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Poor draw in an under the floor system.

Rick Sherman


Joined: Mar 17, 2012
Posts: 8
I have been living with a variation of the RMS all winter in an insulated yurt. See http://sundogecovillage.org/sundogecovillage.org/Projects/Pages/Yurt_Project.html.

The heated mass has kept the 300 sq' space at an even 50-55 degrees all winter with inside temps up into the 70's if we burn longer than two hours. The issue for me is that we have not succeeded in getting the stove to draw without the use of an electric fan to pull the smoke and heat through. It burns wood down to fine white ash but smokes more than I would have anticipated at the outside stack. Over all the stove has been reasonably efficient for fuel consumption, considering we are using beetle kill Ponderosa Pine

At first the six inch system extended across the 20' space and back under the stove and out the back. With four elbows in place the distance was over 40'. I recently shortened the system to less than 20' and replaced two 90 degree elbows with a curved plenum box. It improved but did not cure the draw issue.

We were never able to get the down draft system to work in the beginning so we created a wood box that burns the wood horizontally. The two departures on the standard design is the horizontal burn chamber and the below the floor exhaust. The drop from the bottom of the barrel to the horizontal pipe run is about 2.5' then about 6' to the plenum. The whole exhaust system is below the level of the fire box.

I love living with the system and I like the ritual of the twice daily burns. The big problem is that the power goes out on a regular basis here. If the fire is full on, we get smoked out when it does. I am determined to overcome this issue and develop the concept further as we add more experimental structures to Sundog.

My question is, is the low exhaust the issue? Should I abandon the heated floor concept and go with a heated bench that exits the barrel and runs horizontally from the there?



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Roy Clarke


Joined: Feb 05, 2012
Posts: 121
I'm guessing because I don't know much, but I think there are likely to be two problems. One is the connection to the exhaust pipe at the bottom of the barrel, is it big enough? The other is the barrel itself. It does not look to have much exposed surface. The cooling over the barrel surface is what provides some of the power to make the gases flow.
Rick Sherman


Joined: Mar 17, 2012
Posts: 8
The barrel is half exposed and half buried in rock and cob. I had not heard about the cold air against the barrel acting as an engine. It is something to look into. The exit from the barrel is a 6" hole into a 6' elbow with a 45 degree bend as it descends under the floor. The exit hole is sealed with cob which is part of the reason for the barrel being partially buried.
Roy Clarke


Joined: Feb 05, 2012
Posts: 121
That exit hole sounds like a candidate for attention. If it goes from the barrel side directly transforming into a 6" hole (ie a hole cut 6" dia in the side of the barrel) it will be a restricted exit for the gases. The gases at this point are hot, and thus have a high viscosity, when they get well into the ducting they are cooler and so can flow easier. Have you read Ianto Evans RMH book?
Kirk Mobert


Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
    
    4
Roy hit it on the head.
You can't just stick a pipe in the side of the barrel. You MUST have a much larger volume at the manifold area. Larger than you would think by a lot.
Almost without doubt, that is your problem.


Build it yourself, make it small, occupy it.
Kirk Mobert


Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
    
    4
This issue is THE MOST common trouble spot for rocket stoves.
It's not properly described or stressed in the book, and it's easy to overlook.
If you didn't already say, I would have asked about it, it's the first place I go when there's trouble.
Rick Sherman


Joined: Mar 17, 2012
Posts: 8
Roy and Kirk,
Thanks for the insight, I had not considered the exit volume as an issue. My understanding was to keep the system, 6" or 8", consistent from the beginning of the j channel all the way through. I need to modify the barrel to be opened from the top for cleaning. I tore it apart this winter after three months of daily use to find that the creosote had filled the interior wall of the barrel with black hoar frost. It has been another three months and likely needing it again. I need a redesign of the exit as well as the top access.
Is there a formula for the amount of space needed for the exit in a 6" system?

I love this process of learning, experimenting and sharing. I should be able to pass on some wisdom once it is all worked out.
Thanks again
Rick



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Roy Clarke


Joined: Feb 05, 2012
Posts: 121
That's typical of too low an air supply where combustion is concerned. It could well be caused by the restriction of your exit hole in the barrel. It may be something else too.
Kirk Mobert


Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
    
    4
This thread over here is a pretty clear discussion about it.
I'd fix the exhaust at the bottom first. If the black gunk comes back, THEN do something different at the intake.. Best to do just one experiment at a time.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
the hoer frost look is pretty normal in stoves that are not getting the proper flow in the system also in stoves that do not have a good gap between the barrel bottom and heat riser. We have also observed this in stoves burning uncured wood over a long period of time (relative to the stove). Also this can happen in a system that has to long of vertical chimney and the flow in the stove is to fast for the burn time. Experiment around a little with each of these and see if one or the other provides an answer.

my bet would be uncured wood and too small of manifold volume.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


Rick Sherman


Joined: Mar 17, 2012
Posts: 8
Thanks Ernie,
I have been using ponerosa pine that has been dead and drying for at least a year. It's not the best fire wood but since we have been hit with pine beetles we have plenty.
I am using a fan so the fuel doesn't linger long enough for a full secondary burn and the exhaust exit is too small according to the responses received here.
What are referring to when you say, "stoves that do not have a good gap between the barrel bottom and heat riser"? If you are referring to the top of the stove where the heat spills over, this one is 1.75".
Thanks for the feed back all, it has been most helpful.
Rick
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
Well P pine would be a good part of it as would the fan. your gap is good. looks like most of it is the restrictions and the dwell time for the combustion gasses.

you shouldn't need a fan for more than a few min as the stove heats up. it sounds like this problem has been around for a while. You want a little slower system for p pine. if you have a vertical chimney i would suggest building it taller till the draw matches that of the fan then reducing the height till all you are getting is steam and CO2 with no hint of smoke. it takes a bit of time but it will get you the best results. as always with this stove just enough is better than to much or to little.
 
I agree. Here's the link: http://stoves2.com
 
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