How to seal the metal bar/bypass switch coming through my heated bench

Hello everyone!
First post on here.
I'm in the process of my first RMH, batchrocket 150mm design. With barrel on top for fast heat (poorly insulated house) and heated bench at the bottom. I want to add a bypass in the chimney part that's in the heated bench to make sure I can start up quickly. As my chimney goes up 9 meters before leaving the house, I feel like it's wise to have the option to heat the chimney quickly in the beginning. The pipe is just entering the bench vertically and I'm going to put it 10cm from the floor in the bench to have enough draft. My idea was to cut a circle out of the pipe just underneath the top part of the bench that's +- same cm² like the 150mm pipe. And then have a pipe around it(same diameter but cut on one side, the other side then the hole) so it can move up and down and like that open the bypass or close it. Then a metal rod welded to it coming out of the bench to move the bypass where desired. One thing, i didn't figure out yet. How do I seal the metal rod coming out of the bench, so there's no smoke/gas coming out of the system. I imagine some kind of rubber seal in the cob where the rod moves through, but can't find anything for that online. Also the rubber has to withstand quite some heat..
How did you guys seal your handles of your bypass systems?
I'll attach some pics of my build so far!
I'm waiting now for a company to put the 8-9 meter long chimney feom the roof and need to do the bypass after that before closing everything and continuing cobbing.
Ohh and what's your advice with cob in inside (warm) surfaces? Do you still add straw or??
Thanks for the help!
Grts from North of France
Ruben

Welcome to Permies Ruben.  Great first post looking forward to seeing how your project goes.
Cheers

Ruben Masson wrote:How do I seal the metal rod coming out of the bench, so there's no smoke/gas coming out of the system. I imagine some kind of rubber seal in the cob where the rod moves through, but can't find anything for that online. Also the rubber has to withstand quite some heat..

Hi Ruben,
Best solution I've seen so far is guiding the rod through a piece of closely fitting copper pipe. Just to get it through bricks and cob and whatever. In order to keep the copper pipe fixed in place, solder a strip of copper plate at right angles to the pipe. This will keep the guide pipe in place. Those wings are meant to be placed in a seam between bricks.

For your information: the whole of the system will operate in underpressure so when running smoke won't come out anywhere. While starting cold that's another matter, but that effect disappears magically once the chimney draw is established.

Just another hunch: mount an elbow high inside the bench. The open end could be used to house the bypass valve. As a rotating one or sliding one, your choice. The rod should be situated horizontally in order to use the winged copper tube, I need to add.

Ruben Masson wrote:  My idea was to cut a circle out of the pipe just underneath the top part of the bench that's +- same cm² like the 150mm pipe. And then have a pipe around it(same diameter but cut on one side, the other side then the hole) so it can move up and down and like that open the bypass or close it. Then a metal rod welded to it coming out of the bench to move the bypass where desired.

I almost did as you described, but found it was difficult to cut a round hole in the side of a stove pipe, and even harder to rig up a sliding sleeve to cover it. Then I read that the bypass does not need to be system size, it can be much smaller and still accomplish the goal of pre-heating your chimney.

Here's the bypass design I came up with. Essentially, I cut a rectangular hole in the stove pipe that goes inside the bell (which would be inside the bench for you) and folded out a flange on both sides. Then made a little sliding cover that hooks on to the flanges and had some connection points for the rod. It is not sealed well around the rod , but doesn't need to be (in my shop) and doesn't seem to leak any smoke when running.

Thanks for the quick reply's!!
I like the copper pipe suggestion! Might have a look if I can repurpose some, maybe some ceramic blanket in between pipe and rod closes it off nicely, I still have a bit left from insulating the riser.
Thanks Matt for your awesome suggestion! Yeah, makes so much sense the bypass doesn't need to be so big. And your way to close it seems quite nice! I might try to combine that with Peter vdb his suggestion with the copper pipe to 'seal' it nicely.
Thanks for the input guys!