Hello, there are two upward vents... the small one, about 5 inches in diameter is the flue pipe, it runs hot, The main warm air flow duct runs about 50 degrees F warmer than the room air.
There is a 'high temperature safety limit' switch with a probe that pokes into the fire box area, that shuts the furnace down if it over heats... it will over heat if there is not enough air flow past the heat exchanger, that can happen if too many vents are shut down or covered with throw rugs etc.
It can also happen if that safety switch has gone bad. the way to test it is to fit a temporary jumper across its two terminals.
Please attach a well focused, close up photo of the wiring diagram glued inside the blower service panel,you can use the paper clip-icon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If there is no paper clip-icon, look for the '+attach files' link at left of your send button.
I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.
We can go from there. When we are done you can leave a bonus if you iike.