New Build - Need some help

James: the firebox is constructed out of 1" #2 firebrick. That rests on top of the full size firebrick.   I'll check my new design with a 42" riser and see what that looks like.

Satamaz: - sorry - I didn't get your first but am grateful for the second.

Hit a snag on the new design.  I'm using the Wisner RMH guide and using their specs the drum really doesn't sit well.  Odd.  I'll double check.  

Jim

LuAnne Welch wrote:The goal is to heat a 20 by 48 foot space as efficiently as possible.  There is no insulation along the walls.  (this is a temp tent situation)
So I'm looking to extract as much heat from the barrel as I can - the hotter the barrel the better.  The riser is insulated, but I could remove a layer of brick without too much effort.  The wood I'm burning is as dry as your granddaddy's scalp, so that is not a concern.
I think I mentioned earlier that I measured the riser from the top of the burn box level of brick and went 48 inches up from there.   If I've over done it, that would present a situation where I'd have to adjust (possible) the height of the manifold barrel to get the right height of the barrel assemble correct, given any changes to the height of the riser and the gap between the rise and the top of the barrel correct.
Jim

Increasing the space (gap) above the heat riser will give the heat more time to extract through the barrel(s) and less through the horizontal pipe. If you go this route then your not going to worry about finding the minimum gap (like in the Wisner's book or the link Satamax provided), you want to go as high as you can with your gap. So you could either lower the height of the heat riser a layer as you mentioned and see how that drafts or go with more barrel height (if you have the vertical room to do this). Something like Peter van den bergs shop heater which is 2 barrels high:

LuAnne Welch wrote:

I stayed with the six inch system, (thinking now I should migrate to 8").  The foundation consists of 1 inch of marble, 1 inch of dura-board rated at 2300 degrees and then a layer of 2.5 firebrick.

During the first trial I noticed the dura-board didn't stand up to the fire inside like I thought it would and scraping out the ash damaged it. So I went with #2 firebrick to line the firebox.  As you can see from the pictures, the main changes I made are I made the riser 48 inches instead of the 32 I had before.  I thought this would help with the draft.  On the manifold I started with an 8 inch outlet for two feet then reduced the ducting to 6 inches the rest of the way.   I modified the ducting so that I only have three bends instead of the previous 5 and increased the chimney to 23 feet (its all I can support).  

I fired it up, and draft is no longer an issue. The problem now is I'm not getting any heat from the barrel.  The temp in the firebox will reach up to 1850, but thus far the best I can get at the top of the barrel is 450 - 500.  Now I'm really confused.  The temp above the Tee in the chimney never got above 100, and the burn is so complete that after a day of burning wood, all that remained was less then 3/4 cup of ash.  There is no smoke appearing at the top of the chimney when it got to 500.  So the question to be asked is where does all the heat go from the firebox to the top of the barrel.   Surely and extra foot of riser would not dissipate the heat that fast.  

You went from the highly insulated board to firebrick. Maybe all your missing heat may now be going through your firebrick to the heater. You would get it back via slow release.

Satamax Antone wrote:

Delta T, the temperature differential  between intake air and exhaust air is far greater in the winter,  and  far smaller in the summer. Hence the sluggish performance for the moment. Even more in a tent.

I don’t think your mass of brick on the base is helping, your fire will not get hot enough while it is trying to heat such a large amount of brick.
I think that if you add  some insulation between the brick base and the bottom of the fire box, you will notice a difference.
How much insulation are you using around the heat riser? Is it lined with insulation board or something?

LuAnne Welch wrote:The goal is to heat a 20 by 48 foot space as efficiently as possible.  There is no insulation along the walls.  (this is a temp tent situation)
So I'm looking to extract as much heat from the barrel as I can - the hotter the barrel the better.  

I would offer that getting as much heat into the thermal mass of the bench, so that it radiates the heat out over many hours, is going to be more efficient than trying to radiate lots of heat from the barrel during a burn and then having to burn more often as it also cools down quickly (just like a wood stove). Once you settle on a size and layout, try to get the mass all in place including some cob/mud/dirt directly on the pipe, so there is no air gaps at all right where the pipes are in contact. Do this while it's still warm out, and fire it up several times to get the mass to dry out (where you use soil/cob) so you don't lose heat to that when you need the heat.

After seeing all the math people get into for the proper CSA of the heat riser torus but also considering possible ash buildup on the top of the heat riser, I'd suggest you just make the gap between 50-100% the height of the system: 3-6 inches for a 6" system, 4-8 inches for the 8" system. The feed/burn/riser ratio of 1/1.5/3 is always measuring the feed and riser from the bottom of the burn tunnel, not the top. So 16"/24"/48" on the 8" system includes the 7" or so from the bottom of the burn tunnel to the top- your riser would be an additional 41", and feed tube 9" above the burn tunnel. The RMH Builders Guide should also mention total length of the extraction pipe, minus 5' for each 90 degree bend and some possible bonus based on the chimney. If you made a little bell at the end instead of 2 90 degree turns, then you don't lose the 10 feet there.

It would appear I have confused some of you with the drawings of the new design of version 3 (8 inch system) vs the current 6 inch system I'm running.

