chimney cracks

91LMS · Jan 6, 2012

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.

pen · Jan 6, 2012

Was there a void or air space left between the flue tiles and the blocks or were the flue tiles mortared right to the concrete blocks as you went up?

pen

91LMS · Jan 6, 2012

it was mortared inbetween the liner and the block where the sections of flu went together. probably 3-4 inches either side, even at that it wasnt really packed solid just backfilled. there was never and "intention" to make it a solid mass and have no air gap. however i am thinking its "too tight" and causing the blocks to crack. wondering if this will hurt the liner or if i am going to need to tear this beast down.

pen · Jan 6, 2012

91LMS said:
it was mortared inbetween the liner and the block where the sections of flu went together. probably 3-4 inches either side, even at that it wasnt really packed solid just backfilled. there was never and "intention" to make it a solid mass and have no air gap. however i am thinking its "too tight" and causing the blocks to crack. wondering if this will hurt the liner or if i am going to need to tear this beast down.

I'm not a mason, but what you just said was my best guess.

Are there 2 separate flues here? Or were you going to try and share 2 wood stoves on one flue?

What are the interior dimensions of the flue tiles? I've seen lots of chimneys with cracks like these. My concern wouldn't be long term stability as much as if it could still protect the home in the event of a chimney fire.

I'm thinking if it were me and there is room, I'd run a SS liner down through it for extra piece of mind.

pen

91LMS · Jan 6, 2012

8X8 inside block dimension and a 6x6" liner. just a single flue chimney. dont want to put a liner it in. its only 2 months old, house doesnt even have the drywall on tongue and groove ceiling in yet. at this point if its going to be an issue i will use it this winter and tear it down and fix it. i have less in materials in the chimney than a liner will cost me. ugggghhhhh i hope i dont have to rebuild this thing!

pen · Jan 6, 2012

I'm betting for no more than 500 you could put a 5.5 inch liner down it. Just something to think about.

Also, w/ 2 appliances, I wouldn't recommend trying to run both at the same time.

Again, I'm not a mason, hopefully someone will chime in who is or else you can find a masons forum online where you can find more experience here.

Sorry, hate to see you in the spot you are in.

pen

begreen · Jan 6, 2012

Bummer. I'm not a mason, but it does sound like thermal expansion is causing the cracks. I think the cement should just be at the corners of the flue pipe if at all. The clay is going to expand and contract at a different rate than cement. If there is a solid mass of cement alongside of the pipe there may be no room for expansion and something has to give. It's also going to help transfer the heat from the tile core to the outside block, which you don't want.

As for how to proceed, the clay tiles should be inspected for cracking as well. It may be good to have a pro do this with a camera. Then you will have to decide whether to put in an insulated stainless liner or start over.

FWIW, I like the idea of insulating the clay tile liner as is goes up, which should help hold it in position.

http://en.allexperts.com/q/Chimney-Fireplaces-3286/concrete-block-chimney-construction.htm
http://extension.missouri.edu/p/G1732
http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/tt/tt980.pdf

raybonz · Jan 6, 2012

91LMS said:
8X8 inside block dimension and a 6x6" liner. just a single flue chimney. dont want to put a liner it in. its only 2 months old, house doesnt even have the drywall on tongue and groove ceiling in yet. at this point if its going to be an issue i will use it this winter and tear it down and fix it. i have less in materials in the chimney than a liner will cost me. ugggghhhhh i hope i dont have to rebuild this thing!

Many towns will not allow a stove install even if your chimney is new unless it has a S/S liner in it.. I will say a liner drafts a lot better as well..

