The brains of the automated control determines how many pieces of equipment the control can handle. The main difference between the 4-function, 6-function and 8-function systems is the circuit board. Replacing the circuit board is very expensive, in some cases almost as expensive as the entire new control system. If you are undecided about whether to get a 4-or 6-function system, we would highly recommend going up to the next size system for a few hundred dollars more. Going with a slightly larger system will allow you to add additional equipment in the future without replacing your circuit board or purchasing an entirely new system.