The short answer
Cast iron sheds motor heat into the pit water and resists cracking, which is why most heavy-duty primary pumps use it. Thermoplastic pumps are lighter, cheaper and won't rust, and they suit basements where the pump rarely runs or as a spare.
How they compare
| Cast iron | Thermoplastic | |
|---|---|---|
| Heat dissipation | Better | Lower |
| Weight | Heavier | Lighter |
| Price | Higher | Lower |
| Corrosion | Coated; can rust if chipped | Won't rust |
Examples
- Cast iron: Zoeller M53, Liberty 257, Wayne CDU800.
- Thermoplastic: Superior 92572 (1/2 HP, three-year warranty), Superior 92372.
Other things that matter more
Switch type, horsepower matched to your head, and warranty often matter more than housing. A cheap pump with a good switch can outlast an expensive one with a switch that snags.
Products mentioned
Questions readers ask
Is cast iron better than plastic for a sump pump?
For a pump that runs often, usually: it dissipates heat better. Plastic is fine for light duty.
Do cast iron sump pumps rust?
They're coated; chips in the coating can rust over time.