Ray

webby3650 · Jan 6, 2012

Do you plan on using the stove and the boiler at the same time? I would say that if the block is cracked, the flue tiles will be too. It's kinda odd to see a clay flue on a new house, around here anyway, people usually go with insulated SS right from the start, especially for a stove. It's more money but you would never have to worry about when the flue will crack. Did you choose clay over other alternatives for some reason? I'm not trying to pick on masonry flues here, alot of people run them, it's just a material that often needs more attention.

webby3650 · Jan 7, 2012

greenteam said:
sounds as if you got some chimney block where they were freshly made and known as green cement they dried and you have cracks due to the drying process.
you know there is a lot of weight to support what did you do for a footing to support it? to be done right code requires a 1" air space between the clay flue and solid masonry walls? that one inch air space acts as your insulation barrier protecting heat transfer
one can use wall ties to support the flue liners place in the mortar bed of the blocks

If the structural integrity of that chimney is compromised that thin metal flue liner is not going to help.

one of my chimneys is made of chimney blocks and is 20 year old and is still in great shape
did you know the make half sections so that you break up the constant mortar joints

and did you know that many manufactures test stoves in 8/8 clay flue lined chimneys why would you line a perfectly good chimney that has been deemed optimal for usage

one other comment masonry chimneys have lasted centuries name one with a metal liner that lasted that long

Umm? SS has not been around for centuries! If it was, be assured our ancestors would have used it. A square clay has never been deemed"optimal" ever! It has been deemed acceptable, by default. Smoke forms in a column as it rises, how would a square be optimal?

webby3650 · Jan 7, 2012

greenteam said:
first of all they are not square but canford .... also clay liners can be round too they cost a little more and they come in various sizes

one can build a chimney block chimney using 6" round clay flue liners or 8"

in a 24' chimney the cost difference between rectangle clay liners and round is less that $50

Yes, but there are very, very few of them out there compared to square, ya, square. We aren't having a geometry class here, we are comparing SS to clay, it can in no way compare to an insulated SS liner, in performance or maintenance.

pen · Jan 7, 2012

Let's keep this thread on track in terms of helping the original poster.

pen

rwhite · Jan 7, 2012

Seems very strange that even if it was thermal expansion that they would all crack in the exact same spot. Even with green block I wouldn't think they would all crack in the same spot. Are they concrete or cinder block? How long did you wait after completion to fire the stove?

webby3650 · Jan 7, 2012

I think since it's a single flue chimney, if it settled, it wouldn't crack vertically unless it's attached to the house. It would bust the thimble, since it is so narrow, I think it would settle evenly. Besides this crack is only at and above the main level thimble? Correct? Is this chimney outside the envelope of the home?

Jackfre · Jan 7, 2012

This chimney did not meet NFPA211 at its construction. It has no air gap between the tiles and surrounding block. Also the tiles are likely mortared in with Portland and not refractory cement. You cannot use the same flue for two appliances. A SS liner will give you better operation than masonry anyway. Nothing I've said doesn't mean the system won't work. I'm assuming you have homeowners insurance. In the event of a calamity your system is a four lane hwy for the insurance company to bale on you.

91LMS · Jan 9, 2012

well i appreciate all of your responses, a couple things i would like to address. no this chimney will not use both appliances at the same time, meaning the boiler and wood stove. the state of maine does NOT say that it is against the law or code to have two appliances on the same flue they just dont recommend it. but yes my flue liner is put together with firestop on the seams, sakrete mortar was used for the rest of the joints. there is no way possible that a block chimney with an 8x8 clay liner (which are made to be used together) will have a 1" air space between the two, i dont care how straight it is !!!

i am not a mason personally(just made an attempt at it, lol) but my grandfather built A LOT of chimneys back when he worked as a mason and he has a hard time to believe this is happening to my chimney. processes and practices have obviously improved over the years however the old school ways still function safely and effectively. i have looked at several chimneys and asked several people about this, some say its normal and know why, they others know why and question whether it will be a problem down the road. their seems to be no visual cracks in the liner, however without a way to wash it and look with a camera its pretty difficult to see. the stove never gets run when i am not home, and i keep my chimneys very clean. at this point i think the best option is to cautiously use it, monitor and make a decision in the spring of tearing it down, repairing or rebuilding. any other thoughts are encouraged, its great to hear the differant opinions and thoughts on what is going on here. i will try to post some pics tomorrow. it seems that will relatively low flue temps (3-400 deg f.) the cracks never grow.

