Wet Vs Dry Systems
Dry systems are simple systems where water is fed directly into a tank via gravity. All of the conveyance pipes must be above the tank inlet for the system to work. In dry systems, the conveyance pipe run is usually very short and the system involves only one or two downspouts.
In a wet system, rainwater collected from the roof goes into air-tight PVC downspouts to underground pipes and is channeled back up to the top of the tank via a riser pipe located near the tank. There must be enough of a drop from the gutter to the top of the tank to allow gravity (through water pressure) to fill the tank. These systems are "wet" systems because they must have a drain mechanism to allow water to drain from the pipes after a rain; otherwise, water would remain standing in the downspouts at the level of water in the tank. In wet systems, most of the conveyance pipe is hidden from view; thus they are more aesthetically pleasing than dry systems. Wet designs are common where the system involves multiple downspouts.