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New by the book build. Back drafting bad!  RSS feed

 
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Put a lot of work into this 8" rocket mass heater. Did it all bay the book. Built the fire Box opening and burn chamber 7"×7 1/2". Used refractory kiln brick for the riser 48" high. 250 gal tank 22" wide 50" high. Have 25' of horizontal pipe in the mass set at 1/8" per foot rise toward outside 5' riser 3' horizontal termination to the outside. Holy crap what a horrible back draft! Had to have hair dryer on high with the feed all but closednup around the hair dryer to get it to draft. 15 minutes, removednhair dryer and POOF! Filled the house with smoke. What's up? Help please!
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Jay, what is your chimney like? From your description it sounds like it may just punch through the wall horizontally and terminate out the wall. If that's the case you will struggle getting it to draft. Can you describe your chimney situation in more detail? Maybe some photos?
Also, how much clearance do you actually have on between your heat riser and bell/tank?
 
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As Caleb suggests, it HAS to be a chimney issue or clearance issue between the "barrel" and heat riser.  The current barrel situation in the picture is terrible.  You need to be able to get in there and inspect/clean.  You've basically made a permanently installed barrel.  What did it look like before the barrel was installed?  If the chimney is ok, that barrel needs to come off.
 
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Hi Jay,Welcome to permies!

We need better pictures and more info.     What I can see is) it appears your feed tube is only 3 bricks tall ? That's only 12-13" tall, should be around 16". If it's too short the fire will want to draft up it, instead of your riser. How long is your burn tunnel roof ?   Should be around 12" . Too long and you will not get a good draft.   Hard to see but you may have air leaks at your feed tube, is your core still wet ? Takes time to dry out & then heat up heavy fire bricks.
Removable lid barrels really are nice if your having problems. Lots of folks have used a tank like yours but its a pain having to remove it for inspection.
What is your chimney like?
 
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Jay Breckheimer
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Eric Hammond wrote:As Caleb suggests, it HAS to be a chimney issue or clearance issue between the "barrel" and heat riser.  The current barrel situation in the picture is terrible.  You need to be able to get in there and inspect/clean.  You've basically made a permanently installed barrel.  What did it look like before the barrel was installed?  If the chimney is ok, that barrel needs to come off.


The clearance is 2" between heat riser and top of barrel. The barrel is exactly the same outside diameter as a 55 gal. Drum. Plenty clearance around sides. Over 3". The top of my barrel is getting cut off for access. A barrel band fits perfectly and clamps down perfectly also. It will have gasket rope between. The reason for using the tank was aesthetics. Wife didn't want 55 gal barrel.

My old wood stove that sat here. Exited the same hole and went up. I did not want to go so far up. I had a East wind yesterday blow in right into the pipe, and it just could not catch a draft even when I added an elbow. It worked great today with a NW wind.

I believe the answer is to abandon side outlet and run class A right through the roof as much as I didn't want to do that.
 
Satamax Antone
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2 Inches top gap is way too small for an 8

8 x 3.1415926  = 25.13 X 2 (gap) 50.26 square inches.

8 x 8 x 3.1415926 = 201.06 /4 = 50,26 square inches; the two numbers are equal, since there is a direction change, the recommended number is at least 1.5 times that figure.

Do you think a direction change, in such a hot, turbulent and fast stream of gases can occur without losses?  Nope. There's the high speed crashing of the gases on the ceiling of the barrel, then the direction change, twice!


See, this is an 8 or 9 inch gap, into a batch rocket.



My opinion, go for 4 inch gap, since you won't be cooking on a round top. So you don't need the high temp spot in the middle of the barrel.

HTH.

By the way, do the same calc for your side gap to flue transition, to see if you are within specs.
 
Jay Breckheimer
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thomas rubino wrote:Hi Jay,Welcome to permies!

We need better pictures and more info.     What I can see is) it appears your feed tube is only 3 bricks tall ? That's only 12-13" tall, should be around 16". If it's too short the fire will want to draft up it, instead of your riser. How long is your burn tunnel roof ?   Should be around 12"
What is your chimney like?



Feed is 16" from top to bottom of burn chamber. I went off Ernie and Ericas design with fire brick. Followed it exactly. Can the feed tube be too high? Wouldn't mind adding a piece of chimney liner to the top.
 
thomas rubino
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Hi Jay;  Yes a feed tube can be to tall. It can compete with the riser to be the chimney.

2" is the recommended (by Ernie) minimum for an 8"  , I went with 2.5" on mine but I have a 55 Gal barrel and the removable lid can flex I did not want it to go too small.  my new stove also an 8" I plan on a 10-12" gap as I am heating brick bells rather than pushing thru pipes
 
Satamax Antone
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Well. I will repeat, any gap bellow 1.5x CSA is bound to have problems.

Check this, it could shed light on m'y senseless babble

 
Eric Hammond
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If I were to do it over, I would make the gap as large as I could and still maintain a 36 inch clearance between the top of the barrel and the combustible ceiling. Seems like that gap should be as large as possible.
 
Satamax Antone
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Eric Hammond wrote:If I were to do it over, I would make the gap as large as I could and still maintain a 36 inch clearance between the top of the barrel and the combustible ceiling. Seems like that gap should be as large as possible.



Depends what you want to do.

Tighter gaps are for cooking. Larger, to heat homes, and avoid problems!
 
Jay Breckheimer
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Satamax Antone wrote:2 Inches top gap is way too small for an 8



So you are saying that this diagram that I followed exactly from Ernie and Ericas book is wrong?
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Jay Breckheimer
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After some re-measuring. Turns out I have a 3" inside gap between top of burn stack and inside top of barrel. So that should help. A little. I have had 2 burns since I wrote this post. Everything seems to be working fine now. Better each time actually. Thank you for all the input. It helped a lot! I will post some better pics once I get the build complete.
 