BlankBlankBlank · Jan 9, 2012

91LMS said:
well i appreciate all of your responses, a couple things i would like to address. no this chimney will not use both appliances at the same time, meaning the boiler and wood stove. the state of maine does NOT say that it is against the law or code to have two appliances on the same flue they just dont recommend it. but yes my flue liner is put together with firestop on the seams, sakrete mortar was used for the rest of the joints. there is no way possible that a block chimney with an 8x8 clay liner (which are made to be used together) will have a 1" air space between the two, i dont care how straight it is i am not a mason personally(just made an attempt at it, lol) but my grandfather built A LOT of chimneys back when he worked as a mason and he has a hard time to believe this is happening to my chimney. processes and practices have obviously improved over the years however the old school ways still function safely and effectively. i have looked at several chimneys and asked several people about this, some say its normal and know why, they others know why and question whether it will be a problem down the road. their seems to be no visual cracks in the liner, however without a way to wash it and look with a camera its pretty difficult to see. the stove never gets run when i am not home, and i keep my chimneys very clean. at this point i think the best option is to cautiously use it, monitor and make a decision in the spring of tearing it down, repairing or rebuilding. any other thoughts are encouraged, its great to hear the differant opinions and thoughts on what is going on here. i will try to post some pics tomorrow. it seems that will relatively low flue temps (3-400 deg f.) the cracks never grow.

91LMS, I'm sorry to hear that your new chimney is cracking. Reading your initial post and responses, the one big problem is that the clay flue liner is mortared to the concrete block. Masonry chimney flue liners are intended to float independently within the concrete block. Usually a wire armature is set in the mortar bed every few courses to guide the flue liner to maintain its center relative to the concrete blocks (or brick for that matter, if bricks are used). This allows the flue to expand and contract independently of the concrete block (or brick).

Another thing to consider is the chimney cap. There should be a gap between the exterior face of the clay chimney flue liner and the chimney cap. This space is then filled with mill pack and then sealed with a high temp silicone caulk. Since a chimney flue liner will lengthen and shorten depending on its temperature, the mill pack and silicone acts both to seal and as an expansion joint. This joint also allows for possible differential settling between the liner and block.

I recommend posting pictures of the cracks, thimble, chimney flashing, and chimney cap. Also, it would help to see a picture of the house framing relative to the chimney. There must be a 2" gap around the chimney where the chimney passes through a ceiling or where a chimney runs up a wall. Some of this might be a little departure from your initial problem but the more information you post, the more accurate the responses will likely be to your issues.

Lastly, a chimney flu cannot be used for both solid and non-solid fuel appliances.

91LMS · Jan 9, 2012

the floating liner design makes perfect sense to me. however not every chimney was built this way and plenty function properly and dont crack or have issues. but at this point pretty difficult to dispute the fact of the thermal expansion of the liner and not sufficient clearance to the blocks were/are my issue. back to when this thing went up, there wasnt a damn flue other than maybe 1 that was square on both ends making it extremely difficult to keep it perfectly straight inside the blocks.....

91LMS · Oct 16, 2012

it was evident to many what happened in my chimney but during the tear down it was beyond obvious. without a question the chimney was too tight, meaning mortar between the liner and blocks. areas where the blocks crack there was 100% contact between the two. i opted to tear down after 1 season and rebuild. my liner actually cracked at the thimble. i am assuming that heat made the liner grow vertically but being solid below made it want to break in half??? other than that there were no cracksin any other tile just blocks above the heat source. most likely people havent made the same mistake as me but if i can help anyone avoid this, i hope that i have helped!

91LMS · Oct 17, 2012

could anyone point me in a direction where i can buy the mill pack and high temp silicone to seal the cap to the liner?

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