RMH diagnosis/optimization help needed

I'm just finishing up my first complete RMH build.

It works, but I feel it could be better.  I'm not much of an engineer, so I can't really tell you anything like the CSA with milimeter precision.

Air flows in the right direction, no smokeback.

The base of the barrel has a tower of bricks and mortar, but was leaky so I covered it with cob. No more leaks.

The bench is a pebble style bench, but to my knowledge, doesn't seem to get warm.

At this point, I've done less than twenty test burns, maybe five this season (it sat incomplete for about a year).

In the bench, I placed as densely as possible:

1. Bricks as tightly against the ducting as I could manage
2. Rocks in the spaces between the bricks
3. Pebbles in the spaces between rocks and bricks.

I used a rubber mallet to settle everything before adding more rocks and pebbles.

After burning about an hour:
-I can touch the top of the barrel with my bare hand without burning myself, but only a tap.
-I can touch the ducting leading into the bench with my bare hand for about 5 seconds before it becomes uncomfortably hot.
-I can touch the ducting leading out of the bench indefinitely with no discomfort.

This tells me heat is being stored in the bench, but I don't feel the bench getting warm.

Also, I feel like the burn chamber should be more rockety. My understanding is that this can be done by raising the height of the heat riser and barrel.

Is there anything I need to be more clear on describing? I can post pictures in a bit.

Your help is appreciated!

Hi Chad; Yes we need pictures and without micrometer precision we need measurements.  A pebble bench is not the best at heating ... it is fast to build though.Cob and rock make a good mass. No way you should be able to touch the top of the barrel... if running properly it should have a glowing orange circle above the riser. Also you shouldn't be able to see your transition area at all it should be cobbed over and it is normal to be able to touch the exhaust after it leaves the mass.  Do you have a copy of the Wisners builders guide ??? If not you should get one... it will help immensely.

I'm still not done cobbing the ducts.

Hi Chad. I can't get my hand near the top of my J tube rocket after around 5 minutes of burning. If the barrel isn't getting hot, it's unlikely that your bench will either.

Have you insulated around the burn tunnel?

The burn tunnel/heat riser is made of firebrick, my thought was that it is insulative enough....

It depends on if you are using 'hard' (clay based) as opposed to vermiculite (insulating) bricks.

If they are clay based bricks and the fire brick is in direct contact with the earth it will reduce the combustion temps in the burn tunnel. The bricks there will constantly shed some heat into the earth which will make the burn less efficient.
When experimenting with my own workshop 'J tube' rocket I found that insulating around the clay based bricks in the burn tunnel made a big difference. Eventually I settled on vermiculite insulating bricks in the burn tunnel with additional backup insulation. The difference in performance between the un insulated clay bricks and the insulated vermiculite bricks was considerable.

Air entrained concrete around the ducts? That's no good. It is insulating.

Satamax Antone wrote:Air entrained concrete around the ducts? That's no good. It is insulating.

I did that on the side, hoping to push the heat more upward than sideways since the ductsa are quite low in the bench... still bad?

Chad Sentman wrote:

Satamax Antone wrote:Air entrained concrete around the ducts? That's no good. It is insulating.

I did that on the side, hoping to push the heat more upward than sideways since the ductsa are quite low in the bench... still bad?

Well, if it's meant that way, nope. That's fine then.

Ideally there is little to no air in the thermal mass surrounding the vent ducting, and if heat isn't being drawn from the duct due to insulating air, that heat will travel further down the duct and eventually out the chimney. If you can get some mud/cob surrounding the duct and embed your rock into that to eliminate air gaps it will work much better at that point.

For the heat riser, is it made from an insulating material or is there insulating material wrapped around the riser? You definitely want the burn tunnel and riser insulated, to increase the efficiency and prevent stalling if the riser heats up to where the temps start to balance.

I would also be very concerned about how close you have wood to the ducting. Even if the wood doesn't catch fire right now, if hot enough the wood gets baked and over time the burning point drops until eventually it will at least smolder. Erika and Ernie Wisner's Builders Manual go into the details of fire safety regarding RMH, but I wouldn't have any wood that close to your barrel and ducting, only non-combustible masonry material and heat shields properly installed to protect the rest.