The dip tube on Marathon (and every other water heater) is a plastic tube extending from the cold water inlet down to about 8" from the bottom of the tank. It directs incoming cold water to the bottom so it can be heated, instead of mixing with hot water at the top. Marathon\'s dip tube uses universal water heater dip tube design and is the most commonly-replaced internal part on any tank water heater.
Symptom: hot water runs out fast
- Yesterday: 25-minute shower no problem
- Today: 5-minute shower turns cold
- Tank size, thermostat setting, elements all normal
- Recovery seems normal but you never get a full tank of hot
This is textbook broken dip tube. Sudden onset = dip tube. Gradual onset would suggest sediment or anode (but Marathon has no anode).
Why dip tubes fail
- Plastic embrittlement over time — 10+ years typical
- 1993-1997 manufacturing defect across many brands — bad plastic batch. Marathon units from that era nearly all eventually failed
- Water hammer — repeated shock loading cracks the tube
- Recirculation systems — constant flow stresses the tube
Replacement procedure
- Power OFF at 240V breaker; verify with voltage tester
- Close cold water inlet shutoff
- Open a hot tap upstairs to break vacuum
- Drain about 5 gallons via the drain valve
- Disconnect the cold water inlet nipple at the top of the tank (union, flex connector, or threaded nipple)
- The dip tube either pulls out with the nipple OR remains seated in the cold inlet opening
- Pull out the dip tube — note its length (typically 40-50")
- Inspect — cracked, split, or only fragments remaining = confirmed failure
- Cut new universal dip tube to match length if needed (most come pre-cut to 40")
- Insert new dip tube into the cold inlet opening
- Reconnect cold inlet nipple
- Restore cold water inlet; refill tank fully; restore power
Marathon-specific consideration
Marathon\'s cold inlet port is a brass threaded insert in the composite tank shell. Treat with care:
- Don\'t overtorque the inlet nipple — hand-tight plus 1 turn is the spec
- Use new Teflon tape on the nipple threads
- Confirm O-ring or gasket condition before reassembly
Dip tube options
- Standard polypropylene: $5-10. Adequate for most installs
- CPVC-equivalent: $10-15. Better hot-water resistance
- Curved-end (anti-sediment): $10-20. Outlet angled to stir sediment as cold water enters — reduces sediment buildup. Recommended for Marathon since the composite tank has no anode rod to scavenge metallic sediment
Cost
- Standard dip tube: $5-15
- Curved anti-sediment dip tube: $10-20
- Plumber labor if hired: $80-150
Bottom line
If "hot water runs out fast" came on suddenly, replace the dip tube before suspecting anything else. Cheap fix, 30 minutes, $5-15 part. Universal water heater dip tubes fit Marathon. Treat the cold inlet brass insert in the composite tank gently — no overtorquing.