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How much heat should the barrel be throwing? Help me I'm losing faith in this whole project.  RSS feed

 
Aaron McKinley
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After a few weeks of finishing the RMHeater I am wondering if it is running as hot as it should.  How many hours does it take for the barrel to feel like a wood stove? I don't feel the radiant heat like I do from the wood stove in the house, with the barrel.  I can put my hand pretty close to the barrel when it is cooking.

What I have done is the typical 8" rmheater with a cinder block box for the exhaust pipe, for a 16x18' greenhouse.  In the last few days I have filled in the box with clay fill. I don't have the time to even think about making cob for the whole thing.  So I have covered the exhaust pipe with clay fill and filled in the gaps as best as I could.

I am coming up against a few issues that are really starting to wear on me.  Number one:  Really do I have to spend hours and hours in the greenhouse just to get this up to a decent temperature?  Yes you burn less wood but you have to babysit this thing and put wood in every ten minutes.  Mind you, I have not experienced the benefits of getting the mass up to a decent temperature yet as I just finished filling it today.

I can't get the image out of my head,  of hooking up a decent wood stove to the mass exhaust part, putting cob around it and firing it up, stoking it and walking away, and coming back in several hours time to put a few more pieces of wood on, then leaving again.  Has anybody done a hybrid like this?

Any thoughts?  I do have a full week off of work, so I can really push this thing.  I have put so much time into this thing and I am not really seeing a great return on it as of yet. I am patient and will tinker with for awhile but I have to move on from this project.  Any RMH anonymous groups out there?

Thank you for the advice already given and the advice yet to come.
 
Satamax Antone
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Well, do you have pics and measurements?

This would be a first step to assess if it's functioning normally.

How's your chimney?
 
Aaron McKinley
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Thank you Satamax,

The stove works and it draws the flame into the burn tunnel but doesn't seem to heat up as much as I expected it to do. 

I did have to move the pipe coming out of the barrel from side to side as I was making the cement block box, a bit.  I did notice today that the cob has cracked in a few places, so
i going to replace the entire ring of cob with fresh cob.  Regardless of the new cracks, it was performing the same way before the cob cracks.

I did have some strong winds yesterday and smoke and fire did reverse and come out of the fire box in short spurts.  Out of the four or five times of firing it up, this only happened yesterday. Any suggestions for the end of the chimney pipe?

rocket-mass-heater-in-greenhouse.jpg
A rocket mass heater in a greenhouse
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Byron Campbell
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Location: US, East Tennessee, north of Knoxville
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Hi Aaron, I'd extend the chimney vertically by another 5 to 7 feet, whatever it takes to make it at least a few feet taller than the roof's peak. The stove will draft better, run much hotter, and smoke back much less if any at all. In wind the stove's draft will actually increase rather than smoke back, with a proper / tall chimney. I use just the standard dome style chimney cap sold for Class-A type metal chimney, works just fine in our 25 ~ 35 MPH wind gusts.

Edit: As far as how hot the barrel gets etc., it will be uncomfortable to stand within 3 feet or so of the barrel at full/peak burn. Without even pushing my stove I've IR'd the barrel's top at in excess of 750 degrees F. when burning moderate amounts of dry high fuel density wood; ash, maple, oak. And it only takes about 15 to 20 minutes for the barrel temperature to reach that maximum value.

When the mass of my stove has cooled to ambient, aka cold start, I'll fire the stove continuously for between 4 to 5 hours to bring the mass up to temperature. From then on I'll fire the stove for 2 hours each morning and each evening to keep it all warm. HTH.
 
Satamax Antone
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Hi Aaron.

As Byron stated, your chimney is not very good. It needs to be vertical on the outside. Better if it is insulated too. (sheet metal and rockwool are your friends, even glasswool) Reaching above the apex. And with a chimney cap. On the inside, you could insulate the chimney with cob, so it is vaguely insulated and stores heat.

Then Your flue transition seems too small to me.

Check this thread

https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures

I wonder how is your top gap and barrel gap too.

HTH.

Max.

 
Aaron McKinley
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Thank you,

I will extend the chimney on Monday to the suggested length and insulation. 

Flue transition  Is that the space from the barrel to the mass area?

Looking forward to tweaking this this week.

 
Glenn Herbert
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Yes, that's what the transition is, commonly called the "manifold". It needs to be spacious so that gases can easily move from the barrel perimeter to the mass duct. The more gradually the gases have to change direction, the better. This is most easily accomplished by making the cross sectional area at least twice the nominal system area/diameter.
 
Aaron McKinley
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So connecting the pipe to the drum isn't the way to go? There must be two schools of thought on this then?  Doesn't the ernie and erica model use the pipe coming out of the barrel?

I'm confused now (does not take much).

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? Not looking forward to reworking this whole area.

Suggestions?
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Aaron McKinley
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The gap between the barrel and the chimney is 2 inches.
 
Byron Campbell
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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.
 
