On a non-A/C car there is no heater control valve.
Hot water is always running through the heater core.

A cable operated door is what blocks the hot air from coming through the vents. It might have a cable out of adjustment, or has come loose at the control panel or heater box.

Some people install a manual heater control valve in a hose to block the hot water from ever going to the heater core.

Kayse can't keep up at all now. lol