So I think you are referring to the Heater control valve (HCV)..It is separate from the AC system.

We have had mixed results from the cheap HCV's...most of which are made in China...It will probably last a few years.

But that may not be your problem. Low coolant, and/or a bad HVAC console are just as likely to casue the condition you describe. The HCV usually fails in the open position. Keeping heat on all the time.