Chimney Flue Draw Interesting behavior

Kind of hard to explain this but here goes.

First of all I have a Masonry Clay lined Chimney in good shape, it was well built by a preacher man that used to live here in the late 60's who was also a mason. It goes right up thru the middle of my ranch style house. He started in the basement and poured 3 ft by 3 ft 6 inch deep concrete pad on the basement floor which the basement floor is most likly another 6 to 8 inches of concrete. I think this is one reason the flue is still in good shape as its not going to be shifting unless there is an earthquake.

So anyhow, when I first got the stove and before I really knew how to operate it I hooked the stove's 6 inch outlet to the masonry chimney using 2 pieces of 6" pipe and one 6" elbow. My chimney is a 7"x11" and has an 8" thimble, and I used an adapter at the thimble. The chimney is 25 foot tall.

My chimney has a clean-out door and even when the flue is cold I can open that door and feel the air being sucked up the chimney when I put my hand in the door.

But back then, I at the time I didnt know if I was operating the stove correctly and I was having trouble getting the stove to heat up and the wood to burn with the door shut. I could get the wood a blazing and everything looked good but no heat was building in the stove and when I shut the door things would eventually go out. Sounds like 100 other stories on this board.

So what I did was I swapped out the 6" pipe and put in 8" pipe all the way to the 8" thimble on my 7"x11" masonry flue. My 6" to 8" adapter was now sitting in the top of the stove's 6" outlet then 8" pipe the rest of the way.

This improved things alot better plus I was at the time learning better how to operate the stove. My thought in going 8" instead of 6" up to the thimble was that I needed to provide a bigger volume of air that was heated to feed the masonry 7x11 flue. The 6" pipe and its smaller volume of air was cooled by the time it flowed up to enter and feed the bigger sized flue. The 6" pipe was single walled and ran for about 6 feet with one 90 degree turn. So heat was being given off that length of pipe. My thought was its volume of heated air being cooler wasnt enough to feed my masonry flue.

So to continue , my 8" pipe gong all the way down to the stove top with its bigger volume of air especially being closer to the hottest part of the system the stove , well I am pushing a bigger sorce of heated air to the entry point into the masonry flue thimble and feeding the 7x11 flue much better.

The results were improved but not as good as I would think running a 6" inch liner insulated all the way.

One other thing is I stuffed the flue just below the thimble with high temp insulation to eliminate the leaking flue clean out door.

So after having some issues with the stove getting too hot I decided to go back to the 6" pipe to see how things would work, well I could hardly get the stove going again it was same ole story trying to get the heat up and close the door.

For some added info here, With the 8" pipe install I have to use the manual flue damper , with the air intake completely closed and the manual flue damper completely closed on the 8" pipe I can still have the stove rockin at 400 flue temps about 12 inches up and stove top temps at 800 - 850. So the draw is good.

I will say even with the 8" the stove is sometimes hard to start as some of you remember my wanting to just throw some big splits on and get it going quickly before leaving for work. What I have to do is put some smaller stuff up front right on the coals to get burning hot and fast to heat up the stove.

Has anyone seen or heard of a situation like this with the 6" vs 8" running to a masonry flue?

wood has been split then tested at 20% or less .

I just wanted to point out that if a person is unable to get a new chimney this solution is an intermediate solution but not the best solution.

Here is a pic of the 8" pipe all the way to the stove top.

Did a shoulder type cold start up today with the 8" pipe back in place, , its 49.8 degrees out side and the thing started up nicely. Maybe not perfect but it was a breeze compared to when I tried it with the 6" pipe installed. So something is going on there with that whole scenario.

I dont use a chimney cap.

I maybe still in spec but running the 6" into the masonry 7x11 flue makes for alot of problems with the stove operation.


Something to chew on.

Old Spark she is right the manual states the stove can operate with a chimney thats 85 square inches. So I am in spec but I wonder if they tested this spec with an insulated flue which most likely they did and not an uninsulated masonry flue. Really the main issue was getting the stove started and up to heat. The 6 inch really struggles to feed the larger masonry flue. I am sure there is alot of variables.

Oldspark you are right I wasnt looking for advice just wanted to strike up analysis of the situation so if others having issues might be able to fix their chimney issues.

I really think and I maybe wrong is that by running the 8" all the way down to the hottest part of the flue system which is the stove top I am getting a larger amount of air volume heated to a hotter level and feeding the bigger uninsulated masonry flue and getting the flow going much better this way. That is why I see better performance with the 8" plus I like the idea that the 8" is providing less resistance (LLigetfa idea) especially at the 90 elbow. The 6" is very much harder to get the stove going , its the same ole story you here on here every day , the stove is burning well with the door open and as soon as I close it the thing dies out......

I hope this helps some of the newbies out their as not everyone has the money to fork out for an insulated stainless steel liner. Everyone's situation is different.

Shari thats ok thanks for the help!!