You are correct, the W211 (and the W164, W212 and W166) all have a massive electrical duct heater in the HVAC system. This is only used while the engine is cold. The climate controller automatically turns on this heater if it is needed for either defrost or comfort heat. Once the engine temperature warms up enough to provide heat, the electric heater is shut down.

The thing is fed with a 200 A fuse so it is huge! I think I saw in the documentation that it is a 1 kW element so comparable power to a household space heater.

Make sure to leave the A/C switch on all year round, so the system can run the a/c compressor to help dehumidify the cabin, even if it is cold outside it will use the a/c if it feels the need to keep the windows clear. But if you turn the A/C button off it cannot run the compressor.

I wonder in the new W213s with the 42 V system how well that heater works with 3.5x the voltage available.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech