Comment by chiph

Gas heat uses two fans - one to blow air to the rooms (often shared with an A/C system), and another smaller one[1] to supply air to the gas burners and the heat exchanger. As part of the safety system, the computer won't open the gas valve and ignite the burners until it knows there is airflow from the small fan.

When GP spun the fan it fooled the computer into thinking it was running and continuing the ignition sequence. It may be that once the burners got everything hot there was enough airflow from the thermals so they didn't have a buildup of CO. Or were just lucky.

[1] The motor is usually generic but has a proprietary bracket, which was a $1500 lesson last year