recently, in the past 3 months, built a chimney in my new home. it is full 8x8 blocks with a clay liner from the basement up till it goes through the roof where it changes to brick. the basement thimble is blocked off right now but eventually will be used by my indoor boiler. the second floor thimble is currently being used with a circulator type stove similiar to the one's sold at aubuchon's. the problem that i am having is the blocks are developing hair line cracks vertically. they are not all in line with each other but in the same spot on every block that is cracked, about 4 inches from the right hand side. it seems that the only blocks that are cracking are the ones that are getting heat from the upstairs stove. i dont see any cracked below that thimble but suspect they will crack once i start using my boiler in the basement. my only thought is that the liner is expanding faster than the block and it causes it to crack. has anyone seen this? i cant seen any visible cracks in any liner but difficult to see where there is some creosote now in the chimney. this chimney will eventually be faced in stone but dont want to progress to that level if it needs to be taken down and redone.... please offer any explanations, theories or suggestions that you may have. also wanted to add that the indoor temp of the building is not freezing so by some odd chance it was taking water in i dont think it would be caused by it freezing and cracking. i do have a concrete cap and dont see where any water could enter between the flue and the block.... my stove stack temps run a max of 400-500 f., typically 3-400.