I found this from GM reports on this condition:
Product reports and Technical Assistance cases have recently been received on vehicles that exhibit a concern with the heater not getting hot enough or maybe just the driver's side not getting hot enough. Back flushing the heater core often resolves the concern but sometimes only temporary. Analysis of returned heater cores as well as the material retrieved from them reveals a restriction resulting from the addition of stop-leak pellets. The Denso high efficiency heater core is designed with smaller core tubes that transfer heat with less volume of coolant flow. Stop-leak material will accumulate in the smaller tubes restricting and eventually stopping the flow of coolant. Although stop-leak pellets have been widely accepted in the industry for past
applications, until further notice, use of these materials is prohibited for the CTS.
As for a plugged heater core, it's possible, but I checked the input and output lines of the heater core under the dash and they are extremely hot to the touch. The passenger side floor heat ( for example ) is really hot.
It just seems like the drivers side heat is being diluted with cold air, but from where?
The HVAC control module controls the actuactor on the drivers side. It is possible that theActuacor is not moving all the way over like it should. A easy test is to remove the actuactor and turn the door all the way by hand and see if it changes. If it blows warm, You have a actuactor or a module bad. If still the same the heater core might be partually clogged. I 've seen them do this before. Here is a picture of the actuactor if needed. Its right under the drivers dash above and to the right of the gas pedal.
You were correct. I took it to a dealer and the service department suggested a backflush before attempting a heater core replacement. A backflush corrected the problem. It's 10 below zero and it heats just fine. The dealer also recommended a complete system flush which we did.
You were right on target!