and particulates.
My friend who owns a Marathon heater remains full of strong-headed opinions, same as before he owned product. No change observed.
Keep in mind following things
1) You do not drink large amounts of hot water from the water heater. Turning on cold water at a faucet does not draw hot water from tank. See image
2) Marathon does not have anode rod. All steel tanks have anode rod. There are health concerns over aluminum anode rods. Read
Marathon energy cost
Rheem makes several grades of  Marathon
*Cost numbers include 30-40-50 gallon residential lifetime tank rated max 150°F.

Estimated costs for other grades of Marathon are not included:
Heavy duty commercial 75 85 and 105 gallon/ rated max 180°F.
75 85 and 105 gallon light duty commercial 3-phase/ rated max 170°F.
50-85-105 gallon storage tank for solar etc/ rated max 180°F.
Cost benefit estimate

$400-500 per year to operate any residential electric water heater

Same for Marathon or ordinary steel-tank electric water heater.
Numbers shown on chart are similar to all tank electric water heaters.*

No energy payback from Marathon
Marathon does not save money vs ordinary box-store electric water heater
Figures are for comparison only, and assume average household of 4.
Lower your cost by reducing consumption, and/or re-wiring water heater for off-peak. Same applies for Marathon or any electric water heater.
Resources:
9 ways to save with water heater
Off-peak options
Cost chart for all types water heater
Ways to increase hot water
Water heater size chart Choose correct size water heater
First hour delivery chart generally applies to all water electric heaters.
First hour rating can be increased by raising thermostat temperature, installing low flow shower head, rewiring for simultaneous operation etc.

Homes with oversized bath or whirlpool bath: Install minimum 80 gallon electric, or 50 gallon gas
Home with 5-7 or more people/ install 2 water heaters, or buy the large commercial heater, or 80 gallon hybrid heater.

Resources:
Two water heaters
How to wire for simultaneous operation
Ways to increase hot water
Ways to save with water heater
First hour delivery is amount of usable hot water from a single continuous draw out of fully heated water heater.
It does not mean the hot water will last for one full hour. First hour shows on yellow energy label, but is approximation. Actual first hour depends on incoming water temperature, thermostat setting, standby loss, pipe length, and other variables.
Recovery Capacity
... is the maximum amount of water that can be re-heated by a water heater with a given temperature rise in one hour.
Recovery is typically measured in gallons per hour (GPH).
Generally, higher wattage elements mean faster recovery rate. So Marathon elements can be changed from 4500 to 5500 watt, or water heater can be wired for simultaneous operation. Recovery rates are also a component of First Hour Rating which is basis for water heater sizing. The issue of failed elements with Marathon means higher wattage is probably not best alternative.
Resources:
Calculate Recovery
How to calculate first hour
How to wire for simultaneous operation
Limited Lifetime Warranty
Tank: Lifetime
Parts: 6 year
Order extra elements.

Marathon Warranty/ Certificate of warranty comes with each product
Marathon warranty, and claim procedure
Rheem offers extended warranty for Marathon and for ordinary steel tanks
Read warranty exclusions page 22
RESIDENTIAL USE: Limited LIFETIME on tank and 6 YEARS parts
Privately owned, single family dwellings – the tank is warranted not to leak “for as long as you own your home”. If the home is sold, the warranty reverts to fifteen years from the date of the installation and the second owner receives coverage for the time remaining. Parts are warranted for 6 years from the date of manufacture or 5 years from the effective date, whichever is greater.
Tank must be maintained as outlined in product manual
Warranty void or diminished if water temperature exceeds 180°F
 Read hot water scald advisory in manual page 15
When temperature is set higher, the first hour delivery increases, but EF factor decreases and tank generally costs more to operate.
Temperatures above 140° will distill more chemicals out of water. Temperature for killing bacteria is 140° Read advisories and cautions for hot water
Any temperature above 120°F can scald quickly and is generally dangerous for household. Typical bath, shower, hot tub etc are 104°F

LIGHT DUTY COMMERCIAL USE/ agricultural
10 YEARS tank /1 YEAR parts
Apartments, condominiums, restrooms in office and commercial buildings, multifamily housing or any light duty commercial or industrial use – the tank is warranted for 10 years from the date of installation. Parts are warranted for 1 year from the date of manufacture.

INDUSTRIAL USE
Any application where water is used as process
5 YEARS tank/ 1 YEAR parts
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 cheap element wrench, while auto parts socket will grab elements better
Marathon SP610070 element socket / Amazon
Marathon dip tube reduces sediment/ does not prevent sediment
Marathon dip tube can help stir sediments off bottom of tank, but this does not prevent sediment from forming.
Most heaters offer same dip tube feature. Read about dip tubes

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 slightly larger.
Full efficiency is restored to all tank-type electric water heaters when sediment removed. Unlike gas water heater, any remaining sediment inside electric heater will not affect efficiency 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. Flush tank every 6 months to help prevent problems.  Read about sediment and flushing
Marathon water heater Marathon tank has 2-1/2" foam insulation
By contrast, most electric water heaters manufactured after April 2015 have thicker insulation to meet the DOE (department of energy standard).
Both Marathon and ordinary steel tank electric water heaters have same EF energy factor and same expected annual cost of operation.

