Chase area around wood fireplace gets too hot

[NVHunter]

My chase is going to be ten or so feet tall with 6" duravent duratech running through it. The chase isn't going to be very big (18"x36") so I'm going to line the inside with rock wool or some other non combustible material while maintaining minimum clearances and then vent the chase at the top. Hopefully this will keep it warm enough but won't have it so hot that it's causing issues with the combustible framing and outside siding.

Don't know what else we can do.

 

[Mr_D]

My chase is going to be ten or so feet tall with 6" duravent duratech running through it. The chase isn't going to be very big (18"x36") so I'm going to line the inside with rock wool or some other non combustible material while maintaining minimum clearances and then vent the chase at the top. Hopefully this will keep it warm enough but won't have it so hot that it's causing issues with the combustible framing and outside siding.

Don't know what else we can do.

That sounds pretty good and there's tons of options since it isn't built yet.

Rock Wool passes air fairly easily so naturally it needs an outer wall that functions as an air seal of some kind to limit air intrusion and heat loss when it's not running. I presume you're planning on drywall protected by rock wool (or other high temp insulation) as a reasonably air tight and insulated wall.
That should be better than the average install.

Usually insulation goes between the studs of course, not on top of the drywall so that portion of it isn't traditional construction in that sense but protecting the materials has to be considered in the design. I'm reading up on MinWool-1200 Industrial board and Thermo-12 Gold insulation to see if it's ideal for this type of application or not.
Both are rated to 1200F and come in all shapes and sizes. Neither is as cheap as Rock Wool though.

 

[NVHunter]

Mr-D have you found anything new with the chase?

I was reading my Duratech chimney manual and it calls for the chase flashing to have 1/2" spacers it sits on for proper ventalation as well as 3/4" of open space all the way around the flashing and the chase structure.

Also the chimney opening on the chase flashing is 11" in diameter, leaving an inch on each side all the way around the chimney for further heat ventoff.

Does your system have this?

I'll try and attach a photo from the manual.

 

[Mr_D]

Mr-D have you found anything new with the chase?

The Duratech chimney manual calls for the chase flashing to have 1/2" spacers it sits on .....and 3/4" of open space all the way around ....
Also the chimney opening on the chase flashing is 11" in diameter, leaving an inch on each side all the way around the chimney ..

Does your system have this?

No. This one doesn't have that much vent clearance.

The chimney has a vent at the top but it could probably use more air flow. You can see the clearance in my photo on page 1 of this thread called "top radiation shield.jpg" and "vent view.jpg". The clearance between the chimney and the radiation shield can be seen and it's not a lot. It also has no 1/2" spacers as NVHunter mentioned, it's tight up against the ceiling and only vents around the pipe.

The RSF Focus 320 install guide calls for 2" clearance to combustibles around the pipe which is fine but the top radiation shield looks like it has only 1/2" of clearance for air flow between the sheet metal and the stove pipe. Also the final exit at the top is limited by a series of small, oval slots visible in the earlier photo's and that's just not much vent area which could be half the problem.

New Chimney Sleeve: (same one oconner mentioned actually)
RSF finally came up with 1 suggestion after 2 months of waiting.
They suggested the local dealer install an ICC brand 11" metal liner around the original 9" (7" ID) stove pipe to lower chase temperatures. Apparently they use a liner around the chimney for installs in Canada but the US doesn't have the same requirement. The additional metal sleeve will vent out the top and I'd expect it to reduce the heat in the chase somewhat. The chimney assembly will then be like a triple wall but the outer section of air goes out the top.
RSF didn't provide any specs on how much the temperature should drop.

It's not clear how the new liner will be vented because if it vents back into the chase at the top all the heat captured by the liner would dump back into the chase. Meanwhile if it only vents the liner portion then the chase doesn't have a vent anymore. The chase probably needs to vent not only for chimney heat but also for summer heat if it's closed in with no ventilation. Any thoughts?

