Bill-
I don't believe there is a bypass valve unless the PO installed one.  I bypass our water heater to drain it for the winter. I remove the inlet and outlet hoses and connect them with a short length of hose of slightly larger diameter, clamping each end. That way, I can run antifreeze through the plumbing without filling the hot water tank. I also flush the system in the spring prior to reattaching the inlet and outlet hoses to the water tank. I could probably add a drain and valve to accomplish the same thing.