It sounds like the winterizing system that was on our 1998 Vetra Grand Tour. The bypass switch controls the solenoid to by pass the water heater, so it doesn't fill with antifreeze. When you hit the switch, you should hear the solenoid valve at the water heater click when it closes. The winterizing switch allows the pump to pick up antifreeze from a bucket and it bypasses the tank. It is a system that makes it very easy to winterize. When you are done putting in the antifreeze, you will turn both switches off. Be sure to drain the water heater of water when the bypass switch is used.
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