My climate control issue is a bit odd, unlike any I have found in a search of the archives. My car heats up just fine, cools in the summer nicely, but at ambient temperatures just above 0 (all temps in this post in Celsius), when the system must blend heat and AC, I eventually get the left vents all blowing cool air and all the right ones blowing hot still. I don't have dual zone CC, and everything is controlled from the pushbutton unit. The car uses a duo-valve, and both valves will open and close fully when asked by the system.
A perusal of the PBU diagnostics indicates that the heater core temperature gradually starts to fall as the interior of the car heats up, then continues to fall as the left vents blow cool and the right ones continue to blow hot, almost as if the system is trying to compensate for the right vents. All temperature sensors appear to be functioning properly, as is the sampler fan. When the system starts to cycle the duo-valve, I get a very strong feeling pressure transient in the hose off one side of the valve and much less from the other. This can be felt by squeezing the hoses.
Does the c280 without dual zone CC indeed have 2 heater cores, one of which feeds one side of the car and the other feeding the other side? Presumably if this is so, Mercedes would have a choice of head units to install to achieve dual zone control?
Has anyone seen this before, and is it most likely to be the duo-valve, as I suspect, with one side's movement sticky and not capable of the rapid response necessary for proper cycling?

The problem was fixed today. The duo-valve was the culprit, as I suspected. I also (finally, after 4 years) replaced the auxiliary coolant pump. The climate control system now works as it should, and it's a wonderful thing.