Using the fan, heater

[BailOut]

Yes, using the heater and fan does use more fuel. The hit is not nearly as large as with a/c but it is there.

The fan requires electricity and the alternator puts more of a load on the engine to produce it. The engine also produces only so much waste heat that makes it into the cooling system, and it often experiences drops in the coolant temperature which causes the ECU to cause the engine to burn more fuel (by running richer and/or at a higher RPM). This means that sometimes you're pulling heat from the system that the engine cannot afford to lose, but other times it is happy to dump excess heat.

Sometimes you simply need to use the heat. One should never allow themselves to get too cold in the car. However, like any other use of energy it is best kept to a minimum. Wear a coat and hat and gloves and you'll find you don't need to use the heat nearly as often.