Aaron McKinley
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Thank you Byron,

It's becoming clearer.  I have a small confession to make.  I don't have the Ernie and Erica book yet as I purchased the first set of DVDs, plus the first video in the second set,  plus the RMH heater book, and all the free plans I could get my hands on. I thought this was enough to fumble my way through. I am looking forward to getting the book for my second mass heater!

OK, I did off set the barrel at the back of the stove but did not off set it on the side that the exhaust, that is my problem! I would say it's closer to two inches instead of four.
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?

Nor did I fold back the T in the barrel (just under an inch protruding into the space between the barrel and heat riser)

I need to strip the cob and redo this.  I'm not happy with the original cob mixture anyways. I had to use fine sand for some of the later batches and it started cracking. 

My wife is going to shake her head.  It's a good thing I have March break off this week to redo my RMH missteps.

Thank you Byron, Glenn, Satamax, for putting me on the right track.

I have purchased some more piping and a cap to extend the chimney (today).  I am going to strip the cob, shift the barrel to get 4 inches between the barrel and heatriser tomorrow, as we are going to get hit with four inches of snow tomorrow.  Snow falling outside, making cob and working in the warm greenhouse, finally getting my RMH running the way it should,  sounds good to me.
 
Dave Lot
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What size is your system again ?

I know my 6 inch rocket - after about 20 minutes or so, can warm me up 12 inches away . .  and if I get too close, after a while it is too much even thru the cloths . . . I know, I tried it in a ice cold shed  . ..

If you are running a 12 inch pipe, you should have double that heat output . . so yes, REALLY HOT . . .

The best thing I can tell anybody - if there stove is not heating up - is - make sure every dimension is the same size . . . think of the size of the chimney - from one end to the other - NO CHANGES . .
It is surprising how a taller chimney will help too, but that has been suggested . . .

As for the gap in between the riser and the top of the barrel,  when I was building my stove, I started with the top of the barrel sitting on top of the riser - yes, touching the top of the riser . .  that way, as you add bricks for spacers, - lifting the barrel up, you know exactly how big the gap is up in there where you can't see . . .

For my stove, I start at freezing ( 25 or 30 F ) . .  and a 2 hour burn will raise it up to 50 F . . . come out the next day, and it's still 50 .. . another 3 hour burn will raise it up to 70 or so . . but then my shop is insulated R20 all around.

In your case, you have alot of windows . . . with the windows in my house, I cut 2 inch styrofoam just big enough to friction fit . . . the windows go from R1 or R3 to R14 or so . . . just a thought . . . cover the windows  with insulation plugs until the growing season starts ?

Also, once my shop gets up over 50 F or so, I can take off my coat and work in the shop . . .  so "constantly adding wood" is not a problem . .  since I am out there playing  .  .. 

Hope this helps.
 
Aaron McKinley
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Thanks Dave,

I'm using an eight inch system, same dimension all the way through, so I should and will get more heat soon (tomorrow) as I make the suggested alterations.

I do have some blue styrofoam insulation I could use on the windows around the wood stove.  Good idea for the winter as these windows are not really adding to the greenhouse, this time of year.

Will report back.
 
Byron Campbell
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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.
 
Peter van den Berg
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There's a simple formula you could use to calculate whether the space between the barrel and the riser is large enough. Take the circumference of the exhaust and multiply that by the gap between barrel and riser. When the result is the same figure (or less) as the cross section area of the system size it is too cramped because the gases need space to go around a 90 degree corner. 150% of system size is acceptable minimum, 200% of system size (or even more) means no restriction at all.  Another way is to use a 12" to 8" reducer, that would resolve the problem without shifting the barrel although the resulting figure is just 150% of system CSA.
In 2014 I used exactly the same method at the Innovators Gathering see https://permies.com/t/40/40007/Results-batch-box-thingy-Innovators and scroll down to my post number 487.

I need to warn you for another similar restriction when shifting the barrel in such a way that the riser is close to the barrel wall. In that case, the gases coming out of the riser can't go as easily through that cramped side, so the result could be another restriction, the top gap this time.

Solution here: enlarge the top gap well above the recommended minimum. The thing will still work beautifully, I tried a top "gap" of 3.5 feet without any discernable negative effect.
 
Satamax Antone
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Byron Campbell
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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
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Thank you everyone,

I have made the appropriate changes: shifted the barrel to allow 3.5 inches between the exhaust pipe opening and the heat riser, eliminated the lip of metal from the exhaust pipe protruding into the 3.5 inch space, upgraded to a 12 inch to 8 inch reducer, added 4 feet of upright chimney pipe plus cap. 

Wow!  This thing is humming now.  Heat has increased, horizontal draw has increased.  I can hear the air going through the system when it is cooking. It takes me a minute to get the entire system going.  At the T at the end of the system, I light half an egg carton, let it burn half way down, put the cap on, light the other half with some pine kindling placed on top in the burn chamber and that is it. It always starts!

I will post some video tonight of the system in action.

My wife and I have all our seedlings in there as of this last weekend.

Many, many thanks, I feel much better about the whole system. Working on a system for longer straight pieces of wood.  It's safe and performance does not suffer.  I'll show pictures.
 
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