Interesting observation

[rjdudley]

So it was a great weekend using my RMH. A lot was going on: we were stacking more thermal cob (never seems to end) and doing a burn to help dry it out.
But, the burn was only so-so. As a matter of fact when it was first lit there was a terrible smell, kinda like a chemical smell. If you've ever used the fire starters it was sort of like that, but, different.
And I'm willing to bet that if I could have seen the stack outside (it was night time) it would have been pouring black smoke out of it.
Then, the burn tunnel starts to "gulp" air. As though it could only take so much oxygen every two seconds or so. Finally after it got nice and hot everything seemed to settle down and draft fine, but, I still needed to cover the feed tube with a brick to help choke the system.
We were going to add more cob, but, I was troubled by something I had seen. I had noticed that the 55 gal. barrel top is concave, not convex, meaning that it looked like the top of the barrel was closer to the top of the heat riser than I had planned. Also, Matt, remember when I did my refractory pour that I said the inside bottom of the heat riser had pushed in a bit. Well, before we tucked this baby all the way in I decided to take apart the barrel and look at the insides.
So we broke the seal (cob) holding the barrel and lifted it off. I'm glad we did. Not because we saw anything glaring, but, because it's nice to see what things look like after half a dozen burns.
The barrel really bothered me and so I got a 2x4 and forced the center back to a convex (rounded) profile. Next I removed the 8" galv. pipe that was the interior of the heat riser. Looking at it after only six burns or so inside the heat riser made me cringe. Even if the buckling at the bottom didn't affect the burn it just frustrated me to leave it like it was. Flawed.
So, my wife told me to remove it and I did.
Much better.
So we cobbed everything back into place, level the 55 gal barrel and fired it up. No lie…best burn since I built it. Never needed to choke it down by covering the feed tube, no smoke back, no fire creep, smooth rocket sound. Nice.
So I just wanted to post this to say that little things can make a huge difference. The clearance between the heat riser and barrel makes a TREMENDOUS difference. The feed tube had been wide open for three burns now and the stack still stays at 150˚ or less.
So a recap for any new folks:
I thought the distance between the heat riser and the barrel was supposed to be 2-3 inches (after reading some other forum posts I've come to believe it can be a little bit more).
I thought MY distance was 2", but, if the barrel bottom (now the barrel top) isn't flat, or worst in my case convex, the you need to adjust for that. It's necessary for the push/pull of the system to work correctly.
Does this make sense? It seems to make to me because I saw the issue, but, I'm not sure it comes across in writing.
Ray

[matt walker]

That's a great post Ray, I'm sure that will be very helpful to folks down the road here. I think you are right on, and I remember posting something about the stage you are going through now with the internal form for the riser. Yep, it only takes a few burns to start taking that thing out, and while it's deteriorating it usually messes up the burn pretty well for a little while. It's really important to remember that that is coming and not get too freaked out when things get a little rocky. You did good taking off the barrel and taking care of it, and I'm glad you posted this here as a reminder.

As for the top gap, yep, I feel that 2" is on the verge of too small already. As you found out, it's very possible for the warping of the barrel bottom to close that gap to where it's a problem if you start out tight. I've successfully run these things with two barrels stacked on a system with a normal sized riser. At that point, it truly is acting as a bell, and it works fine. So yeah, in my opinion 2" is too close, and there's no such thing as too much top gap.

[matt walker]

Oh, and Ray, don't be afraid to add an 1" or two to the barrel landing to get yourself a little breathing room up there. You also learned a great lesson in how easy it is to take off the barrel and then slap a little more cob around it and keep going.

Are you, like I was, in constant wonder about how perfectly suitable plain old cob is for the sealing and landings and stuff? It so funny how we think on these little connections and stuff, and make them so complicated in our heads, then get the mud going and it all becomes clear.

[rjdudley]

Matt, I gotta tell ya that this whole cob thing amazes me on soooooooo many levels.
I never understood it's value. Never. Once you get to working with it a person begins to develop a much better sense of all that it's capable of doing. I no longer have ANY fear of cracking off the old and rebuilding. Thanks again.
So why isn't the clearance between the heat riser and the barrel discussed more? It surely seems to be as important as the whole burn tunnel/feed tube CSA issue?
There must be a clearance between the heat rise and the barrel, right? What should that be? Or….is more fine. Just don't go smaller?

[matt walker]

You just made me smile Ray, I clearly recall my neighbor and I putting mine together and just laughing, wide eyed at how easy it all was once we started in with the mud. We agonized over details, and "how will we seal that", or span this or,....anyway, yeah. You really just gotta get your hands dirty then it all becomes clear.

As for the gap, well, anywhere there is real, two sided discussion on rocket mass heaters there is a general consensus on the way I put it up there. No such thing as too big, etc. Sometimes the real information takes a while to find though, and you know, these things are evolving quickly as well. What was the one true way a few years ago may not be relevant any longer.

[mannytheseacow]

[matt walker]

Manny, so glad to hear you are finally warming up and getting past that birthing stage. Your post just reminded me....you two, Ray and Manny, you guys are at this stage which I have some advice about. And it sounds kinda crazy, but give it a try. Cover those benches! I know, you are trying to dry them out and it seems totally backwards, but if you have some towels or old blankets or something like that, cover up your benches while you burn it dry. Or, at least try it a bit and tell me if I'm nuts.

What I found was that by covering the bench and insulating the heat in it lost heat a lot slower but retained it's ability to lose moisture. My theory (ready for this crazy?) is that the fabric, once the whole bench is steaming dry, becomes sorta part of the layer of cob that is transpiring moisture. In other words, it's passing the moisture to the air just as quickly as if it weren't there, but the big advantage is that it insulates and rather than the top layer of cob having that evaporative cooling effect keeping it's temp down that calorie transaction happens at the top layer, the boundary between fabric and air. Anyway, I found it to be really effective in getting my bench temp up in those early days.

How are those steamy windows boys?

[mannytheseacow]

Thanks for the advice, Matt. I remembered reading that in one of your other posts, too. I've got a huge alpaca wool blanket from Peru that was fairly inexpensive. I found some old couch cushions and I'm going to take them and the blanket to a friend later this week to sew the blanket into a sort of "sleeping bag" that the cushions can be slipped into to make one long comfy cushion. I'm a little bit on the fence because the bench is really nice with just that wool blanket on it but I think the cushions will give some added insulation.

I agree that the wool really wicks the moisture while retaining the heat. Earlier this weekend I could see steam raising from the blanket. At one point there was literally a steady stream of water running out of the clean out under the barrel, too. I also see a trickle of water running from the joint in the exit flue often.

Oh, and at some point I'm going to plaster this thing? .....eh

.