Cracks in CMU Chimney

[techinstructor]

Our new home has a 40-foot, concrete block chimney with two ceramic flues. The chimney originates in our basement and goes straight up through the main floor and the attic floor and extends about 4’ above the roof. The chimney’s outside dimensions are 32” x 40” and each flue is 9”x12”. The chimney appears to be plum, but there are up to ¼” differences in the edges of the face of the blocks in many places. There are also many places where the mortar has been patched. We used a mirror to look inside one of the flues and found it to be splattered with mortar.

The chimney was completed in March of 2009. We began using one of the flues in October, starting out with small fires. We are using a woodstove in the basement flue to heat the house while we do the finish work. We have not yet had any fires that would use the second flue.

After about a week of use, we noticed cracks in the mortar and in the block of the chimney. There are small cracks above where the flue thimble is located in the basement on the side of the chimney where the woodstove is located. On the main floor there are cracks on the side of the chimney and along the center of the front. The front cracks run through the block and then along the mortar. The largest cracks are in the center of the chimney on the attic floor. My husband has noticed that the cracks have widened as we continue to use the chimney.

http://lh6.ggpht.com/_JWULuIxmfCo/SvXFmebzAZI/AAAAAAAADHU/eAdfQVXLb6Y/s640/PA210019.JPG

We’ve already paid the contractor for the work on the chimney but have not yet had an HVAC inspection done. Here are the questions that I have:


What factors, other than heat, could have caused the chimney cracks?
What are the long term ramifications?
Can surface bonding cement, such as Surewall or Quikwall, be used to strengthen the chimney? How will this material react to heat?
Will the chimney pass HVAC inspection in its present condition?
What can be done about the cracks, short of tearing it down and having it rebuilt?

Thank you in advance for your replies.

[Don_P]

I've seen a chimney crack opposite the thimble when the flue tiles were mortared in solid between them and the block. As the liner heated and expanded there was nowhere for it to expand to and so the chimney cracked. It progressively got worse and was replaced.

I did try to patch a cracked chimney with surewall once, don't know if it would have worked, the inspector said he understood what I was trying to do but that it had to come down. YMMV

[rwanders]

I agree, cracks in chimney will only get worse and the chances of a chimney fire or flue gas seeping into the home will only get better-----would not feel good about assuming that liability from the builder.

[gkdesigntech]

The crack shown in the photo is very typical for any CMU construction and could have been caused by any number of things. The imporant element of the chimney is the liner. Provided the liner is in good condition, there isn't really any danger from the crack that's shown. The outer CMU on a chimney is just for thermal isolation from the liner and isn't expected to hold up if the liner has any cracks or breaks. I would have a chimney cleaning company do an inspection, preferrably with a TV camera.

[gkdesigntech]

Oh, I forgot to mention - I doubt if there's any bonding cement that's rated for high temperature exposure, but you'd definitely want to check before using it. Even if you did successfully repair the crack, whatever stresses that caused it, if still present, will just cause another crack somewhere else.