What is causing cold air to blow from passenger side when heat is on?

Will I need a mechanic to fix this problem? I have been told that it could be the heating element. Do the C230's have that (w/o them checking of course). I have been told it could be the mono-valve needs cleaning out as well. Going strictly with your answer what would be the estimate of getting this repaired.

no way to estimate.best thing to do first is to find a good,local,independent,trusted repair shop that is going to be truthful with you and not wipe out your bank account! these cars are expensive to work on.depending on how involved and time consuming it's gonna be,will depend on price to fix it.