Herbie question

[JKDMan]

I have a 220 DT with dual overflows both have a 1" and a 3/4" hole drilled in them. Im currently using durso standpipes and am thinking about switching to the herbie style drain. My question is regarding which hole to use as main and which as back up drain. I have read it both ways where the 1" is used as main and 3/4" is used as backup and viceversa. Currently i dont have a very powerful return pump. its a mag 12 with 40ish inches of head pressure on it...
I was thinking of doing it both ways. one overflow with the 1 inch as main and 3/4 asbackup and in the other the 1 Inch would be the backup.
Does this seem like a bad idea and i should just do it the same in both or would this be fine. I am going to be upgrading to a stronger retun pump in the near future a mag 24 possibly.

[EuroGTI]

Use the 3/4 as the main. Especially if the pump is on the smaller side.

[Breadman03]

Use the 3/4 as the main. Especially if the pump is on the smaller side.

3/4 as main. The reasoning is that if there is too much water for it, the emergency can handle all that flow and then some.

[JKDMan]

and when i switch to a bigger pump? most of the bulkhead flow rate charts i see say 3/4 in can only take about 480 gph . say i wanted to go to a mag 24 up from the mag 12 for example. would i then make the 1 in the mains?

[ca1ore]

and when i switch to a bigger pump? most of the bulkhead flow rate charts i see say 3/4 in can only take about 480 gph . say i wanted to go to a mag 24 up from the mag 12 for example. would i then make the 1 in the mains?

Always use the smaller pipe as your main siphon; the larger one for the open channel backup. Open channels handle less flow than a siphon, so in the event the siphon were to clog, the open channel would be able to handle the flow.

Didn't bother to hunt down the flow chart, but depending upon the drop, a 3/4" drain operating as a siphon ought to handle far more than 480 gph. Although personally I think the Mag24 is a horrible pump, and far more than you need, I'd think dual 3/4" siphons would handle it just fine.

[Breadman03]

and when i switch to a bigger pump? most of the bulkhead flow rate charts i see say 3/4 in can only take about 480 gph . say i wanted to go to a mag 24 up from the mag 12 for example. would i then make the 1 in the mains?

Check this RC thread (http://www.reefcentral.com/forums/showthread.php?t=1815517), along with the post from BeanAnimal, who designed the BA overflow and might just be considered The Foremost Expert.

According to the chart, 3/4", Sch 80 with 12" of head can flow 616*.85=523GPH. Head to BeanAnimal's site and check his calculator for a much better answer.