New firestop to be installed:
At my suggestion the local dealer agreed to work with me to install a firestop half way up the chase since it has no firestop in their original recommended design.
With the chase open from the fireplace all the way up to the chase top 27 feet above we lose a lot of conditioned air out of the vent at the top.
That's why I pursued having a firestop installed, not only for safety but also to buffer the conditioned air blowing out through the chase top.

The tall chimney with no bends and a tall chase with no firestop causes a continuous draw on the house making the whole structure negative pressure. This makes portions of the house colder (like an open fireplace) as outside air has to replace all the air that goes out the top. The house is fairly well sealed (maybe too much) but with that much negative pressure the house now cools off a lot faster than it used to with the influx of outside air getting pulled in. The indoor humidity also dropped from 60% last year to 30% this year due to the high air exchange rates. I'm hoping a midpoint firestop that buffers the air will fix the heat loss

Sealed radiation shield ?
The dealer plans to install a vented firestop with a 2" air gap around the chimney. The firestop gets installed about 10ft above the fireplace and the new 11" metal liner would extend from the firestop up to the chase top. I proposed they install a sealed radiation barrier at the firestop instead to limit the house air going out the chase top. RSF makes one just for that purpose for efficient houses where it maintains the vapor barrier integrity. Then if necessary we add a small intake air vent just above the firestop to supply outside air for the new 11" liner to vent with.

The dealer plans on using a vented firestop which can only vent by consuming conditioned house air unless I somehow convince them to use the sealed version instead. Their plan would waste house air 24x7 forever whether the fireplace is on or not.

Chase wall protection
It looks like they'll have to remove the chimney and top radiation shield to get the new 11" liner installed, When the chimney is out there would be room to install wall protection in the event that the new liner/vent doesn't lower the wall temps enough. I'm considering installing 2" mineral wool board such as IIG "Minwool 1200" or Roxul RHT-80 board to line the chase walls. 2" rock wool board at R-4 per inch would lower temps enough to protect the drywall by dropping it about 50°F which would bring the drywall temps back into the safe range below 125°F. Rock Wool board is rated for 1200F operation and doesn't melt till 2000°F. The Roxul version costs only $.40 cents/ board foot at 1" thick or 80 cents for the 2" version so this is cheap insurance. I can do the whole chase for $100 + shipping,

The rockwool board would add a significant safety factor for the chimney and also protect the drywall from long term heating so it's fire rating remains intact.

If the dealer agree's and my logic is on track the chase redesign accomplishes 5 things:

• Sealed Firestop - adds better protection for the house and keeps conditioned air in the house rather than venting it out the chase top.
• Metal chimney liner lowers chase temps to manageable levels. (might require dedicated intake air vent).
• 1200°F Rockwool boards for chase wall adds protection for fire events and lowers drywall temps to safe levels below 125°F.
• Upper chase area can be cooled with outside air without affecting the house or conditioned air.
• Lower chase heat can be redirected back into the living space via passive vents for more efficient heating.

I know this level of planning, safety, and thermal efficiency is past what the dealer will care about but it should be a good overall design.
We'll see. Right now we're waiting for the proposed 11" liner to arrive.

 

[E Laszlo]

Wow, the suspense is killing me! Any updates? I'm building an interior chase in a new construction house right now. After reading this, I'd like to incorporate some of your proposed designs... if they work. Keep us posted!

 

[Mr_D]

It worked!

The high temperature's inside the upper chase area are resolved.
This is perhaps one of the few installs that always has a warm chimney but it never gets too hot either.

The modifications I mentioned before have been completed and the chase walls now runs about 100°F cooler than before.
Original chase wall temps were hitting 210°F with the firebox loaded at 60% full and the air damper set at medium.
Running the air damper fully open or burning a larger load of wood such as is shown in their product brochure would have easily hit 230F or more inside the house walls.
Now it runs 90-110°F inside the chase enclosure and the heat loss of conditioned air exiting out the top is also resolved.

