@Chinyoti
@Retaliatior
Turning on the AC switch when the heater is on would have no effect other than AC would be working all the time and not tripping. But if you have fogging of windows in extreme cold weather or rain, switching on the AC, while the heater is also on, suddenly clears the fog.
@Poster. The knob should be on red for heater to work. Also set the recirculation on (it is a different knob and the positions are indicated in the shape of arrows). Then there is a third knob for the air direction. You can set the air to come from dash vents, near the windsheild and on the foot or on the foot and the face and some other positions.
Also another point is to note that heater will not start working immediately like AC because it gets heat from the water circulating around the engine. It takes some time for the water to get hot and only then the heater gives heat.
Even now if you do not feel any heat, kindly check that water pipes from engine bay are connected with the heater core in the cabin.