[Izkata]

The original comment was unclear whether the fan kept spinning while the furnace was running, or if all it did was bypass the safety and the fan didn't continue to spin while the furnace ran. They clarified in their response it kept spinning.

[MPSimmons]

It seemed obvious to me that this was bearing stiction and that manually rotating it during the start allowed the fan to spin on its own after that, but I could be wrong and maybe the fan was dead entirely?

[modriano]

Yeah, that would be my assumption too (based on my admittedly incomplete personal experience where I got my furnace running by manually spinning my draft inducer motor, which kept spinning).

As exhausting the combustion products is a critical safety feature, I would be surprised if any furnace was designed such that it could possibly keep running if the draft inducer motor stopped. It seems like it would be trivially easy to make a circuit such that gas valves could only open if the draft inducer motor + fan wasn't spinning.