If it is 90 degrees inside and 90 degrees outside, will the flue draft?

When the indoor and outdoor temperatures are equal, there is no temperature difference to create the conditions necessary for flue gas to rise and generate draft. Draft relies on the principle that warmer air is less dense and will rise, creating a negative pressure that draws air through the flue. If both the inside and outside temperature is 90 degrees, the air density is similar, thus no rising action occurs, and the flue will not draft effectively.

In this scenario, even factors like wind or humidity will not contribute to draft since the fundamental requirement of a temperature differential is absent.