I have a 2011 C30 2.5T R-Design that I only drive in winter, I've had it for 2 years and have had no work done to it in that time other than oil changes. Just took it out for the season and noticed that when it's on idle it only blows cold air, I didn't notice this in years past. All the fan speed and air location selections work.
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Last few weeks here it has been hovering around -5°C. When warming the car up the temp gauge rises at a normal rate, and stops bang in the middle. It doesn't fluctuate at all between city, Highway, and spirited driving. The car will only start to blow warm air through the cabin vents when the engine revs. Set to 25°C, On idle it'll blow cold air until you hover it around 2k rpm then it's nice and comfortable. Flooring it will make it pump out lovely heat through the vents, when you stop at a set of lights you can feel the air get progressively cooler the longer you're on idle.

Any ideas?
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This is usually a sign of an air bubble in the cooling system. Check all the cooling hoses for a leak and a drop in the coolant leave in the header tank.