Has anyone got a feeling for how much the auxilliary heater on a TDI affects the fuel consumption? My other half keeps the Econ button switched on all the time, in the interests of economy - which is fair enough on a short journey, when the Aux heater doesn't have time to do anything. I have noticed that the water temp gauge seems to climb faster when the Aux heater is on, so perhaps by getting the engine up to temperature faster it will actually aid fuel economy [?]