JW51, I don't know about 54' but my 49', which has the same heater, has a valve control knob on the dash to control flow of coolant through the core. I can shut it off completely or adjust the flow. It has an air filled flat canister about the size of the diameter of soup can with a copper tube back to the valve that will help regulate the flow based on cabin temperature. Kind of an automatic temp control. All factory.
