Rocks in the vent pipe?

Helo folks,

So I'm working on the construction of my first rocket stove + mass heater.

I was wondering.

Would it be helpful to add some rocks within the vent pipe of the bench in order to absorb more heat? I feel like that 6 inch pipe lets escape still too much heat!!

I don't think it's a great idea. It will disrupt airflow within the duct, decreasing the efficiency of the stove. If your exhaust temps are too high at the end of the bench, you could either add more bench length or put a bell at the end of the run, before the exit chimney.

Here's a picture.

Charles Laferriere wrote:Here's a picture.

Ah, did you mean underneath the duct? After reading your name it became clear English might not be your first language.  Rocks in the mass can work from what I understand, but it is incredibly important to make sure the mass is monolithic, and please, do NOT USE river rocks. The moisture in them can cause them to explode if they heat up too much. I personally wouldn't use them at all, but that's just me. By the way, what kind of insulation is on the ground under the bench? And where is the stove?

Yeah I meant rocks INSIDE the vent.

I'm not sure if these are river rocks? They are store bought rocks called "crushed stone".

The insulation is:
1 part clay
1 part sand
1/2 cow dung
(eye balled)

The stove is in the basement. I made one test run. It worked well! I was thinking to build a hho torch to get the fire started. It'll save on the propane.

Is your mass already applied over the ducting? If the photo you posted is current, I would hesitate to say you need more heat extraction without the thermal mass in place.  

Yeah the job is pretty much done now.

In that case, I'd still advise against rocks in the pipe. I'd go for a bell to extract more heat. Check this out, if you haven't already: http://batchrocket.eu/en/building#belltheory

Charles, don't use rocks in the pipe. This is a foolish idea, to say the least. Sorry.

The only way to do this is a bell filled with rocks, with enough space between them, so the flow of gasses is not disrupted too much. Far away from what you wanted to do.