On the 6 inch platform, the base of the fire-tube is 1 inch of #2 firebrick.  (this stuff is supposed to be much more heat resistant than normal firebrick).  The brick below that RMH (the same as in the drawing) is simply for insulation (along with the lower levels of dura-board and marble) so I don't toast the carpet underneath.

In the future system (8 inch per the drawing) the layer of firebrick will now become the base of the fire-tube (see new drawing).  Again the goal is not to heat the bricks but contain the heat of the fire-tube.  I posted the pictures because the RMH guide is just a bit off, because when I added the drum, it appears there will be a conflict between the drum and the feed tube using the specs in the guide.

I've modified the new design to measure the riser from the bottom of the burn tunnel, and that seems to be good advice.  

The only remaining question I have now is, I received counsel that I should have the exit from the drum about 4 inches from the bottom of the barrel, I did that on this latest build (5 inch) but that seems pretty high and will cause a lot of excess mass to fill in the box and since heat rises I wonder if I should lower the exit somewhat?   Any thoughts?   My issue is I'll have to fill in about 6 inches of mass (pebbles in this case) just to reach the bottom of the exit tube.  It just seems like a lot, that's all.

Jim

The nature of the experiment demanded a six inch system and since weight was a factor I purchased a set of plans from Matt Walker for his ceramic RMH core build.

Matt Walker recommends a bell type bench. My thoughts on this type of build is that instead of 2 runs of round pipe a single run or rectangular air duct in the bench would reduce flow restriction. The exit flue could even run down inside the barrel to an elbow at the bottom of the duct which could be directly attached to bottom manifold portion.

Hi Jim, it could be I have got this wrong and I have misread your post but it appears you have built your fire directly on top of a mass of dense bricks?

The effect from this will be a poor performance from your heater because the bricks will have to become super hot all the way through before the temperatures can rise high enough in the fire box for the system to work at its best.
insulation in the right areas will make dramatic differences to the overall performance of your design and stop the bricks wicking away the heat from the critical areas.

The fire box and the heat riser really needs to be encased in insulation, that way you can concentrate  the heat from the fire in the right places.
If you want you rocket fire to work more efficiently and operate as a smokeless economical heater, then you need to use insulation in the right places.

So rather than trying to heat a brick mass under the fire, you need to concentrate on getting the fire super hot inside the fire box and tunnel.

The actual brick base is fine it just need isolating with a piece of ceramic board under the fire box.

The riser is equally important and needs to be heavily insulated, that is very easy to do with ceramic fibre.
In fact you don’t need any bricks to build the riser, just a metal tube lined with ceramic fibre matting.
This method is often termed as a “5 minute riser” due to the simplicity of constructing.

Proper core proportioning is either along centerlines as mentioned, or along outside edges, so heat riser height would be measured from floor of burn tunnel, not ceiling. Likewise, burn tunnel length is total inside floor length.

Fox,

Going forward on the 8 inch build.
Yes, I had intended to build the burn tunnel on the top of the dense brick.  I added the non-essential bricks only to protect the dura-board underneath. (the white layer in the drawing)   If I eliminate the non-essential bricks I'm back to the minimum the RMH guide calls for. Taking into account what you mentioned, if I make those bricks 1.25 inches thick instead of the 2.5 I'd normally get, those bricks would come up to temperature faster and I'd be left with the 2300 degree rated dura-board and the marble base to insulate the floor.

Then insulate the riser and the fire box with dura-board inside of the drum and I should be good to go. Do I have the thinking correct at this point?

Jim

Glenn,

Roger that on the core proportioning.   The fire tube is as follows:  
Length 24 inches
Width 7.5
Height 7inches.

Measuring from the bottom of the burn tunnel the rise will be 47.5 inches.

Jim

All,

I have taken all the advise I have received and now provide you with the following design and ask for your comments before I begin this last build.

For the foundation I have:

Marble 1.25 Inches thick
Firebrick 2.5 inches thick
Dura-board 1 inch thick rated at 2300 degrees.

For the base of the firetube i have:

Firebrick at 1.25 inches
the first layer on top of that is 2.8 inches thick

the fire tube dimensions are:

Length 24 inches
width  7.5 inches
height is 7 inches

The width and height dimensions are also those for the feed tube and the riser
except the feed tube length will be 16 inches
and the riser will end up at 47.5 inches in height

The gap between the riser and the top of the barrel is 3 inches.

The exit pipe from the drum is set at 8 inches and begins 2 inches from the bottom of the barrel
The gap from the first row of the firetube and the drum exit is 3.6 inches

I had to move the drum 1 inch to the rear of he RMH to accommodate the feed tube

If you see an error in this design, please let me know.  I appreciate all of your comments to this point.  

Thanks again for your time.

Jim

small correction

The first row on the base of the fire-tube is 2.5 inches thick - sorry fat finger syndrome kicked in there

Jim

Jim, too much mass under the dura board, which is not insulative.

Best way, use insulating firebricks.  Or even better superwool board.  Or vermiculite board, under those bricks.  You coule get by with rigidized rockwool/roxul, with your dura board on top, for example.