Choosing the Right Size Overflow Fitting:
There are two popular sizes of overflow fittings. 3" and 4". The first thing you should look at when trying to decide what size you need is the size of your roof. The reason for this is; the larger the collection area, the more water flowing into the tank. When the water tank is full, this equates to more water that needs to exit the tank.
Installing an Overflow System in 6 Simple Steps:Although there are several designs of overflow systems available, each design will work very similar.