Yeh it depends if you talking about the aux cabin heater or the aux coolant heater. The Webasto heater in the 3.0d (UK model at least, but I assume it’s all Northern Europe models) is there to help the engine warm up quickly and there more get into its most efficient running state quickly. Eg. My car will go from cold to hot coolant within 2-3 miles. Don’t see any smoke from the exhaust ever though even on a cold day. I can smell the exhaust fumes on that side though on a cold day

You can tell which heater you have, if you have a auxiliary cabin heater you will have a little fob that you can turn it on when the car is parked up.

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E53 3.0d Sport 119,000 Miles
E61 535d Estate - Hybrid turbos and Dark-side intercooler...389hp,792nm