Yes, the system needs to be burped. Take the top hose off the radiator and fill with 50/50 coolant/distilled water mix. I start the engine with the heater set to "high". Of course this is after a very thorough flush of the coolant. I've used both the prestone back flush kit (only install using a spare heater hose when you are backflushing, then reinstall the original heater hose because Prestone backflush kits are known to leak during everyday driving) and the famous Mercedes Citric flush (some people have had problems with the citric flush but it seems to go fine for me).

After all that, the car may need a new heater control valve (monovalve) repair kit which is a lot less money than an entire new heater control valve. It is my recommendation to stay away from the MTC brand and go with genuine Mercedes though a bit more $$$$$$, your stress level will remain lower.

http://www.pelicanparts.com/cgi-bin/ksearch/PEL_search_2014.cgi?command=show_part_page&please_wait=N&SUPERCAT_FLAG=Y&make=MBZ&model=3306§ion=ventht&page=1&bookmark=2&part_number=000-835-06-44-MBZ