The Rheem Marathon is unique in the entire residential water heater market: it has a limited-lifetime tank warranty. Where every other Rheem caps the tank warranty at 6–12 years, the Marathon's warranty runs as long as the original owner owns the home. The reason is the tank itself — Marathon uses a seamless polybutene plastic tank instead of glass-lined steel. Plastic doesn't corrode.
The three Marathon models
- Marathon 40G (MR40245) — 2–3 person households
- Marathon 50G (MR50245) — 3–4 person households, volume pick
- Marathon 75G (MR75245) — 5–7 person households
All Marathon models are electric only. Rheem does not make a gas Marathon — the polybutene tank doesn't handle the direct-fired heat exposure required for atmospheric gas combustion.
Why the polybutene tank matters
Standard water heaters fail when the steel tank wall corrodes through. The anode rod (see our anode rod page) slows this corrosion, but eventually all steel-tanked water heaters reach end of life through tank-shell failure. Average life: 10–14 years.
Polybutene plastic doesn't corrode. It also doesn't conduct heat into the surrounding air the way steel does — Marathon's tank insulation is built into the tank shell itself. Net effect: a Marathon installed in 2003 is, statistically, still running in 2026. Many Marathon installs from the 1990s are still going.
What the Marathon trades
The Marathon's lifetime warranty isn't free. Trade-offs:
- Higher upfront cost — Marathon 50G runs $1,200–$1,500 vs $549–$699 for the standard Performance 50G electric. Pay 2x upfront.
- Electric only — no gas variant in the Marathon line
- 0.93 UEF resistance electric — same as Performance electric. Marathon does NOT have heat-pump efficiency. For heat-pump efficiency at all-electric, see ProTerra.
- Heavier unit — the polybutene tank with built-in insulation is heavier than a standard steel tank
- Specialized parts — Marathon elements and thermostats are not universally interchangeable with standard Rheem electrics
Marathon vs ProTerra — the electrification decision
If you're committed to all-electric and committed to staying in the home long-term (15+ years), the two real choices are:
- Marathon: lifetime tank warranty, 0.93 UEF resistance electric. Pay more upfront, run the same operating cost as standard resistance electric.
- ProTerra: 10-year warranty, 3.45 UEF heat pump. Pay similar upfront (after IRA tax credit), but operating cost is one-third of Marathon.
For pure economics, ProTerra wins — the operating savings over 10–15 years dwarf the warranty value of Marathon's lifetime coverage. Marathon wins on: install locations that can't support a heat pump (no basement air capacity, condensate drain issues), homeowners who hate the idea of any failure point (compressor failure on ProTerra is a real possibility, plastic tank rupture on Marathon is essentially impossible), and very high water-quality concerns (no anode rod means no aluminum-zinc or magnesium dissolution into hot water).
Marathon vs Performance Platinum 50G Electric
The volume comparison for Marathon shoppers. Performance Platinum 50G Electric at $799–$999 ships 12-year warranty and EcoNet WiFi. Marathon 50G at $1,200–$1,500 ships limited-lifetime tank warranty but only 6-year parts warranty (the parts last; the tank just doesn't fail to begin with).
Choose Marathon when you genuinely value the lifetime warranty over the upfront price difference. Choose Performance Platinum when you're comfortable with 12-year coverage and want the WiFi/EcoNet feature set.
Marathon maintenance
Marathon maintenance is simpler than standard tanks because there's no anode rod to replace. Routine items:
- Annual flush — same procedure as a standard tank
- T&P valve test annually — same as any tank
- Element check year 6+ — Marathon elements are specialized; verify with Rheem if replacement is needed
- Inspect the polybutene tank annually for any visible damage or cracking (rare)
Bottom line
The Rheem Marathon is the only residential water heater on the market with a lifetime tank warranty. It's the right pick for: long-horizon owners (15+ years) who can't use a heat pump, water-quality-sensitive households, and anyone who values warranty over operating-cost optimization. For long-horizon electrification with the best operating economics, the ProTerra still wins. See the Marathon 50G page for the volume model.