Double skin batchbox question

I've seen a number of people build double skin batchbox heaters, but I've never really heard them explain the space between the two walls.  I've seen people leave no space, some leave a space the thickness of a piece of cardboard, other people leave a bigger space.  Anyone have any thoughts?  

Hi Trace;
Within reason, there is not much difference in how large the gap is.
The main idea is to provide safety without exhaust escaping into your building.
The other is to add mass.
The most common gap is 1/4" or so, the thickness of cardboard.
By using cardboard the mortar cannot bridge between the bells, the cardboard can be removed as you build or it can be left in place where it will char away.
Some folks will place copper heating coils between for water heating so they gap 1"-3" to allow room for the piping.

thomas rubino wrote:Hi Trace;
Within reason, there is not much difference in how large the gap is.
The main idea is to provide safety without exhaust escaping into your building.
The other is to add mass.
The most common gap is 1/4" or so, the thickness of cardboard.
By using cardboard the mortar cannot bridge between the bells, the cardboard can be removed as you build or it can be left in place where it will char away.
Some folks will place copper heating coils between for water heating so they gap 1"-3" to allow room for the piping.

For those contemplating a double skin bell, is simply making the inner skin as strait and true as possible (and exhaust tight)  This allows you to be very creative with your second (mass) skin bell.  As simple as just adding a little tile, large brick, granite or soap stone. And letting the gap take care of difference of coefficient of expansion of each bell.  As the temp reading between the bells will never be the same,  Each can expand and contract in there own ways. And not rubbing or installing stress on each other.

Really the sky is the limit in incorporating your second bell, the gap that Tom listed above is the key.  By the way, my expansion on granite in 48" from 85 degrees to full warm of 250  was only 1/32"  (.08 mm) But it can be for certain that the inner bell was different and mostly likely more.

if your core and bell is built on a shelf/base and if this base has the extra room around it, you can be building your second bell as your drying out the first bell.

Don't let a second skin hold you back.  More can be good, but less is never bad. As long as the first one gets built.

Because I am using 2 kinds of bricks, I would have a gap of 6 mm (1/4th inch) where the wider bricks are, and a gap of around 15 mm (0.6 inch) where the smaller bricks are. You are saying this will not make a noticeable difference in heat transfer, Scott and Thomas?

Any gap at all breaks conductive heat transfer. A small gap, around 1/2" or less, will allow only radiative transfer, while a large gap will also allow convective transfer with air circulating and moving heat.

My cob outer skin, 6-7" thick, is actually placed right against the uneven face of the brick inner skin, and has not shown visible thermal expansion cracking in about 8 years, only the web of tiny permanent drying cracks.

Meaning that in this section with a larger gap, heat will also move upwards - less effectively transfering to the outer skin at lower heights?

Heat would circulate, and would warm the upper outer skin more, but not as big a difference as you will already have from the internal gas temperature being much higher at the top.

With alternating wide and narrow bricks, you would not have a clear channel for circulation, and I would not expect it to make a noticeable difference.

Thank you for your input Glenn, in the added picture the 'void' in the middle will have an exterior skin with the slighly wider bricks, leaving about a brick wide on both sides which would have the bigger gap. I will see if I can break the convection on a few places by making a small ridge with lime mortar/plaster.

Julian Adam wrote:Because I am using 2 kinds of bricks, I would have a gap of 6 mm (1/4th inch) where the wider bricks are, and a gap of around 15 mm (0.6 inch) where the smaller bricks are. You are saying this will not make a noticeable difference in heat transfer, Scott and Thomas?

I got behind on my replies, sorry.  

Julian, I presume the gap difference would happen because you want to keep the face of the bricks on the outside flush with each other?  And rightly so, And I think Glen said it well, that things might not be quite as uniform, but... will you be able to really tell?  That is always the question, but honestly I think so little of change will be made that your good to go.    After all the first bell is the work horse, collection what we give it, The second one is to help us extend even more collected heat,  READ  extend, not more.  We can hold more, but the fire will not produce more that it did the first time.

Best of success.

Scott Weinberg wrote:
I got behind on my replies, sorry.  

Please don't apologize, there is no need!

Scott Weinberg wrote:
Julian, I presume the gap difference would happen because you want to keep the face of the bricks on the outside flush with each other?  And rightly so, And I think Glen said it well, that things might not be quite as uniform, but... will you be able to really tell?  That is always the question.

You are completely right, I want to keep the outside flush. You are also right that it probably does not matter a great deal, but I'm a bit perfectionist so I like to exclude as much (big) mistakes as possible, knowing full well that once built, changes to my heater will be very hard as it will be in my living room and I have a girlfriend to keep happy..