On my '99 Suburban K1500 when working on HVAC I discovered the solenoid, which controls the heater control valve, was bad. When I replaced the solenoid I found that the heater was always on, in that by design the heater control valve is normally open and allows heated engine coolant to always flow to the heater core in the passenger compartment. This also seems to be the case for the rear heater. Does anyone know why the HVAC would be designed this way. If I lived in Alaska this might be reasonable but since I live in south Florida it does not work so well for me.
Bill

 

An update to my above post. I am thinking of installing a manual valve to cut coolant flow to the front and rear heater cores. Also can anyone tell me when the existing heater control valve is supposed to be activated to cut the flow to the heater core?

 

The solenoid comes on and applies vacuum to the heater coolant flow valve to bypass the heater core when the selector switch is in any of the a/c positions. checked this yesterday.