Guys, hope you don't mind if I join the conversation. The renewed interest in OAKs lately has grabbed my attention. My setup is similar to the OP and alternativeheat. P68 sitting on hearth, 3" duravent coming out the back into a T in the fireplace, and from there it's adapted to 4" flex pipe which runs all the way up to the top inside the original fireplace flue, which is 8" stainless steel. No clay, no mortar. Just the stainless pipe. The fireplace is in the center of the house, not on an outside wall. The 4" flex pipe exits the top by extending about 1" beyond a block-off plate attached to the 8" flue. Topping the whole thing off is the original 8" screened flue cap which does a great job at keeping pests and weather out. This has been my setup for 6 years and it has worked very well. I clean the 4" flue from the top every fall, using one of the flexible rod dryer vent cleaning kits. There is hardly any ash when I do the annual cleaning (a testament to using good quality pellets, I guess).
Now, for the OAK part. I've never had one, and until reading all the recent OAK related threads, never really thought I needed one. My house is very tight, and I do realize that I am drawing cold air in with this setup. Heck, I have a wood stove, and that draws cold air in too. No such thing as an OAK for that, so when both stoves are running, that's a lot of cold air coming in. It's worth stating that although at times we can feel cold air coming in specific areas (it has to come from somewhere, right?), in general we have never felt cold because of it - the stoves do keep the house very warm despite the cold air coming in.
Now, I have to say I really love the idea of the Selkirk DT, and if I had to do it all over again, I would go that route if feasible. But my existing setup cost a lot of money and I just can't justify ripping and replacing it, too much investment already. So...If I wanted to install an OAK, I think my only option is to run 3" OAK flex pipe all the way to the top, snaking it inside the 8" flue - alongside the 4" flue pipe that's already there (probably some fun challenges awaiting there). I have read about 2" OAK flex pipe, but the Harman manual mentions 3" pipe? Anyway, that should effectively provide the same benefit as the DT, at least with respect to pre-heating the air, since it will run the full flue length with the heated flue vent. From what I have read, once the OAK flex pipe exits the top, I would need to take steps to ensure it terminates at least 12" away from the 4" exhaust flue to prevent the possibility of drawing exhaust air right back into the stove.
Do I have this right? Am I over complicating the situation, or is there a better way? As much as I would like to do this myself, I don't trust that I would do it right, so I am likely going to hire a local stove/chimney/flue guy to do it. In terms of time and materials, what should a job like this cost (roughly)?
