Flow directions in engine cooling hoses (both early and B-series engines)
Cooling loop schematic for earlier engines (e.g., M-25/XP/XPA)
The yellow arrows show the two hose barbs where, OEM, the engine had a "thermostat bypass hose." Thsi connection provided a constant small flow through the engine block even when the thermostst is closed (before the engine coolant reaches operating temperature.) The purpose is to prevent hot spots in the engine block that could damage gaskets or warp the head. Once the thermostat opens the bypass befomes a redundant flow path. Catalina (and other manufacturers) removes the bypass hose and connects the domestic water heater hoses -- thereby just creating a much longer flow path that still bypasses the thermostat and provides the same protection against hot spots. The benefit of doing this is that the domestic water heats FAST because it doesn't depend on waiting until the thermostat opens up. Now, water will heat while idling, which was about impossible when in other models the water heater was plumbed in series with the heat exchanger.
Coolant flow on the B-series engines (M-35B shown)
--KWKloeber 2 February 2019