The other day in the freezing fog I stopped to go in a shop. I had already driven 15 miles and everything was nicely up to normal temperature (80C on the temp guage). So I decided to switch on the residual heat system to keep it toasty while in the shop. When I pressed the button, it started blowing warm air and all seemed well.....so off I went into the shop. 20 minutes or so later, I got back in the car and discovered it was blowing quite cool. When I started the engine, I noted that the temperature guage had dropped to about 60C, but within 5 seconds I was feeling much warmer air being blown through the vents. So, I am wondering if the residual heat system is faulty, or was the weather just too cold to cope? My understanding is that the system pumps hot water from the engine into the heater matrix when the engine is off. Yet my engine was clearly hotter than the heater matrix when I returned to the car. Anybody know exactly how it works?