newbie piping questions - what size return - what size overflow?

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Originally Posted by Milad

The current holes are about 8 inches from top and about 4 inches from the side

As MrOM cautions re low return holes

To find the size of sump you'll need to handle the just back flow if you lost power and the check valves not working, it's the 8" x the width x the length of tank divided by 231 for gallons. You'll probably find it 50+ gallons.

Not ideal but could still use the holes for return, just have your return line go up to the top of the tank then back down to the holes and have a small siphon break at the top. Line can extend fr the siphon break to over the tank.