The C5 Series electronic controls now feature our fifth generation electronic power board, the B8 power board. The new B8 board feature 'zero-crossing' technology. This means that the relays are now only engaged at the optimal zero crossing of AC power, which is easier on the relays, and easier on the components in the spa. C5 control boxes ship with our dual-voltage transformers. There are 7 different models in the C5 Series, all feature the same electronics, but offer different heater configurations. The most common units are the C5-B (Bottom Heater) and C5-T (Top Heater), which feature 15" long 2" flo-thru style heaters mounted either inside the bottom of the box (C5-B) or to the top back of the box (C5-T). There is a model with a 'remote' heater, the C5-R, which features the same 15" long 2" flo-thru style heater on a 4' cable for tight installations. There is the C5-G which is designed for use with external gas heaters, and the C5-L which is designed with a special low-flow heater for use on spas with small circulation pumps. Finally, we also have the C5-V and the C5-H systems which feature our low flow heater on a 4' cable and mounting brackets that allow the heater to be mounted vertically (C5-V) or horizontally (C5-H).