Linered Block Chimney Install Questions...

billw... I checked out the links at the bottom of your posts and have a couple questions about your chimney install.

A couple years ago in a previous life 2004bc (bc - before coal) when I was burning wood I built a block chimney similar to yours. It seems as though I started my liner installation much the same way by mortering it in place to keep the bottom pieces stable and stationary for the stack above. At the end of that burn season I was not burning wood daily because the house was too warm but one particularly cold day I started a fire. The usual practice was to open the ash door and start the fire... once the fire was going good (about 20 - 30 min) I would close the ash door and then walk away checking the fire every hour or so until I knew everything was stabilized and such.

This was a wood furnace that I had ducted into the oil furnace ductwork. After about 45 min I noted that the blower hadn't shut off at all indicating that there was a lot of heat in the system. When I opened the basement door the first thing that I noted was that wretched smell of something very hot. When I walked around the wood furnace, the flue pipe was cherry red all the way to the thimble. I always kept a bucket of ashes nearby for emergencies like this so I smothered the fire with them and got the situation under control.

The short version is that the fire box rusted out and caused the fire to burn out of control with the extra air.

Here's my question... after that long story. I went outside to see if there was fire coming out of the chimney, which there was none. But at that time I noticed a big crack at the base of the chimney starting about two or three blocks up and continuing down below ground level. I was baffled and disappointed that it seemed as though the money and the blood sweat and tears was wasted and the chimney was ruined.

I stopped burning wood that year but I still ponder why the chimney cracked. Some suggest that because I mortered the liner in place down low and the clay liner expands more than the cement block that this is what cracked the chimney... there was no room for the clay liner to expand.

Does anyone out there in NEPA land have any experience with this sort of thing?

If the crack went bleow grade perhaps the fire had nothing to do with it and it was a footer failure or a soil bearing problem. The footer could be intack but the soil beneath failed and freeze thaw cycle brought it to the surface. Just a possibility. Other than that I would say the heat played a part.

frank

Footer failure is possible but if you didn't notice any cracks before the fire, the extreme heat and expanding of the flue liner probably cracked the block. The flue lining should be "free floating" of the block and preferably a refratory mortar between flue sections especially when using a woodburner. Possibly use a mirror to check the condition of that flue lining if you would ever decide to switch back to wood. The cracks in the block are best sealed with a silicone or polyurethane based caulk to prevent further damage.

oldAA130,
Is your coal boiler connected to this flue? I think the chimney (should) have been able to withstand the heat if it was constructed properly.
It could be a footing (foundation) problem as was mentioned above.
How big is the footing you poured for the chimney? Can you tell if the chimney has sunk at all?

Cracks in masonry or concrete slabs are often blamed on soil material settling or some sort of footer failure, but are actually most often caused by expansion and contraction and then worsened by moisture penetrating the cracks and freezing. An expanding flue often will cause chimney damage if it is mudded in tight to the block. Most often it happens at the top where the flue is usually mudded tight to the top block or cap to form a seal. It is common to see the top 2-3 mortar joints on a block chimney cracked due to this.

This happened at a house I moved out of about a year ago. The house heating system was switched to oil boiler after I left (wood burner was removed prior to leaving).

The footer was solid. I replaced a chimney that stood on the footer for 50 years. The only thing it could've been was the heat expanding when the wood burner overheated. I saw the attachment at the bottom of posts from billw and it sounded like he installed his first liner very similar to mine. I was just curious if he or anyone else had experienced a similar problem.

At my parents house as a youngin' I remember the chimney going up the back of dad's house was cracked all the way up the middle. This goes as far back as I can remember. Is it normal/common for the block to crack?

So how do you keep the bottom piece of liner steady? And how would refractory mortar be any different than regular mortar if it fills the gap between the clay liner and the blocks? Wouldn't you sitll have the same issues if you experienced a runaway/overheating situation? I'm not sure I understand this.

I was going to use the insulating beads when I built this chimney but the cost was much too high.

Thanks guys for the feedback.

Tom

refractory mortar between flue sections, not between flue and block. sorry if I was unclear