It would take longer than that , but can build up rapidly IF moisture content in wood is high, and you constantly burn low trying to save wood. This unit having a thermostat will allow the flue temps to be higher when calling for heat. The basics are keeping the flue temperature ABOVE 250* all the way to the top. This is the condensation point of any moisture going up the flue that condensates on flue walls allowing smoke particles to stick. This is more critical when first loading since temps are down and the fresh fuel load is not burning hot enough to burn smoke off. Mid burn should allow enough up the stack, again depending on flue size (square area you're trying to keep above 250*). In the coal stage, there is little to no moisture and smoke, so a cooler flue is not a problem. Once you understand these basics, YOU can make a poor installation safer. YOU can also make a safe installation unsafe with poor burning practice as well.
It doesn't seem like much of a difference going from a 6 inch round pipe to an 8, but when you use the formula pi X radius square to figure square inch area of flue, you find it almost doubles in square area. So a 8 X 10 flue being 80 square inches compared to the 6 inch round outlet or 28.26 square inches requires MANY times the amount of heat to be left up trying to keep three times that square area hot. Measuring the existing flue is critical so you know approximately how much needs to be left up. Now you see the need for a liner the same size of the stove outlet so the square inch area of flue you're heating remains low, and the insulation around liner keeps it hot inside.
Another benefit is not heating the mass of the chimney. Open burning in the fireplace relied on the mass helping to radiate inside the building. This mass also radiates UPWARD allowing a greater loss, and as the fire dies, inside warmth then is absorbed by the masonry mass and allowed to rise up out the roof as well. Once you know what your loss is all the way up in a liner or flue the correct size, a thermometer on the pipe where it dumps into chimney flue is a good gauge of the exhaust temperature to run. As an example 350 may cool to 250 in an insulated liner at the top, compared to needing 600* exhausted into a much larger flue that you may have now. An inside chimney vs. exterior, outdoor temps, and wind are all factors that change inside flue temp and creosote build up. Now you see how an 8 X 8 indoor chimney is radically different than a 8 X 10 outdoor chimney. You're not in an area that's going to chill it down in -0* f. temps either.
This is not only a safety reason, the rising exhaust also creates the low pressure area in the flue, pipe, and stove which allows higher barometric pressure to push INTO the stove intake to make it burn. So the less "vacuum" in the stove created by cooler chimney, the slower the burn and less heat output from the stove.
The chimney is the engine that runs the stove. A larger flue doesn't mean you have a larger engine. It means it's capable of running a larger stove. So the flue size and height is much more critical than the stove.
So if you're serious about heating with wood or coal, get a liner.
