Minimum distance from riser to barrel?

I've been reading Peter van den Berg's "Batch Box Rocket" info (and other RMH stuff) and was wondering about the minimum/maximum (if there is such a thing) distance between the top of the riser and the barrel. Seems like I've seen something like a 3 inch minimum, but for distance farther than that there doesn't seem to be any limits with the riser opening into much larger/taller bells. It seems that the barrel would last a lot longer if it is taller (farther away and cooler) from the top of the riser. On his site Peter shows a three barrel system I'm assuming burnt quite well. I was thinking about making a 2-barrel stack instead of three due to an 8 foot ceiling height with a ceiling heat shield.

Hi Leslie;
It is suggested that 2.5" as a minimum clearance.
Indeed with a bell it is common to have 12" plus.
I use a 7" batch with a double barrel and a brick bell in my poorly insulated shop building.
Using double barrels releases quite a bit of instant radiant heat.
Having a "mass" as well will give you the long-term heat holding that RMHs are known for.

If you are building this in an insulated home then a double barrel might be more radiant heat than you want.
Remember an RMH burns wide open, and there is a tremendous amount of heat generated, it could be more than you want in a smaller area.

EDIT)  Barrels last indefinitely, I still use the first barrel I purchased in 2013 and it is as good as new.

Hello Leslie,

The information on Peter's batch rocket site is not intended to be equally transferable to other types of heating devices other than batch boxes.
If I remember correctly, Peter has said a 12" minimum  gap over the riser is needed for batch boxes.

Also, here is Matt Walkers take on barrel longevity:

barrel longevity

I know he talked about top gap in one of his stove chat episodes but can't find it.
Basically, I remember him saying that it was a myth that the gap had to be precisely so or somehow the stove wouldn't work properly/efficiently.
2.5" was considered absolute minimum, whereas maximum was mostly only limited to the height of your ceiling.
Also, that unless your cooking or boiling water on it all the time, more gap is better as this is a prime spot for a restriction to happen. Fly ash builds up on top of the riser and chokes the flow of gasses.

In general, the top gap for a J-tube is much smaller than for a batchrocket. So what has been said about allowances for a J-tube doesn't apply to the batchrocket. The latter one is much more picky about restrictions in the smoke path. Logically when you look at it, its burn rate is much, much higher as compared to the J-tube so a pinched spot will play up much earlier.

Due to experience, the minimum top gap for a batchrocket has shrunk from 2 times system size to one time system size. To clarify: a 6" system would need a top gap of 6" as minimum. As for a maximum: there doesn't seem to be such a thing. The 3-barrel workshop heater that's showed on the batchrocket site sported a top gap of more than a barrel height. In an early experiment I stretched that size to 5 feet, with no apparent penalty.

Thanks Peter and Gerry,

The house is fairly tight new construction with 6" walls. I'll be heating about 2900 sqft of CBS construction in the basement with about 50% of that being below grade space with open trusses to the main level. There is also a central heat pump with all the ducting in the basement to circulate the air. If the basement ceiling (main level floor) is 100 degrees that will limit the amount of any additional heat needed on the main level.

I was just wondering because I see flue pipe that looks like lacework over not too many seasons. Just thinking increasing the distance from the top of the riser to the barrel top by a few inches would dramatically reduce the heat on that surface and reduce the potential for spalling  and improve longevity. When I saw the 3-barrel setup with original paint remaining on the barrels that fire must really cool quite a bit in that larger volume. I've got barrels with removable tops which will make seasonal checks and cleanout a lot easier.

I watched the Matt Walker video in your link which makes some sense. The chemistry of the post-rocket-burn may be less corrosive in the first place??

Leslie Walper wrote:When I saw the 3-barrel setup with original paint remaining on the barrels that fire must really cool quite a bit in that larger volume.

What you see in the picture of the three barrel workshop heater is a shadow of the original paint. This paint happened to be water based, the binder burned off eventually and drove me out of the workshop. To such an extent I had to open the large garage doors fore and aft to get rid of the stench.
What remained was some of the colours, which could be wiped off as dust.
Please, don't think you can go away with it, those barrels can get very hot, except the lower half of the bottom one closest to the exhaust. The top of the tower didn't get red hot, though, but that's all.

Leslie Walper wrote:I watched the Matt Walker video in your link which makes some sense. The chemistry of the post-rocket-burn may be less corrosive in the first place??

What's happening when the heater is started up first: the barrel(s) will be lined with a very thin layer of wood tar, caused by the moist in the core. Wood tar together with moist and oxygen is very corrosive, you might say. But in this instance, the barrel will get dry every time the heater is fired, the tar is excluding the very little bit of oxygen that remains in the exhaust gases. As long as the barrel won't get red hot, the tar won't burn off.

All the above according to my opinion.

Peter,

I'm planning to build an 8" batch box recycling some of the components from the Scandinavian style mass heater that I deconstructed. I'm thinking I'll use the front doors, air intakes, cleanouts, and etc. for the "box" portion, adding a barrel over the riser and probably include an adjacent bell for heat capture. In your discussion of the P-channel you give dimensions for a 15 cm system (60x20x2mm). Would you use that same size material for the 20 cm? There also doesn't seem to be consensus on placement of the P-channel -- in the ceiling or floor of the box, except that the lower position might be more easily replaced if needed. With my already available large front doors and my ideas for attaching same to the front of the box, it wouldn't be too much trouble to simply remove the door and minimal facing material for maintenance.

EDIT: Reading on in your material I do see that the horizontal floor channel dimensions are recommended to be 8.25% of the CSA of the riser with the vertical part being 5.4% of the riser. The more recent edit of that material gives the horizontal part being "close to twice as large as the stub, CSA wise."

Leslie Walper wrote:In your discussion of the P-channel you give dimensions for a 15 cm system (60x20x2mm). Would you use that same size material for the 20 cm?

No, the secondary air channel should be scaled up also, with the same factor. Keep in mind, the P-channel should be (slightly) wider than the port in order to avoid leaking along the sides. For a 200 mm system I'd use 80x25x2 mm. The inside of the channel is about 5% of system cross section area. While upscaling, all measurements need to go up, no exceptions.

Leslie Walper wrote:There also doesn't seem to be consensus on placement of the P-channel -- in the ceiling or floor of the box, except that the lower position might be more easily replaced if needed.

The P-channel is an overhead channel of specific dimensions, delivering fresh air right in front of the top half of the port. The floor channel is different in the sense of placement and measurements, but it's delivering fresh air to the top half of the port, also. So there is a choice, both are doing the same thing by slightly different means.
There is consensus where to place either of those, the development of the floor channel was done because it could be replaced much easier.

Leslie Walper wrote:EDIT: Reading on in your material I do see that the horizontal floor channel dimensions are recommended to be 8.25% of the CSA of the riser with the vertical part being 5.4% of the riser. The more recent edit of that material gives the horizontal part being "close to twice as large as the stub, CSA wise."

The reason for the edit is a practical one. When the vertical part (stub) of the floor channel could be obtained but in some instances the horizontal part (feed) proved to be much harder to find. I tested the 1:2 configuration and found out it worked even better. So now using the same size tube for stub and feed, two tubes side by side for the latter, is recommended.

Please keep as close to the recommended values as you can, the whole of the design is a very tight one. Using other proportions of whatever part could lead to disappointment.