2011 w204 heater not working correctly

[sceh]

Recently I need to put the heater temperature on 28c to get any decent heat at all. If I have had the aircon on and then switch it off, the heater blows cold for ages before signs of life.
Any ideas why? I initially thought it might be short of coolant but there would be other warnings for that.

thanks

 

[190]

I think it's normal for cool air to continue after the air con is switched off. I turn it off a mile or two from home to help the condenser dry out and it does keep on blowing cold air for a few minutes.