rmh Exhaust options?

Hello all,
I am in the process of building a 6 inch system batch box. My prototype which was outside worked wonderfully, but because this is a basement build, I am having draft issues. The exhaust is a plunger style inside of an open bell bench. Currently running at about 45 degrees outside to the window. My question is, would it be possible to tap into my prefab fireplace chimney? It is on the second floor right above the rmh. The main issue I see is that it's an 8 inch chimney compared to my 6 inch RMH. I'm worried I won't be able to produce enough heat to push all that cold air up and out 3 stories. (The house is 2 stories with a very high roof, so basically 3 stories to the top of the chimney).
I was stubborn and tried dry stacking the bricks, then cobbing the inside and outside. It worked in my prototype, so I was hopeful! It's very leaky because my cob cracked while drying (way too fast).
Something I noticed is the firebox has draft during the coaling stage, even when the exhaust is blocked off. Does this mean it's being pulled up the riser and then exiting into the room in the cracks in the mass? None of my CO alarms have gone off, so far. Should I start over or is it possible to cob over the whole mass to make it more air tight? I know I need to solve the chimney issues first. Let me know!

Hi Wes,

My comments come only from limited experience in running RMH's, so this will be more about the chimney and your outside exhaust works may be similar to ours.  Our old farmhouse has a basement, main floor and second floor.  A woodstove on the main floor connects to black stovepipe rising about 4 ft off of the stove before exiting out the wall into class A chimney (thus, 2 bends in the exhaust stream).  From there running up parallel to the outside wall, the chimney pipe runs past the second floor, through the roof overhang, and up another 15+ ft. where it ends in a wind-reducing cap (photo below).  

Were I attempting an installation similar to yours, I would indeed attempt splicing of the exhaust stream into that of the external class A chimney, but would be worried as you are about cold air column wanting access to the basement.  What I *can* say at this point is that we have minimal issues reversing the draft in the stove on the main floor except under high wind circumstances and if I haven't 'pre-heated' the chimney with a quick crumpled paper burn to warm up the chimney walls.  When a chimney that tall *is* preheated, it acts like a straw pulling that rising air column up the insulated pipe.  So in the build you describe, I would think it of utmost importance to have some way of pre-heating a tall stack in order to get good draft from the point of the stove to the top of the chimney.  There will be others to weigh in on the stove itself and the draft particulars of a basement build, but just commenting from the side of tall chimneys in drafty/windy surroundings.  Good luck!

John,
Thanks for the reply. That's very helpful. Is that another chimney in the middle of your house? If so, why did you not use that instead of running class A? My chimney is covered by wood and vinyl siding to match the house. I need to find a contractor or someone to come out and help me understand how it's built, and where to make the cuts.

Wes Khan wrote:John,
Thanks for the reply. That's very helpful. Is that another chimney in the middle of your house? If so, why did you not use that instead of running class A? My chimney is covered by wood and vinyl siding to match the house. I need to find a contractor or someone to come out and help me understand how it's built, and where to make the cuts.

I can't say if the middle chimney was original from the time of the house build in 1915...??... At any rate, if we did not install a modern chimney liner in that old brick chimney, insurance would have said "No way!..".  That chimney was apparently fine for heating oil and propane (and probably RMH) exhaust expulsion, but not for woodstove in this era. So our original woodstove that we installed at the time of home purchase was professionally installed to meet code....and they recommended an exterior wall-mounted class A chimney.  (They advised against a chimney liner as the house had been jacked up several times for new foundations after flooding and the chimney was no longer very straight! )  The exterior class A chimney has been a robust install for sure and just a few months back I swapped out our original stove purchase with a wood cookstove that has a baking oven compartment.  This did not require more than a black stovepipe collar replacement as the remainder of the pipe seemed fine and I was able to get the new stove into place and hooked up more easily than anticipated.  

I have my RMH in my basement. Early season burning, if not done right, will smoke out my house. You either have to get the riser really warm before your attempt to start the first true fire, so to preheat the whole system. Or, what I tried this year was where my pipe meets the wall (old existing chimney that runs outside and up 20 feet) I used an electric heater and got the pipe really warm, so to create a draft through the system. Also you need to crack a window when you first start so to draw fresh air into your system. If you don't you will experience negative pressure and you will get back flow. I have been burning for 4 years now. I really don't think the 30 ft chimney will affect the system if it has a straight up run. You just need it warmed up enough to push up a cold chimney.

Jason Speaks wrote:....... I have been burning for 4 years now. I really don't think the 30 ft chimney will affect the system if it has a straight up run. You just need it warmed up enough to push up a cold chimney.

Jason, this brings up a point that I have wondered about for some time for those with basements and typical forced-air, ducted furnaces (propane/gas/fuel oil, etc).  A basement-dwelling RMH would heat up the basement area nicely and, with the added insulating value of earth along the outside of the walls, may even heat up the concrete/cinder block foundation as added heat-storing mass.  Presumably, then, the heat would rise on its own up through registers into the rest of the home.  But additionally, most furnaces with which I am familiar have "blower mode" where the furnace fan (without using the actual petroleum-based heat source) can transfer ambient air from the basement up through the ductwork of the home.  So would this work to draw RMH-heated air up through the ducts to heat the home? Or would drawing on this air mass cause issues with proper operation/draft of the RMH?  Thanks!.....

Jason Speaks wrote: I used an electric heater and got the pipe really warm, so to create a draft through the system. Also you need to crack a window when you first start so to draw fresh air into your system. If you don't you will experience negative pressure and you will get back flow. I have been burning for 4 years now. I really don't think the 30 ft chimney will affect the system if it has a straight up run. You just need it warmed up enough to push up a cold chimney.

I stuck a lard candle in the exhaust to warm it up on the first test burn, but then decided to cold start an entire batch full of wood. Boy, was that a mistake. Lol. That's what's got me on the idea of cobbing the bottom section.
I'm in the process of redoing the exhaust. It's going to be closer to the barrel for more heat exchange. Hopefully.

John,
I have already asked an expert (Matt Walker) about using return ducts and he said they had luck with return ducts near the ceiling above the RMH. That's also my end goal. Once I have something that doesn't throw smoke everywhere.