Internal exit flue chimney in BMH, bypass required?

Hello I'm Kees from Holland and I'm planning to make a Batch rocket mass heater.

One thing isn't clear for me despite searching a lot on this forum and watching Ytube vids. I hope to get answers here and would be very grateful.

I would build a RMH  based on a 177cm2 riser channel, it would be a sidewinder. (I use the exact measurements who Peter Berg recommended)
All the necessary information I have are from batchrocket.eu and has been applied.
So my ISA should be 5.3 m2, which i calculated exactly like this (innerside of bell is H: 173cm/D: 54cm / W: 90cm. And  minus the fire chamber hole on the front side ofcourse.

My half brick Bell wall under riser level are hard brick stone (18x9x4cm), above that I use Fire bricks up to 1200 degrees (a double layer with superwool between it as recommended)

Now I would build a bricked internal chimney for the exit flue. First the hard stone brick and above riser level the Firebrick to above ceiling of the Bell.

The main question is: do I need to make a bypass in the bricked chimney? Otherwise I have maybe problems with starting up the heater and the smoke coming back in the living room because the internal chimney is too cold.
I prefer to do not because I think i'ts more sensitive for damage.

Thanks for answerring me.

Hi kees;
If I am reading this correctly, you are building a single-skin bell.
You plan on constructing the exit chimney from bricks, starting inside the bell?
If your chimney is inside the bell, how would you control a bypass?
It would be much easier to have an external metal stovepipe and connect a bypass to it.

In my opinion, without a bypass, a cold start with an internal brick chimney would likely cause smoke back into your shop.
I recommend using a bypass on all stratification chambers.

Yes that's correct, bricked inside the bell.

It should be possible; on a dutch forum was a guy that made from steel a bypass in the inner chimney, but it was a bit complicated so I prefer to avoid it.

Yes your right, a external flue pipe is much easier but the wife don't like the view ;(

Oke thankyou for your opinion. I'm a little bit confused now because I thought I had read on a dutch forum a post from Peter vd Berg who said if you don't exceed the preferred ISA of 55.97 Sq Ft., it would be possible without a bypass. But maybe this is only meant for a chimney outside the Bell.

I have also a little question about the riser firebrick thickness.

Would you recommend a firebrick of 2.17 inch or a 1.18 inch? I think the thinner brick is better to heating up the riser more quickly, but the thicker
one has a better longevity?

The Sidewinder sketchup version of Peter shows a thick brick, but multiple youtube build video's shows thinner bricks. So I'm confused.

I prefer using a larger brick; the heat-up time inside the riser is reasonably quick.
Split bricks crack  much more easily than a full-size brick.

Perhaps, you could use a metal exhaust pipe externally, making a bypass installation easy.
Once in place, you could brick false walls to cover over the unsightly pipe.

Normally, a first-generation batch rocket should be able to start without any smoke in the room. Under a few conditions: the stove must be completely dry, and all building moisture must have been removed. And there must be a sufficient air supply, which in turn depends on the house's construction. In a reasonably airtight house, you can open a window a crack as long as the fire is burning. In a passive house like ours, this is a bit more difficult, which we solved by deliberately setting one of the balanced ventilation presets to an unbalanced setting. The intake fan then runs 7% faster than the exhaust fan, just enough to compensate for the chimney exhaust. When the stove is closed, we return the ventilation to normal.

There are, of course, other options. One is to build the internal chimney into a corner of the bell. This makes installing a bypass much easier. Or you can create a small, narrow opening between the bell and the chimney. This creates a small, permanent bypass, and the main airflow automatically takes the path of least resistance as the stove reaches operating temperature. The flue pipe is no larger than 5 to 10% of the chimney itself.
These features are especially useful when the stove is still wet and therefore quite resistant.

About split firebricks or not: f you use good refractory mortar, 30 mm is perfectly doable. A smaller mass does indeed heat up faster, especially if it's covered in superwool.

I noticed you also posted the same questions on ecologieforum.eu. I post on three forums, but I'm not going to answer double posts. I will answer your questions on one forum, your choice which one.

Het Peter thankyou for the answers.
It's easier for me if we talk on th dutch ecologieforum, so I prefer that.

thomas rubino wrote:I prefer using a larger brick; the heat-up time inside the riser is reasonably quick.
Split bricks crack  much more easily than a full-size brick.

Perhaps, you could use a metal exhaust pipe externally, making a bypass installation easy.
Once in place, you could brick false walls to cover over the unsightly pipe.

Okay thanks. Then I wil use the full size bricks.

Yes that's also possible, a lot of thinking I have to do now. :)