After waiting 4 months for RSF fireplaces and the local dealer to come up with a solution they implemented a combination of solutions.
Then the local dealer tried to charge me an additional $1800 in labor and parts on their 2nd service call to fix the issues that have been there since it was installed last October. Fortunately I was able get the labor portion of the 2nd bill removed since they can't really charge 26 hours labor to come back and fix it since it was not installed in a safe and code complient manner in the first place. They were trying to charge for installing inadequate venting and removing it as well as the time to install the right one plus all the original and adjusted components as well. They wouldn't have had to come back at all if they knew more about the heat generated by these systems and gave adequate install guidelines but they had never really tested and evaluted an installed wood burning fireplace before.

This install took a lot of research and calculations regarding convection and conductive heat transfer and high temp material choices. The local dealers install crew also learned a lot and while they may not routinely use all of what was implemented I'm sure that future installs in this area will be safer due to what we learned on this one.

Here is how we fixed it.

New Chase top with better ventilated flashing
The original top vent had rows of small holes with minimal surface area. The new ventilated flashing at the top of the chase has at least 5x the surface area. This should have been used from the beginning.

Chase wall protection
I lined the chase walls with 2" of Rock Wool 2000°F rated Insulation. The R-8.5 insulation value (at 2") protects the drywall from chimney radiation and dropped the surface temperature of the drywall 50°F by itself, almost down to safe levels. 210°F readings are now down to 160°F after this modification. The rock wool was cut for a snug fit and held in place with long decks screws capped with 1.5" washers.

11" ICC brand metal liner
We installed an 11" metal liner pipe around the original 9" (7" ID) dual wall stove pipe. This is an original, optional factory part that they routinely install in Canada but not in the US due to less stringent requirements here. The metal shields the walls from the chimney heat radiation and provides a 1" air buffer around the chimney that allows the heat to go out the top through the new ventilated flashing.
This retrofit lowered the walls temps another 40F down to around 125F now.
We added seven 1" air intake holes around the bottom perimeter of the new 11" liner just above the firestop. This allows it to pull air from the chase area into the liner and exit out the top as it's heated. Upper chase makeup air comes in through a new 4" outside air vent located half way up the chase exterior wall just above the firestop and that air gets pulled into the bottom of the new liner via convection through the 1" air holes we added. In actual use the 4" side air vent was larger than needed and it chilled the chase area considerably. I reduced the upper chase air intake vent down to a 2x3" size and that's working good.

New top air vent for the chase area
The new upper chase air intake needs somewhere to vent the heated chase air into so we also added a 5" round, screened air vent that goes out the chase top. It exits through the ventilated flashing. This allows any heated air to exit out the top using convection only and it works fine. There's no humidity issues inside the chase anymore because conditioned room air is no longer being dumped into the chase as cooling for around the chimney system as was required in the original,inefficient, inadequate design. The new design with a dedicated chase air intake just above the sealed firestop resolved the problem of major conditioned air loss out the chase.
Now it now uses outside air just like the stove combustion itself.
The new air vent design and other adjustments maintains a chase temperatures around 90- 115°F with no fans or automatic electronics needed. The chimney pipe itself is still nice and warm too due to the new liner keeping a 1' warm air buffer around the chimney. I measured 515°F coming out the top of the chimney pipe above the roof while the flue temp at 1ft above the fireplace was at 600°F during a typical size fire. 515°F coming out the top should maintain a nice clean chimney.

New Firestop
A Firestop was installed half way up the chase. It's constructed of 2x4 framing with a floor of Durock board backed with 4" of industrial rock wool. This component was missing in the original design and now it serves 2 purposes, it blocks conditioned air from exiting the chase and of course it serves as a fire stop as well.
It's a 2000°F rated firestop to go along with the fireproof walls.

Air vents... Lastly I need to install some passive air vents that allow the chase area in the living room to dump it's extra heat back into the room. I see temps at the top of this section around 110-120F and that warm air may as well contribute to heating the room rather than staying in the chase. The air vents will be just below the cathedral ceiling with another vent near floor level.

 

[E Laszlo]

Well done... and thanks for the update!!