Water heater blanket does not void warranty
Water heater insulation blanket can be installed on any tank-type electric water heater, including Marathon, but not hybrid electric water heaters, or tankless electric.
Insulation blanket can cover entire tank and pipes, but cannot cover or obstruct TP valve.
Resource
TP valve overflow tube installation code

Larger image
''... tough polyethylene exterior resists dents and scratches, is resistant to moist locations and can be cleaned with soap and water and/or graffiti remover.
Polyurethane insulation helps reduce energy consumption.
'Seamlessly blow-molded polybutene tank surrounded by a filament wound fiberglass outer tank' does not rust or deteriorate over time, and requires no anode rod. Source   Source

Resources:
Marathon-parts-sheet-2011.pdf
Commercial parts-sheet-marathon-storage.pdf
Rheem-light-duty-3-phase-Marathon-parts-list.pdf
Marathon-heavy-duty-parts.pdf
Marathon does not have anode rod because tank is non-metallic
Anode rods are installed in steel tanks to prevent rusting, but not needed for Marathon

Marathon anode advantage: Homes that must replace anode rod frequently because of odor problems will benefit from Marathon water heater.
Marathon does not eliminate all water heater odors, or eliminate water problems such as hard water, etc. If odor is present at all hot water faucets, and not at cold, then problem is inside water heater. Other causes for tank odors include dirt inside the line or inside the tank, or water supply problems.

Typical anode problems that are solved by installing Marathon:
1) Anode rod prevents rusting.  If anode inside steel-tank is not inspected and replaced each 2-4 years, then steel tank will rust, eventually causing tank failure. Marathon tank will not rust and solves this problem.

2) Harmless bacteria, often found in well water, can react with anode rod inside steel tank. This reaction results in odor, requiring anode rod replacement, and/or periodic cleaning of tank with hydrogen peroxide. Marathon solves this problem.

3) If a steel-tank water heater has not been used for several weeks, then certain bacteria can react with anode rod to produce hydrogen gas, which is flammable and explosive, and can ignite when faucet is first turned on and then light switch turned ON. Marathon solves this rare tank problem, but does NOT prevent natural gas from entering water well due to local hydrocarbon drilling and fracking.
Resource:
Anode rods
Marathon water heater connections




Larger image
Water connections: Cold and Hot must be connected to correct side, or water heater will not function correctly. If unsure which pipe is hot: Before disconnecting old heater, run hot water for a few moments and feel which pipe is hot. Note also that incoming cold water pipe usually has shut off valve.

Vacuum relief valve
: Do not connect anything to relief valve, except cold water connects at top. "Certain conditions produce vacuum or negative pressure condition inside tank. Negative pressure can cause tank to fail. To prevent problems, make sure water heater is full of water before turning on power. Run each hot faucet to remove air, then briefly open TP valve on water heater to make sure last of air is gone, then turn on power.
Buy:
Vacuum relief valve

Electric connection: Typical electrical connection is made with flexible conduit.
Use 10 gauge wire and 30 amp breaker for elements larger than 3800 watts.
Read label on side of tank to check element wattage. Wattage also printed on end of each element.
Marathon requires 10 gauge wire and 30 amp breaker 

Resources
Read page 10-11 manual
See wire sizes
See basic water heater circuit

TP valve: All water heaters today have temperature-pressure relief valve to prevent high-temperature causing tank failure.

Resources:
Marathon-residential parts-sheet-2011.pdf
Commercial parts-sheet-marathon-storage.pdf
How to install TP valve overflow tube:
Applies to all water heaters
Use overflow pipe off old heater. Connections do not have to be water-tight, but joints must be secure so pressurized scalding hot water is released downward away from people. Overflow tube must be same diameter as valve discharge connection. Do not downsize pipe into smaller pipe. Overflow tube can have maximum 4 elbows and 30 feet length. End of overflow tube cannot be threaded. Tube cannot be lower than top edge of drip pan. Tube must terminate 6" above open drain. TP valve can draw water back into water heater if overflow tube is under water. Do not cap off TP valve or obstruct overflow tube.  Overflow tube cannot have shut off valve installed, or any obstructive valve. TP valve is first line of safety for water heater. TP valve releases water when pressure or temperature is too high, and prevents possible water heater explosion  
Resources
TP valve overflow code
How to test and replace TP valve
Water heater element Marathon tank must be full of water before power is turned ON
All electric water heaters must be full of water before power is turned ON.
Otherwise elements burn out instantly.
Fill tank until bathtub tap runs full with no air gaps, then open Marathon TP valve for a moment to bleed last of air from tank. Then turn power ON. Marathon will heat 20 gallons per hour, so expect tank to be fully heated in 2-3 hours depending on tank size.
Remove Marathon elements using 1-7/8" socket
Read manual for advisories
Page 14 manual
Required maintenance Maintain Marathon water heater
Warranty void or diminished if tank improperly maintained.
For example if household water pressure exceeds 80-100 psi limit, or water temperature exceeds 180 degrees, or if TP valve not opened and tested each year.
Page 16 manual
Marathon manual says to flush water heater each month, typical metal-tank water heaters say once per six months.
Troubleshoot Marathon water heater
Page 17 manual

