View Full Version : Cracks in flue

Phillip Stojanik
07-14-2007, 08:34 PM
The problem here is obvious. I am curious though as to what might have caused this kind of cracking?

I'd like to hear from anyone about this but Bob H. or Dale F.'s comments would be most appreciated!

James Duffin
07-14-2007, 08:36 PM
You got a picture?

Phillip Stojanik
07-14-2007, 08:37 PM
OK I guess the photo would help!

James Duffin
07-14-2007, 08:45 PM
As clean as the flue is I would guess it is used for gas appliances. Could the cracks be from overheating?

Phillip Stojanik
07-14-2007, 09:00 PM
As clean as the flue is I would guess it is used for gas appliances. Could the cracks be from overheating?

Actually its a wood burning fireplace but does have a gas starter rail installed. I was thinking about high heat since there were no cracks to be found in the brick chimney structure itself. Even still, a clay flue liner should be able to withstand quite a bit if heat without cracking.

Other issues include a lack of a rain cap/spark arrestor but I don't see this cracking to be the result of moisture damage due to a lack of a rain cap (I stand to be corrected though if others know better!)

Moisture damage was however apparent in the firebox in the form of efflorescence and deteriorating mortar joints in the firebrick. I attribute this to the lack of a rain cap over many years (along with a very rusty damper assembly).

Bruce Breedlove
07-14-2007, 09:18 PM
Any evidence of a previous chimney fire?

Phillip Stojanik
07-14-2007, 09:40 PM
Any evidence of a previous chimney fire?

Nope. All original construction 40+ years old

Nick Ostrowski
07-15-2007, 12:21 AM
That's an odd look for the inside of a fireplace flue in that it appears there is a terra cotta liner entering the bottom of the flue horizontally (or am I seeing things).

As for the cracks, it may have been a one-time overheating of the flue interior from an oversized fire? The interior looks to have barely been used.

wayne soper
07-15-2007, 03:42 AM
It almost looks to me that the chimney moved, or shook ( earthquake)? due to the even cracking through different sections.

Phillip Stojanik
07-15-2007, 04:01 AM
No earth quakes here on the gulf coast, just hurricanes and tropical storms.

Jim Gecz
07-15-2007, 04:40 AM
Bet the smart kids in the class are squirming in their seats!

My take on it is a quick and intensely hot chimney fire. I see dark discoloration at the bottom flue section. Perhaps a creosote coating in that section flared. Looks like heat signatures, seen on the upper most section left side. Fast and hot meets cold and brittle resulting in thermal shock cracks.

Is this for a wood stove? Isn’t that a chimney connector sticking way too far into the bottom?

Thanks for the photo challenge!

Eric Barker
07-15-2007, 06:14 AM
I would tend to lean toward sudden thermal expansion. It is possible to crack tiles when starting up the fire. If the fire is built up too quickly the tiles are exposed to sudden high temps which can certainly damage them.

Bob Harper
07-15-2007, 06:43 AM
Cracked flues can be tricky and the popular answer is that it suffered a sudden thermal shock such as a creosote or "chimney fire". What you must rule out are damage prior and during construction such as mishandling and seismic events. One cracked tile alone may be from construction such as kicking the tiles off the delivery truck or dropping them. However, longitudinal cracks in the corners are more indicative of thermal shock. The fact these cracks extend from one tile down to the next would seem to fit the thermal event vs. mishandling. Seismic damage is Dale's specialty since he lives out there so I'll defer to him on that. There appear to be dark stains in the cracks, which would indicate creosote deposits while the tile opens up during heating much like a clam shell. This would seem to indicate there was the initial event then continued use afterwards.

Chimney fires are rarely reported or observed by homeowners. They tend to last about 20 minutes so you could be in the shower and never know it. Often, the homeowner notices the chimney is suddenly filling the house with smoke where it used to draw just fine. They call a sweep, who removes a huge plug of pyrolized creosote. When creosote burns, it can puff out like a croissant, blocking the flue. Many sweeps are keen to spot this though some aren't. Let's say this chimney was swept. If there is no rain cap, the water can rise the creosote off much as you see here. BTW, wet creosote run off can seep into those cracks without needing heat to induce enough hoop stress to open the clam shell.

Yes, that 8x8 thimble is protruding into the flue. It should be cemented flue with the inner face of the flue wall. It also appears to be damaged.

So, was this taken at the top of the chimney? Are you telling us this is a 6 foot tall 13"x13" chimney? That's a functional problem right there as it is way too short to generate sufficient draft. What about roof clearance? A woodstove with a 6" collar cannot vent into such an oversized flue. Doesn't appear to have a cleanout either.

