Masonry chimney trouble

[bama]

I built a new chimney to go with the wood furnace I put in my garage. It has a 6" clay flue and is about 15 feet tall. This is the 3rd chimney I have built, so I figure I have the bugs worked out. My dad showed my how and his chimney's on the farm are going on 40 years without a problem.

Here is the problem I have noticed: I have a separation of the mortar from the brick between the two thimble halves of my masonry brick. This separation cannot be seen until the chimney gets warm. The top half of the thimble block raises up to about 1/8" gap when I get a good fire going.

The flue is nice and straight and I used the "special" flue cement to glue the section ends together. There are no cracks in that area(I reached in and felt all around). I also noticed the three blocks above it have a hairline crack around the mortar joints and there is a hairline crack running vertical up the outside of the same three blocks.

It has TREMENDOUS draw for air, so I am thinking it might be getting too hot in that area and it can't handle the expansion. If I open my cleanout, the stove still has a great draw for air! When the fire is going well(after about an hour), the blocks in question get very warm(almost hot), yet my chimney is clean and I am burning dry wood. I do not use this every day. It is only used when I work in the garage, so it goes from 0 degrees(or colder) to 70 degrees in about an hour(or less).

Does anyone have any idea as to why this occurred? If I need to, I will disassemble and redo that portion of the chimney, but first I need to know why it cracked. If I get a chance, I will take some pictures while the crack shows. Once the garage is at room temperature, the crack disappears and the blocks cool down to being just warm.

 

[olyman]

well--after conversing with a block co the other day--they told me the clay liners CANNOT be against the block hardly at all--and mortar cant be locked to the block--they said they will break--or do as your saying--wondered why i was having the same problem--the liners start to expand with heat--and if anything is against them--they break--not break whats tight against them---

 

[bama]

Maybe that is what happened. I put a generous amount of the cement on each liner end. Maybe enough squished out to get close to the block. I was very careful about keeping the liner in the center, but this is the first time I used the high temp cement.

 

[Sprig]

well--after conversing with a block co the other day--they told me the clay liners CANNOT be against the block hardly at all--and mortar cant be locked to the block--they said they will break--or do as your saying--wondered why i was having the same problem--the liners start to expand with heat--and if anything is against them--they break--not break whats tight against them---

Yup, different expansion rates with glues, mortar being similar to clay will have less of this effect. Just my unworthy glab fer now,

 

[bama]

My plan is to wait until it warms up(May, probably) and chisel the mortar out of my top block. Then I glide a 8 foot flat iron (3 inches wide by 3/16 thick) up and down between the block and flue so I can break off the cement that is sticking out. The chimney is only about 12-14 feet tall and my thimble is 5 feet off the ground.