No control valves are used. Engine coolant is circulated through the heater hoses to the heater core at all times. The temperature is controlled buy a blend door actuator under the dash. Here is what they look like. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.