Satamax Antone
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Yes definitely.

I have explained the calculation. Refer to the link i have posted.

Turbulence in an elbow, boundary layer not wanting to change direction, high speed gases not wanting to change direction, all that means that if CSA and gap surface, (ring projection if you prefer) are equal, your "flow" is chocked. It takes a huge amount of energy to make the gases change direction. Turbulences form on each edge of the heat riser, which are a good inch in diameter, gases crash in the barrel ceiling. All that means there are lots of turbulences.  And the flow of gases can't pass that easily. Plus, gases at atmospheric pressure, are considered as incompressible. So that can't even save the functioning.

See here, in my workshop batch, running in thermal runaway mode.



Do you think this could be chocked within two little inches ? Or am i talking completely gibberish?

This could well illustrate what happens exiting a heat riser. And this is not sharp orifice, but a funnel end.




 
thomas rubino
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That's Great Jay !  Warming up your stove , straightening out your chimney, you shouldn't have any more issues.  3" gap is more than enough for your J tube, mine has been 2.5" for 5 years now.
A batch burns much more violently, 2" (as recommended by Ernie for a J tube)  would be much too close.  

Excellent video and information Max.

Keep your pictures coming Jay.
 
Satamax Antone
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If it's three inches, i shut it!

Thomas, thanks.


This one is even better,



If you immagine what happens in, a T, the two blue regions are increased, even more turbulent, and the gases crashing on a flat surface instead of a sharp point, create even more turbulence. Mixing with the stalling blue adds more and more drag on the gases.
 
Satamax Antone
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thomas rubino wrote: mine has been 2.5" for 5 years now.



Thomas, you don't want to try it at 4, for the sake of science?  
 
Eric Hammond
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Jay, you might also try cracking open a window to let fresh air into the house..  air can't leave the chimney without some more air to replace it.  Dryer running or bathroom fan could make the condition worse.
 
thomas rubino
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Well, Max ; this next spring / summer I am replacing my fireclay perlite riser in the greenhouse .I would do it right away but my wife says I must finish the new one before I can take apart hers... My current riser is 53", from the bottom of the tunnel to the top of the riser.  The new 5 minute riser will have a 57" rise.  
My barrel will need to be raised up to fit... so I may very well try a larger gap at that time. If I don't like it I can always drop it back down.
My new build in the shop I have a 12" gap planned. That way IF I decide later to change it to a batch it will be easy...
 
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Can I ask what may be a stupid question here, though I think Satamax already addressed it with his comment about tighter gaps being for cooking and larger for heating homes?  In my 6 inch system my gap ended up being 3+ inches (I forget the exact amount and don't really want to open the barrel to find out right now.)  My RMH heater seems to be running great to me, easy to start.  Temperatures as it's going up the chimney are higher than I'd like, but I suspect this is because the whole thing is smaller than it could be due to a lack of space in my home, and I don't have all the mass in place yet.

Anyway, my question/concern is that I understood that the top of the barrel would get to around 800 degrees F when it was burning good.  If my gap was larger than 2 inches would this then lower what that barrel top temperature is likely to get?  I'm generally reading high 300's to mid 400's, occasionally up into the 500's.  My concern is that I've got something wrong and I'm not getting a hot enough burn to avoid creosote developing in the heat exchange bench and chimney.
 
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Hi Jay, I was just wondering about the top gap. When you say you have a 3” gap, where are you measuring that from?
What I mean is.. your cylinder has a domed  top so if the gap is 3” at the very top will it close down due to the domes radius?
Or to put it a different way, what is the gap on the extreamly outside of the heat riser?

Re the gap, I can only comment on my own experiments, so my results probably only apply to my own design.

I did plenty of test and found that on my 6” system,  2” did not work very well, 3” did work but 4” seemed to give the best compromise of draft and heat on my cooking plate.
The biggest issues I had was how long it took to dry out the refactory, even in mid summer it must of been 10 burns before the system was really kicked  ass! and reaching over 1000f on the top plate.
 
Satamax Antone
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Fox, the gap is the tightest space between top and heat riser. Exterior, interior, mid way, whatever! Just the tightest gap. If you do a trumpet bell end, you can reduce the gap, since the diameter is larger, and so is the circumference.  

David, i think theoretically, in a perfect burn, you could reach 900 celcius at the top of the heat riser. So, a third of that on the cooking area would be amazing. On my batch, the walls of the "barrel" get around 285/295C°But that's approximate. And cooking plate was 191° last night. But the cooking plate is on the firebox. Not on top of the riser.
 
Fox James
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Thanks Max, although I do understand that, I was trying to put across something else, perhaps a picture will help me explain.
I understand that the bigger circumference on the outside of the riser will increase  the potential flow area but perhaps it might still effect flow?
I have no idea how Jay finished the riser so i am just adding a sujestion....

E01DDC25-CB8C-47A3-89B0-AD9C8AB10732.jpeg
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Satamax Antone
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Yes, the 2 inch in the centre of the dome isn't enough for a six incher already.

Imagine the 1 inch is at 10 diameter.

10*pi = 31.41 square inches.

6*6*pi/4 = 28,27 sqin. So the 31.41 isn't at the recomended 1.5* CSA.
 
David Huang
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Thanks so much Satamax for that information.  It sounds like my heater is getting hot enough during a good burn then.  There have been so many little new things to learn while doing this project it's a relief to have some corroboration that the results are correct.
 
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