Is main drain necessary?

[mas985]

Turning off the main drain adds head loss to the suction (i.e two lines vs three) which in turn lowers flow rate but usually not by a lot. You should expect a slight drop in pressure because of this. However, the amount of total flow rate lost is less than the flow rate gain by the remaining skimmer so you come out well ahead from that perspective. I think overall, your pool will probably end up cleaner without the main drain, than with it.

But given your run time (12 hrs), you might want to read the article in my signature on pump run time. Most pools only need about 3-4 hours per day. You can save a lot of energy and cost by reducing pump run time.