I'd recommend a Level II inspection. They would be a lot better off if they tore this chimney down and replaced it with a listed factory chimney of proper size and height.


Dale W. Feb
07-15-2007, 12:45 PM
There are only two events that would leave this pattern on the clay liners, lightning strike or chimney fire. You already eliminated a lightning strike as you stated that there is no damage to the exterior brickwork. Always look closely for repairs on the exterior brickwork for both lightning strikes and chimney fires. The lack of creosote deposits leans toward a chimney fire. The one key bit of evidence is, how many time has this fireplace/chimney been used? We should ask the current home owner and look within the smoke chamber and firebox area for evidence of use and deposits. If they have used the fireplace more than twice then there has been a chimney fire. There will be deposits visible on this liner (more than we see). A chimney sweep will remove excess amounts of deposits but will not CLEAN the chimney. There are no chemicals that can clean the clay liner to this degree. There would also be evidence of chemical use (possible etching or uneven streaks). To clean the liner surface as viewed within your photograph, we would have to use the best cleaner around, FIRE. Fire is the only cleaner that will result in the current condition.

In this case, "lack of fuel for thought"

Rick Hurst
07-15-2007, 01:57 PM

I may be way off base with this, but have you ever heard of or seen a flue cracked as such and be related to possible structural movement?

Just curious.

Phillip Stojanik
07-15-2007, 06:51 PM
Thanks guys. I appreciate all of your replies.

Dale W. Feb
07-15-2007, 07:33 PM

Yes there are times that product stress will result in damage. However, this damage is usually isolated to a section of the system. The section affected is usually the weakest point of the system or the area with the most stress. This picture indicates a fairly consistent failure of the product which would eliminate that possibility.

Bob Harper
07-15-2007, 09:51 PM
An example of what Dale is referring to is when a large tree or brach hits the chimney causing it to shear at the roofline. I saw this a lot after hurricane Hugo. Also, on large chimneys that are high aspect/ ratio there is considerable wind pressure, which can cause damage right at the roofline. One interesting bit of storm damage was where a tree limb struck the side of a chimney with enough force that the inertia caused the flue liner to pop directly in line with where that big branch hit, just like a hammer on a plaster wall.

Dale W. Feb
07-17-2007, 10:53 PM
Bob, Your medical experience might also help us. I was watching the discovery channel and found out that a high impact head trauma can result in a correlating bruise on the opposing side of the skull. I cannot remember the name of it but it’s a good example of the tree branch impact. This action on a chimney is less likely when the chimney wall is hollow verses solid masonry throughout. The solid material acts as a conductor. It’s like the difference between and electrical current running through tin verses gold. It doesn’t eliminate the possibility but it does reduce the probability.

Any input Dr. B ?

Bob Harper
07-18-2007, 04:25 AM
You just had to ask me a medical question---now you're in for it!

Dale is referring to a "contra-coup" injury to the brain. Think of a bell clapper. The bell goes one way but the clapper the opposite. In a contracoup brain injury, there is an impact to the head, let's say the right parietal lobe (over your right ear). Normally, you would expect damage to the right lobe to affect your left side similar to stroke victims. (yes, our brains are all cross-wired, not just mine). However, instead of the victim showing weakness to their left side on an injury to the right side of the head as you would expect, they show right sided weakness. That means when the impact occurred, the brain sloshed around inside the skulll (you have almost one millimeter of extra space--some argue I have a lot more space than that) and the brain literally slams into the opposite wall of the skull causing the damage away from the initial impact.

If a flue tile is installed per the code with a 1/2"-4" airspace, there is the potential for some movement inside the chimney. As it swings over, one of the first things it can do is open a mortar joint much like a bird's beak. I think Dale is referring to damage a little more impressive that just an open mortar joint though. For you left Coasters, you don't have the airspace due to seismic requirements. In this case, the inertia may be transmitted over to the opposite side of the chimney.

There Dale, that's the SHORT version. I tried to use my brevity but I still can't find it!

Phillip Stojanik
07-18-2007, 06:52 AM
OK Bob,

You're now my go-to guy here at inspectionnews for brain injuries too!

"Certified Hearth Professional"? Is that "r" a typo? Was it supposed to be "Certified Health Professional? :D

wayne soper
07-18-2007, 07:15 AM
Hey BOB, Is it "contra coup" when you speak to a broker and your words go in one ear and right out the other. Or is that "contra foo"?

Bob Harper
07-18-2007, 04:12 PM
With Realtors, I think it would be "contra-pooh" ;-)