i want to have a certain part of the level fade to a white daylight colour.
Though i only want it on a specific spot.

It should be the same as how you see those furnaces like in this picture:

Valve made the furnace by putting orange or yellow or whatever in the back, and then putting textures on the sides that made it look like it was glowing. There is probably some type of texture or overlay that would work in your situation. You should probably try using func_dustcloud to enhance the effect as well.

You could realize it with light_spots. Set the constant value to 1, the quadratic value to 0 and set the two fade distances (50% and 0% fade distance).