Factorio Forums

I think that overflow and/or top-up valves should have their thresholds adjusted so that they work better together. Currently, they both fill at 80%, which means that if you have a top-up valve and an overflow valve later, you're never going to actually reasonably stop input. If the fill is below 80% between the valves, then the top-up valve will be open, so it will let fluid in until it's above 80% at which point the top-up valve closes, but the output valve opens, and then the process repeats on output. There's no stable point (except maybe at exactly 80%?)

The reason this is relevant is because you can't use them to easily prioritize inputs when you need to void excess fluid. You can use a top-up valve to prioritize inputs or an overflow to void excess, but you can't effectively do both (assuming that the lower-priority input has something else you want to do with it, like making a side product).

there are mods that offer this, but in general I use a check valve with a pump in this case.

In any situation where you'd want to use both valves, pumps with circuits will almost certainly work better (and with much more fine control).