Questions for the chimney sweep / installer

[Ashful]

So, I have another chimney sweep coming out to look at my ill-fated chimney. The one that the last company tried to convince me there was wood inside, and then later agreed there was not. Now they're claiming I can't use the chimney because there are combustables (roof rafters and sheathing) too close to the OUTSIDE of the chimney.

Again, the goal is to install a new 6" liner for a Jotul Firelight 12. I assume it will be an insulated liner, as this is an old and rough stone chimney built into an exterior wall. Flue diameter is enormous (18"-ish), although it does have a slight jog near the roof line. Should be no problem for any liner I will need. Chimney height is 29 feet from the floor of the firebox, if I recall correctly. The stack is topped by a large slab of flagstone, setting on brick pedestals.

I am also looking to have a 6" insulated liner installed in my much shorter 8" ID clay tile lined exterior masonry chimney on my other Jotul Firelight 12. I believe that clay flue is only 13 feet, plus 6 feet of stove pipe from the blockoff plate down to the stove. This chimney is also topped with flagstone sitting on brick pillasters.

What are the questions I should be asking this guy? I don't know a whole lot of particulars on things like blockoff plates, caps, liner vs. double wall chimney, etc., etc. I believe this guy is dead-honest, as I've dealt with him on more than one occasion before, but I'm not sure he's as well equipped to deal with a complicated install as the first company was. While the taller flue is ony 29 feet from the floor of the firebox, it's more than 45 feet above the ground outside. That likely means erecting some serious scaffolding, to deal with the large slab of flag stone topping the chimney.

 

[webbie]

95% of masonry chimneys are built incorrectly - in fact, even if they were built correctly, the code has changed and they would not pass muster. But we have to use common sense in these matters!

In a nutshell, if you install a 6" ss round liner which is insulated, it should properly upgrade the vast majority of masonry chimneys to a very safe level. The exception might be a chimney, as you mention, with a wood beam actually inside the chimney structure....

Let me illustrate what would have to happen to make some wood combust on the outside of a 5 inch plus masonry wall......

Firstly, the 6" pipe you are putting down the chimney is round - so it radiates evenly in all directions 360 and, even if it touched masonry (it does not, because of insulation...and proper installation plus rough chimney interiors also means plenty of air around it), the amount of heat which would have to soak through is almost off the charts.

I have monitored wood burning chimneys in the old days...before liners were common and in interior chimneys, and the outside of the chimney structure almost never got too hot to touch. Now, take a better chimney - add in the insulation power of air (which is why double wall pipe which is air cooled has close clearances), add in insulation and add in almost no direct flat-to-flat radiation pattern, and you can see what I am getting at!

I guess the most important things to ask are:
1. Can he use rigid pipe? This is better, IMHO. What brand? (you can check on the rep of the particular brand).
2. Is he insulating it to UL standards and/or is it pre-insulated? (either is OK).
3. Is he going to make a sheet metal plate for just above the lintel area of the fireplace (block off)? This will certainly make sure you don't lost heat up the chimney structure.

 

[Ashful]

Thanks, webbie. I agree with everything you said. The outside of this chimney will not get hot. In additon to all of the factors you list, it's heat sunk to an enormous 18" thick solid stone wall. However, the installer is going by what's tested, approved, and code, not by common sense.

I was going to request that the blockoff plate be installed rather high, approximately 8 - 9 feet off the firebox floor. This is because I have a thimble thru the front wall of the fireplace, and want to ensure that thimble remains below the block-off plate. I want to experiment with venting the firebox using this thimble to move trapped hot air out of the fireplace and into the room.