Why won't my furnace run on the generator?
Your portable generator has ratings for starting watts and running watts. When you first turn it on, it starts with a higher number of watts then reduces to the amount of running watts. I am wondering if there is a mismatch in watts needed by the furnace and those provided by the generator. Make and model might help us discern this. 8000 watts might just be the starting level, while the generator provides only 5000 (arbitrary number) watts as running watts, underpowering the furnace.