[llbbdd]

Can you elaborate? I interpreted the same as the other comment that the blower fan just needed a hand start and kept going after the furnace started up. What you're saying only makes sense to me if the spinning the fan by hand allowed the furnace to start by bypassing the safety at startup, but wouldn't that mean that if the exhaust fan was stopped during normal operation (blockage etc) that the furnace would just keep going, dumping CO into the home?

[andrewthornton]

It wasn't bypassing, I was just helping start because of what I believe to have been a bearing issue.

[doubled112]

It’s a pretty normal trick to try while troubleshooting a rotating part.

Helping something start is not likely to ruin your day (unless you get caught in a rotating part)

[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

[llbbdd]

In the case I described then, what would normally happen if the fan is blocked after ignition? Like if I had a fully functioning system, booted it up, and then blocked the fan from spinning with a stick, the computer is going to keep the gas valve open and fill my house with CO?