Byron Campbell

For my PvdB 6" batchbox I went with an IFB (Insulated Fire Brick) heat riser (2600° F. rating) sourced from a local Harbison Walker refractories outlet (many locations scattered around the US). Anyway, three seasons down and the heat riser is going strong, and is expected to last for a good many years yet to come. It took three cases of IFB for the heat riser construction, and the cost at that time was in the range of $120.

Velocity stack photo attached.

From what I've observed in my own stove, pulsing occurs from a momentary pressure increase inside the firebox, when the burning wood is producing more combustible gas than the draw (draft) can pull through the port into the riser to feed the secondary heat riser's combustion zone. The greatest tendency of pulsing is when all the wood is catching at once and out-gassing like crazy. A portion of the excess gasses cause a flareup inside the firebox, firebox pressure increases briefly and that pressure simultaneously pushes gasses towards both the port and the primary intake. The velocity stack mitigates this effect by encouraging improved firebox throughput, especially handy for burning hardwoods that tend to "coal" heavily.

Jay Hughart wrote:Ok I scored 25lb of Hawthorn bond fireclay. Should I just mix it to a mortar like consistency with water or do I need to add anything to it? Sand pea gravel portland cement ect? I think the clay alone will be fine but I have never actually worked with it and chipping the motar from the top of the barrel was dusty and messy wanna get it right this time! Thanks for everyones help and input!

Just like making clay-sand cob, one part fireclay to between 3.5 ~ 4.0 parts masonry sand. Use as little water as possible, to cut down on cracking as it cures. One method I prefer to use, first mix fireclay with water to form "clay slip", about the consistency of pancake batter. Let it sit overnight to insure the fireclay has completely absorbed the water. Then mix in masonry sand to form a stiff cob that can be formed to the desired shape etc. Such a surface made from this will conduct heat well and boil water, but it will be subject to abrasion and wearing away with heavy use like dragging pots across the surface.

Matthew, I agree with Max that your stove more resembles an ever decreasing-CSA flue run system. BTW, that is very similar to my own stove. If your first bell was much larger, then it would be more like Peter's stove.

One thing I noticed about my batch-box is that there is much less tendency of pulsing when the burn is occurring just in front of the port to no further forward than about 6 inches in front of the primary air intake. Through experimentation I also discovered that a velocity stack added to the primary air intake, to promote straight-line primary air flow into the base of the burning wood, to encourage the burn at the back of the firebox to just in front of the port, significantly improves performance under all conditions from a full firebox load to the coaling stage of the burn.

The velocity stack protrudes through the firebox door's primary air intake into the firebox by 2 inches, thus directing primary air straight into the firebox's V-floor. As a test I made a temporary experimental velocity stack from coffee can tin, formed into a square tube tightly fitting the primary air cutout in the firebox door. The velocity stack is flush with the outside face of the firebox door, with a half inch wide 90 degree edge-lip to keep it from being pushed through the primary inlet while using a poker to tend the fire (shove embers and coals back away from the primary air intake). Anyway, maybe give this a try and see what effect it has for your stove.

Sounds good Matthew. Leaks indeed cause a performance hit.

I don't notice any smoke when burning seasoned wood, not after startup, only steam that vanishes within 4 to 6 feet of the chimney cap. The door has a viewing glass and I can see clear back through the port into the inside of the base of the heat riser. When secondary combustion is occurring, the inside the heat riser is illuminated in golden yellow light. Pretty cool, er uh hot.

My usual start-up routine is to first burn a handful of kindling to verify the draft and warm up the fire box a bit, using wood of about 1" diameter and thinner. All my wood is cut to a length that provides the recommended clearances between the fuel and the primary and fuel to rear port (about 2"). Once the kindling catches the secondary combustion inside the heat riser begins in less than a minute or so, noticeable by a change in sound as the rocket roars to life. After burning down to gently flaming coals and the secondary combustion subsides, I'll load in a full batch of 1" to 2" diameter sticks first followed by 3" to 4" diameter ones, filling the firebox to 3/4 full. Secondary combustion lights off within seconds of the firebox door being shut. HTH.

