I was driving this evening with the fan only mode in the hvac selection. For the first 15 minutes of the drive it was fan half way speed just as usual only drawing 3%, then it shoot up to 80%, heater turned on and warm air started to come out, the temp was set to 70f. Is this normal behavior? I thought the FAN only was supposed to keep resistance heat off since it's the biggest energy hog. It's a late model 2012 Volt.