How much backflow is a sump-pump check valve meant to prevent?

The potential drain-back depends on how much water is in the discharge line, so both its length and diameter matter; a wider pipe holds more water per foot. Vertical lift affects the pump’s workload, but lift alone doesn’t tell you how much water will return. I’d weigh the valve’s resistance against the pump and the whole discharge layout, rather than treating either pipe size or lift as decisive by itself.

A rise after shutdown could fit drain-back through a valve that isn’t sealing, while slower emptying during a pump cycle could fit added restriction. Neither clue is specific by itself: inflow to the pit, pump performance, and other parts of the discharge path can also affect what you see. I’d treat the timing and pattern as clues to investigate, not proof that the valve is at fault.

I’d give the backflow benefit more weight when the discharge line could return enough water to make the pump cycle again. If little water returns and pump performance is already a concern, restriction and upkeep would matter more to me. That makes the valve a layout-dependent choice, not an automatic one.

Imagine a long, wide discharge line where enough water drains back after shutdown to trigger another cycle. Limiting that return could make the valve’s upkeep worthwhile. A substantial lift by itself wouldn’t establish the same benefit; I’d still want to know how much water can return and whether that actually affects cycling.