Since smoke particles sticking to the flue walls are what form creosote, during the coaling stage you can go below the critical 250* flue temperature. Exterior chimneys cool much more than Interior, or “through the roof” straight up chimneys. So you have to allow more heat out of the stove for them. The increase in size of the connector elbow to 8 inch is a sign you probably have a 8 inch chimney outside, but you would need to remove the clean out cap if there is one outside to measure the flue size if you don’t know what it is. Pipe inside normally stays 6 inch for a 6 inch chimney.
When the flue is hot enough, water vapor from combustion is expelled from the top harmlessly. When allowed to cool, or not burning hard enough to generate the heat necessary to stay above condensing point, the smoke particles stick to the wet flue walls. So I use the term “when smoke is present”.
Since it is a guess how much cooling you have at the top of chimney, you can only burn hot and check creosote formation frequently. To get a good idea of the temperature needed at the bottom, an IR thermometer can be used on the inside of flue wall about a foot down to get a temperature reading and compare to the temperature of pipe wall at bottom so you know how much cooling you have in your particular system. An IR thermometer will also show you the drastic temperature difference from the 6 inch coming out of stove to the 8 inch pipe connecting to chimney. You just have to run with the air open more than you would with the correct size chimney, extending burn time and radiating more heat into the building than letting it up the stack.
