Coaly,
Well yes it is a second home. Why the 6"insulated liner as it burns hotter with less build up in the chimney?
The chimney is the engine that makes any stove work. The hot rising exhaust gases moving up the chimney flue causes a low pressure area inside the chimney, pipe and stove. This is measured as draft. This lower pressure area allows the higher atmospheric air pressure to PUSH into the stove intakes. This is what makes it work by adding oxygen into the intake openings.
Every stove has a required draft needed at the flue collar. The greater the temperature differential between inside and outside of chimney, the lower the pressure inside, so the greater the draft. Velocity has a lot to do with the pressure, and the smaller diameter causes the gases to rise faster, again, lowering the pressure.
The larger flue you have is about twice the diameter of the 6 inch stove outlet. When hot exhaust gases expand into the larger area they cool drastically as they expand. They can drop almost by half their temperature. Remember, the draft required to get the proper air mix into the stove is due to the temperature differential, which by allowing the flue gases to cool is detrimental to draft.
The other problem allowing the flue gases to cool in the chimney is the water vapor created by combustion condenses on the cooler flue walls. This is where the smaller diameter and insulation around the liner keeps the flue walls hotter. The critical temperature is 250*f when water vapor from combustion condenses on flue walls. This allows smoke particles when present to stick. The mixture of smoke particles and water vapor is pyrolegenious acid. Mainly wood alcohol and acetic acid. In liquid form, this is harmless. When allowed to bake on flue walls, this fluid forms creosote.
So the creosote issue is actually due to hydrogen in the fuel turning into water vapor when burned. The molecular ratio of hydrogen to water is 9. The hydrogen turns to 9 times the amount of water. There is 6% hydrogen in oven dry wood. So the hydrogen in every pound of oven dry wood creates .54 pounds of water when burned. The chimney needs to stay hot enough to the top, to expel this water as steam before it condenses inside the flue. An older stove that loses more heat up the chimney can do this easier. Newer, more efficient stoves lose less heat up, so the smaller 6 inch exhaust can’t heat the much larger area of the inside of your chimney, which is twice the size it should be. (6 inch round being 28.26 sq. inches vs 8x8 =64 sq. inches)
Add the water in wood with a 25% moisture content and you’re trying to get another quarter pound of water out the chimney, while the higher water vapor caries more BTU up the stack along with it. You can see how detrimental higher moisture content in the wood becomes.
The critical temperature to maintain above 250*f is necessary while smoke is present in all older stoves with much more particulate. The more smoke particles burned in the stove, the less critical this temperature becomes for creosote formation, but you still need the correct internal temps to create the correct draft. So you can see how much more older stoves benefit by keeping the flue gas hotter by insulating flues.
This being a second home just means less fuel burned creating less creosote and the chance of burning harder to heat the home when you get there, causing the flue temps to be higher with less formation, and creating the stronger draft needed to make the stove work better.
Since the chimney is the engine that makes the stove go, think of buying a stove like buying the body of a new car. You’re still using the old less efficient engine. The larger the flue diameter the more capacity the chimney has to move more BTU. You won’t have that excess wasted heat anymore. Not giving the chimney the fuel it needs to generate the needed draft. So your old chimney is like a big engine in a school bus. It is capable of more power, but needs more fuel to get the power out of it. Heat is the fuel that makes the chimney engine go. You need to match the engine to the car just like you need to match the chimney to the stove.