Hi Matthew, my batch-box (firebrick combustion unit, IFB heat riser, 6" system size) had the identical pulsation to your stove, at exactly the same frequency, with small flame bouncing out of the primary air. Likewise, slightly opening the firebox door stopped the pulsation and the stove behaved itself. The stove is going into its third heating season and doing very well, after a bit of fine tuning. Each heater build will naturally be a little different; construction materials, flu path, chimney draft, wind effects on the chimney, fuel-wood burning characteristics, cold start-up time, and etc.

I'm burning mostly ash and beech, and the primary air opening ended up needing to be 3.125 x 3.125 inches to correct the problem. I got an idea of how much larger to make the primary air intake by observing how much, on average, that the door needed to be cracked open over the course of the first heating season. In windy conditions that significantly increase chimney draft, I'll close the primary air by one third to one half.

For very close to the same heat output of an 8-inch J-tube RMH, a 6-inch batch-box is the equivalent. An 8-inch batch-box will run super hot, dump a lot of BTU, enough to heat a small warehouse.

To keep a PvdB batch-box going continuously for hours at a time will require reloading it during the coaling stage, about every 45 minutes.  Matt Walker has been working on extending the burn time of the batch-box out to several hours while retaining clean burning characteristics, details this thread:

http://donkey32.proboards.com/thread/1433/pre-port-tube-secondary-arrangement?page=1

Aaron, I'll pass along a few thoughts that will hopefully help save time and effort on the reworking of the exhaust "manifold" area. The 1/2 barrel manifold works best with  smaller diameter heat risers, so as not to inadvertently build in an unwanted exhaust restriction. With that in mind:

1) Let's say the 1/2 barrel is shifted right up against the heat riser directly opposite the exhaust cutout in order to get 4" of clearance. As Peter mentioned, it would be best to increase the riser to barrel top gap, since the exhaust gas path is now being narrowed out of the top of the riser (no longer a full 360 degree exit path). 4" or more top gap would be the minimum, I would think. The 2" top gap value commonly suggested makes for a hot cooking surface on the barrel's flat top, but that's not important for stoves built primarily for heating. The riser top gap on my stove is currently 13" (I experiment a lot). You could try it first without the 12" to 8" exhaust port reducer, but that's an excellent suggestion via Peter, and will improve the performance of the stove.

2) Let's say that shifting the 1/2 barrel produces only about 3" of exhaust port to riser side clearance. I'd definitely do as Peter suggests, make the 1/2 barrel exhaust cutout larger and fit a 12" to 8" reducer between the 1/2 barrel and the Tee, and also increase the riser to barrel top gap to at least 6" or more.

Aaron McKinley wrote:
Question, if I shift the barrel to get four inches on the exhaust side that will put the barrel and the heater riser (with a two-inch perlite ring) very close on the opposite side. Is that OK?

It is okay, and as they say "a necessary evil".  Yeah, there's an exhaust restriction working against that stove for sure. With the Tee protruding 1" into the 1/2 barrel manifold, and only a 2" gap between the exhaust cutout and riser, the effect is that there's only about 1" for the exhaust gases to squeeze through. No wonder the barrel is not heating up. Once that restriction is corrected, along with the vertical chimney extension, be prepare for some serious heat! With the stove up and running I'd monitor the temperature of anything flammable within about 3 feet of the barrel; wood framing, window frames, greenhouse plastic, etc.

Aaron McKinley wrote:

So right now I have an 8 inch T coming out of the drum with foil tape to seal it, then cob to seal it.  This will not work?

Hi Aaron, you're using Ernie's 1/2 barrel "manifold" technique, in which the recommendation is to have 4" of space between the exhaust hole cutout in the 1/2 barrel to the heat riser. The minimum, according to their book "Rocket Mass Heater Builders Guide", is around 3" but the closer to 4" the better when going straight in (perpendicular connection) in order to keep exhaust gas restriction to a minimum. An angled or offset connection works even better, as it requires a somewhat oval hole cutout in the 1/2 barrel, which has a larger cross section thus less drag on exhaust gases. One spot to take care with is where your Tee connects to the 1/2 barrel "manifold", to insure a flush non-inside-protruding connection etc. Perhaps you've done that, since it's in the Wisner's book and plans.

BTW, it is okay to offset the barrel to get the required 1/2 barrel manifold exhaust hole to heat riser clearance.