Duraplus through wall thimble acceptable temperature

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New Member
Oct 31, 2023
I just installed a Duraplus triple wall chimney with their through-the-wall kit. In order to avoid having to cut through the roof eves for the vertical section of the new chimney, I used a horizontal 24 inch section of the triple wall, going through the wall thimble to the T connecting of the exterior vertical chimney. (Duratech told me 24 inches horizontally was OK). My old chimney installation had used a SafeT thimble with about 2 inches of rock wool sandwiched between stainless walls, with stove pipe running inside to a masonary/stone chimney. That thimble never felt more than slightly warm even with a hot fire.

On the new Duraplus installation, the horizontal section of triple wall was installed through the wall thimble per directions, touching the interior facing of the thimble. The interior stove pipe is secure onto the provided adapter, but since these are slip connections (with short screws added for security) I also used furnace cement at the joints to make sure there could be no leakage of gases. The Duraplus thimble itself is simply a round metal drum with two sections that telescope together so you can adjust for house wall thickness, but the thimble has no insulation. I presumed that the triple wall section going through the wall, which obviously DOES have insulation, along with the air gap between the triple wall and the thimble wall, would prevent any excessive heat from reaching either the metal wall of the thimble or, most importantly, the wall studs. With my first moderately hot fire today (thermometer measured around 325-350 at the stove collar outlet) I was surprised that the top of the thimble (which sticks the required 2+ inches into the room) felt hot. I didn't take a thermometer reading there, but it was too hot to hold my hand there more than 5 seconds, so probably at least 150 degrees. The bottom of the thimble inside was only warm to touch.

I was even more surprised when I went outside that the top of the triple wall horizontal section as it exited the exterior house wall also felt uncomfortably hot on top, even though the outside air temp was around 32 degrees. Eight inches further out from the exterior wall it only felt warm but not hot. So my question is whether I should buy a section of 1" thick ceramic fiber blanket, take off the outside cover plate of the thimble, and wrap the horizontal triple wall with it, still leaving an air gap between the insulation blanket and the wall of the thimble. After reading about pyrolysis lowering the combustion temperature of wood studs, maybe I've gotten a little paranoid. I would contact Duratech but their techncial guy, who was friendly and answered my initial installation questions, has never returned my subsequent voicemail requests. I think at least one other poster on this forum had a similar experience - as he put it "you get one question per customer from this company".

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