I installed a system here in Kansas, haven't had much rain since it was built so it's not full.
I am fairly confident freezing will not be a problem because I put the vent pipe higher than the water level.
The highest point of the main pipe DETERMINES THE WATER LEVEL (Normal Pool) and is buried in the dam. When the water gets higher than that point it runs through by gravity like a traditional system. Since it's buried in the dam freezing is not a problem.
Only when there is enough overflow to run a full pipe will the siphon start. The siphon is not necessary to control the water at all, it's just happens after enough overflow fills the pipe.
Once started the siphon would draw the water down past Normal Pool, which is bad, so a vent is installed to break the siphon.
Since the vent is not necessary for water release it can be installed inches, or even feet, above normal pool.
When the pond is overflowing and the pipe is running full, the amount of water running through the pipe will increase when the vent is submerged. The outlet of the main pipe being lower will draw or suck increasing the speed of the water through the main pipe (siphon).
