Heater Control Valve Requirement

On A bodies, the non ac cars circulate coolant all the time.

On factory ac A body cars the under hood control valve turns off the coolant flow.

If you have aftermarket air, you need to install the system the way it was designed for it to work as designed.

In any of the above cases you can always install a $5 heater hose manual shutoff to keep the hot water out of the passenger compartment during the summer if you want.