Its not a code of the silt in the system, just that the TDI does produce very much waist heat, the cabin heater gets the hot water from the engine first, even before the thermostat, so the small amount of heat being generated by the engine is feed into the cabin heater which dully extracts the heat and returns cool/cold water to the engine.
This will continue with the engine running cool / cold until either the cabin warm up and therefore pulls less heat from the engine (allowing it warm up and ultimately open the stat) or the engine revs increase, thus creating more surplus heat
This is where on the early cars the Webasto Aux heater helps greatly, as on full power it adds 5.5 Kw of heat into the system, or half this on half power. The Webasto kicks in automatically at less than 7.5 Deg C unless the heater control is set to ECON in which case the Webasto is disabled
On my own A2 at 8 deg C outside temp. on my 8 miles commute on B roads to work and the cabin heater on full the temp gauge will not move past 70 deg C when I have got to work, in the warmer weather at say 14 Deg C I hit 90 Deg C at about 5 miles all be it with the heater no medium
Now if the outside temp is < 7 deg C I hit 90Deg C in 3 miles, and 80 in about 1.5 miles, the cabin is toasty warm in 3 or 4 mins - that is what 5.5Kw of additional heating does for you
Cheers,