Replacement parts and element installation for Marathon
Page 18 manual
marathon water heater Marathon can resist water
Installation in damp or wet location reduces warranty, but tank will not rust.
If water heater is flooded, the electrical parts can be replaced and water heater put back into service as soon as surfaces, covers and conduit channels are dry.

'Car-bumper tough polyethylene exterior' is resistant to moist locations and can be cleaned with soap and water and/or graffiti remover
'Seamlessly blow-molded polybutene tank surrounded by a filament wound fiberglass inner tank' never rusts or deteriorates under normal operating conditions.

Steel tank heaters: recommend  replacement after flood
Resource
Read about Flooded water heater

2008 Sell Sheet with model numbers
2012 Sell Sheet with model numbers
Rheem Marathon parts list
Rheem marathon parts list 2011/ pdf
Marathon storage tank parts
Marathon troubleshoot walk-thru/ pdf
Rheem parts warranty claim form
Marathon at Rheem
Marathon selection at Rheem
Marathon use and care guide
Marathon warranty, and claim procedure

Efficiency .90-.92 EUF/ same efficiency as ordinary steel tank heater.

Storage tank available for solar, geothermal
and indirect heating 50-85-105 gallon with back up elements
Pressure relief valve
2000 watt element
SP610130 3800 watt upper element
4500 watt upper or lower element

Rheem titanium Marathon element

Advantage of electric water heater vrs gas water heater (Applies to non-hybrid electric water heaters)
Electric prices are stable in most areas of US, but likely to increase over next years. Electric heater avoids price fluctuations of NG and LP gas.
Electric water heaters can be installed anywhere inside a home as long as area does not flood. No air supply required, while gas water heaters require fresh air for combustion. Electric heaters can be installed in closet, or below grade, or in attic.
Gas water heaters can have FVIR problems and cannot be exposed to flammable vapors, or dust, or stored household chemicals.
Maintaining electric water heater is easier than gas, and is simple DIY repair. Gas water heaters require yearly service cleaning the air intake, burner and combustion parts, and while ordinary 40-50 gallon gas heater can be repaired DIY, there is danger from incorrect repair.
Electric heater are 'not as dangerous.' No danger of CO poisoning, or gas leak. No need for CO detector or gas detector.
Electric water heaters need no vent pipe, and do not draw air into house that will cause heat-air systems to run more.
Electric heaters are easier to install. Unlike gas water heaters, electric water heaters can be installed in direct contact with carpet, and personal belongings without creating fire hazard, assuming no obstruction over TP valve and wiring is up to code and wiring is contained inside approved conduit..
Electric heaters can be controlled by a timer and home automation. See timers
You can direct-connect ordinary electric heater to solar panels and use sun to heat hot water.
Connect AC water heater to low voltage DC    Connect AC water heater to high voltage DC
Disadvantage of Electric water heater vrs gas water heater
Burning coal to produce electricity produces more greenhouse gas than burning NG or LP gas. However power plant gasses are released at one central location instead of venting out at each home. Offsetting the lower carbon from burning natural gas, is that HVAC unit runs more because gas water heaters draw air into house from outdoors.
Yearly operating cost for electric water heater is usually higher than gas.
Gas heats water faster than electric, so gas water heaters have higher recovery and first-hour delivery.
Difference between gas and electric water heater EF energy factors
Electric water heaters have higher EF energy factor rating than gas water heaters. Higher EF is better.
Gas water heaters have lower EF, or worse EF than electric water heaters. But gas water heaters cost less per year to operate, despite gas water heaters having worse EF.
This is generally because burning gas releases more BTUs into water than electric. Gas heats water faster than electric.
Electric water heaters have higher EF rating because the element is immersed in water, and no heat is wasted up a vent pipe like gas water heater.
Gas water heaters can increase EF rating by recycling more heat from combustion by-product. This means less waste up the vent pipe. Read Effex water heater review

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