Connecting Duravent Chimney Adapter to Existing Masonry Chimney Thimble Pipe

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New Member
Oct 3, 2021
I'm glad to have found this forum. I recently purchased an old home that previously had a wood burning stove vented into the masonry chimney in the living room. I just purchased a ComfortBilt HP61 pellet stove and tried to hire a professional installer to connect the pellet stove to the masonry chimney thimble but none of the 7 installers I've contacted so far will install the stove because I didn't buy it from them. I've been waiting for my local heating and cooling company to get back to me (they said they could help) but it looks like they are going to let me down also. It's looking like I'll have to do the install myself. I don't have any experience doing this so I'd be deeply grateful for your help. I have done extensive Google searches and have been scouring this forum but haven't come up with a solution yet. It's getting cold here in the upper Northwest and I'm hoping to get this installed and working ASAP.

I want to vent my pellet stove into the existing masonry chimney. My masonry chimney is between 20 and 30 feet high and has an existing 6 inch oval stainless steel liner installed (I have no idea which brand the liner is). A chimney sweep and ComfortBilt both recommend that I use 4 inch pellet stove pipe up to the chimney thimble for better air flow given the height of my chimney and the size of the liner. I have all of the Duravent PelletVent pipe to go from the stove to the thimble. I also have a 4 inch to 6 inch Duravent chimney adapter (4PVL-X6) to connect the 90 degree Duravent elbow to the thimble. My issue is that the inside diameter of the Duravent chimney adapter is 6 1/8" but the outer diameter of my thimble pipe is approximately 5.75" so sliding the chimney adapter over the existing thimble pipe has quite a big gap at the connection. The thimble pipe coming out of the chimney is also not perfectly round because the round thimble pipe where it connects inside to the chimney liner looks to have been forced into the oval opening of the liner. This causes the gap between the thimble and the chimney adapter to be uneven where they need to connect.

I need to find a good way to be able to attach the chimney adapter to my existing thimble pipe so that it won't leak and still be removeable so that a chimney sweep can vacuum out the chimney after sweeping it. I contacted Duravent who suggests that I cut my existing thimble pipe shorter so that the chimney adapter goes over the thimble pipe and butts up against my trim ring, secured with screws (see the attached image showing my thimble pipe and Duravent's sketch showing cutting the thimble pipe and putting their adapter over the pipe). As explained earlier, there is a large gap due to the difference in size between the thimble and the chimney adapter and the fact that my thimble is not perfectly round. Do you have any ideas on how I can make a good connection between the two without using a bunch of hi temp silicone to fill the gaps (which would prevent a chimney sweep from removing the chimney adapter later to vacuum the chimney)? I was looking at using some kind of other adapter to make up the gap difference and found the Duravent 6DFS-X55 5 1/2" to 6" increaser/reducer (inner diameters) but I'm not sure that this would work without having the adapter here physically to try it. Perhaps you might have a better idea that I've haven't thought of yet.

Many thanks for your help!

Thimble 1.jpg Thimble 2.jpg Chimney Adapter 1.jpg Chimney Adapter 2.jpg Chimney Adapter to Thimble Sketch.png
I think that I have a solution that will provide an adequate connection to my existing chimney thimble. I bought a stove pipe coupler from the hardware store and thanks to the help of my neighbor, we modified the pipe connector to become a reducer on the one end by removing several of the pipe coupler rivets. This allows the one end to pivot on the remaining rivet and reduce which now fits inside of my chimney pipe thimble. The other end fits fine into the Duravent chimney adapter (which will be caulked and screwed together on that end). Where the other reducer end goes into the chimney thimble pipe, that will be secured with screws and the joint will be wrapped with silicone tape to allow easy removal when the chimney needs vacuumed through the thimble. A previous owner who had the liner installed didn't install a cleanout tee at the bottom where the chimney cleanout door is so the liner must be vacuumed through the thimble unfortunately. We did also cut the thimble shorter so that it doesn't stick out so far into the room which will look nicer.

Modified Pipe Coupler.jpg Chimney Adapter and Pipe Coupler.jpg
I use a leaf blower with a modified end to simply blow out any ash from my Harman P61's vent pipe to a 6" x 30' metalbestes chimney.
That's what I do. I remove the cover in the back of the ash pan compartment, wedge my shop vac nozzle into the exhaust port, pack a shop rag around it to plug any leaks, put shop vac on blow and go outside to watch the dust show!
My masonry chimney doesn't have a cleanout tee at the base of the stainless steel chimney liner so I'll need to disconnect my pellet pipe from the chimney thimble to vacuum the base of the liner out along with removing the pellet pipe tee cap where connects to the exhaust pipe on the stove to clean the pellet pipe itself.