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Marathon 40 Gallon Water Heater

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Quick Overview

MR40245 Electric Water Heater


Marathon water heaters are consistently considered the best electric water heaters on the market. The MR40245 Rheem Marathon water heater features a high efficiency design and a lightweight tank that cannot rust through or corrode. The MR40245 is more durable than other electric 40 gallon water heaters. Rheem backs up the engineered quality of Marathon water heaters: Each Marathon water heater comes with a limited lifetime warranty against water heater leaks.

Marathon water heaters are built without using steel or other metals. Rheem Marathon water heaters are made of cross-fiber polybutene, which is ten times stronger than steel. Hard water, heavy in minerals, cannot damage the water heater tank. As a result Marathon water heaters last much longer than standard steel electric water heaters.

Rheem Marathon water heater tanks, built free of metal, eliminate the need for sacrificial anode rods. Sacrificial anodes disintegrate over time and cause a sulfuric rotten water smell. Marathon water heaters are often used to replace water heaters with sacrificial anodes that cause the water to smell rotten. The 40 gallon Marathon water heaters (MR40245) use two water heater elements to heat the water to operating temperature:

  • Upper Marathon water heater element. The upper water heater element in the MR40245 is made of copper and is fused to protect against damage from dry firing the water heater. Dry firing can cause damage to the water heater element and tank. The upper water heater element is rated 4500 watts.
  • Lower Marathon water heater element. The lower water heater element in the MR40245 is made of titanium to provide added resistance against lime and sediment build up compared to steel or copper elements.

MR40245 Rheem Marathon water heaters can hold 40 gallons of hot water. The 40 gallon water heater size makes them a good choice for installations in homes with 2 -3 family members. The MR40245 Marathon water heater can deliver an average of 52 gallons of hot water per hour (first hour rating).

If you have a smaller household, home, or apartment you may also want to consider 30 gallon Rheem Marathon water heaters (MR30245), which can deliver 42 gallons of hot water per hour (first hour rating).

Slightly larger 50 gallon Rheem Marathon water heaters (MR50245) are readily available for households with 3-5 residents. The MR50245 can deliver up to 61 gallons of hot water per hour (first hour rating). Both MR30245 and MR50245 should be considered when replacing 40 gallon hot water heaters.


  • Seamless, blow-molded, polybutene tank impervious to rust and corrosion.
  • Multiple layers of filament wound fiberglass to give tank extra strength.
  • A recessed drain valve so it's out of the way of brooms and scrubbers.
  • Tough molded polyethylene outer shell to resist against dents and scratches.
  • High temperature polysulfone dip tube.
  • Factory installed conduit between element control boxes.
  • All plastic tank eliminating the need for an anode rod.
  • Manufactured in America.

Energy Savings

The 40 gallon Rheem Marathon water heater (MR40245) is unrivaled in its energy savings. All Rheem Marathon water heaters use immersion electric water heater elements which apply 100% of the energy they use towards heating water stored in the tank. . Rheem Marathon water heaters experience very little heat losses due to high-tech Envirofoam insulation that surrounds the water heater tank. The Envirofoam insulation traps the heat inside the tank and keeps the water hotter for longer periods of time without the water heater’s elements cycling on and off to re-heat the water.

Standard electric water heaters suffer from high standby losses; up to 20% of the energy used in old standard electric water heaters is used to make up for heat losses. Rheem Marathon water heaters have standby losses of 5% or less per 24 hours. Replacing a standard electric water heater with a Rheem Marathon water heater can result in significant energy savings for a home owner.


Capacity: 40 gallons
Element Size: 4500 Watts (240 Volts)
Tank Height: 61 1/2
Height to Top of Connections: 65 1/2 Inches
Tank Diameter: 21 3/4 Inches
Weight: 90 lbs
Standards: Meets or exceeds ANSI standard and is UL listed. 
Energy Factor:

All Marathon water heaters come complete with factory installed temperature and pressure relief valve, replaceable brass drain valve, and vacuum relief valve. The connections for cold inlet and hot outlet are 3/4 inch NTP, and should be roughed in at 5 3/4 inches on center.

This 40 gallon marathon electric water heater (MR40245) should be taken into consideration for replacing standard 40 gallon hot water heaters.


Rheem Marathon water heaters come with a residential limited lifetime tank warranty as long as you own your home. If the home is sold, the warranty transfers to 15 years from the date of installation and the next owner receives coverage for the time remaining. All elements and parts are warranted for 6 years from manufacture date against factory defects.

Additional Information

SKU MR40245
Manufacturer Rheem
Energy Source Electric
Tank Size (gal) 40
Certifications None
First Hour Rating (gal) 50 - 59

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