I believe most modern cars are designed to immediately circulate coolant from the engine cylinder block through the cabin heater even though the thermostat is shut and there's no circulation through the radiator. The block starts to heat up fairly quickly, so demisting is quickly available. Very much a safety and comfort feature.Not sure if it uses the same system at all ..... but I have noticed on my Insignia, especially this morning -7 ...... that when I put the windscreen blowers on (when the car is cold) they instantly blow warm air (not hot, just luke warm) onto the screen and the screen clears from ice/frost in under a minute !! I really was actually surprised by this, saves me scraping the windscreen now (Y)
I have also found by the time I am at the end of my road, the cabin has already started to warm up, it does seem miles better than my VXR heaters, and there weren't too bad.
The only time I've noticed a car do that before is when the thermostat is stuck open. All the Astras that I, my dad and my friends have had have all stayed exactly on 90C once they've had time to warm up, except when the thermostat got stuck on my dad's old one then it behaved like you describe. They were all petrols though.I noticed today that my 2.0 CDTi doesn't seem produce enough waste heat on idle to warm my car up :lol:
I pulled off a short trip up the motorway today where the engine temp had climbed to ~80c. After 2 minutes in very slow traffic with my climate control at 22c, my engine temp had dropped to 70c.