Are you talking about the plubming lines??
Bottom fitting is COLD Inlet to the water heater
Top fitting is the HOT Outlet to the RV Hot plumbing lines
Any line connecting COld/Hot would be 'Bypass' line
OR are you talking about the Engine Coolant Lines to water heater tank?
Some units have an optional 'motor aid' that allows Hot engine coolant to flow thru WH tank and heat the water while driving
Engine coolant lines FROM and back TO engine
Is it time for YOUR Medication or Mine?
Dodge 3500 w/Tractor Motor & NUWA 5vr
US NAVY---USS Decatur DDG-31