RMH troubleshooting

Ho guys. I decided to build a test RMH in my brothers little walipini. Everything worked out fine, however, as I've heard someone say this, my RMH is not "rockety" enough and there is still smoke coming out of the chimney which leads me to believe the RMH is not generating enough temperature.

Now, we dont have 8 or 6 inch pipes, we only have 5" pipes here. I calculated that we can have about 5-6m of thermal mass. Here are some pics from the build>


testing


the heat riser was tilted so later we rebuilt it


laying down the pipes


thermal mass


So the RMH is functioning. There are no leaks. At times it starts pulling air much faster, other times not. The barrel is hot, but not as hot as I'd expect.

I suspect the problem might be in the gap between the top of the heat riser and the top of the barrel. I think it turned out about 8cm, and I think it should have been 4-5.
Any suggestions?

Stevan,
I'd think the burn tunnel is way too long.
Is there insulation around the whole of the core?
Leave that 8 cm top gap, the recommended size is the absolute minimum, there's nothing to be gained by a smaller gap.
When all the above is in order, you best look at the manifold, the transition from the barrel to the bench pipe. That transition should be much wider than you might think.
And, most important, the cross section area of the heater core should be the same as that 5" pipe.

Your build does not follow the basic geometry which is recommended. That's the first thing I would fix. See page 28 of "The Book" by Evans. The basic geometry is 1:2:4, or 1:2:3, depending how you wish to measure it.

FeedTube:BurnChamber:FireRiser

The Burn Chamber is to be twice as long as the height of the Feed Tube. The height of the Fire Riser, you can measure either against the Feed Tube or the Burn Chamber. It's minimum recommended heights are either three times as tall as the feed tube, or twice as tall as the Burn Chamber.

Looking at your pictures I'd say your Burn Chamber is way too long. Most people have a horizontal length of three or four bricks on edge, where you have what, ten? And in the picture there is no way that Fire Riser is more than twice as tall as that Burn Chamber is long. The pictures did not show the finished height of the Feed Tube.

What are your measurements?

I don't think the upper barrel gap is your biggest problem. It may or may not be ideal, but a number of people report that increasing that height changes where the temperature is at is maximum in the barrel. That is not at all the same thing as saying it effects the total output of heat from the Fire Riser into the barrel. Personally, I don't see how the height of the gap at the top of the barrel can effect heat output, provided it is at least tall enough to permit air to flow unrestricted into the barrel (that's where the minimum height recommendations come from, as I understand it, at the ball park of 1.2 to 2-inches; at 2.54 cm per inch, that's roughly 3.8 to 5.1 cm, right?)

So there is a minimum height. Peter vdB has built double barrel systems in his batch-box experiments, and they have very large top gaps (measured in feet!) and they produce a lot of heat. My 6-inch system has about a 3-inch gap if I recall correctly, and it draws very well and get plenty hot. I've had it set up if I recall correctly even with a 4-inch gap at the top of the barrel, and it ran plenty hot. I personally am leaning toward a gap on the order of 2 or 3 inches between the top of the barrel and the top of the Fire Riser as my target when designing a J-style rocket heater. And 8 cm is just a small amount more than three inches, so I don't think that's your problem, in fact, I expect that part is just fine.

And insulate the fire box, it is most important that the Burn Chamber and the Fire Riser be insulated. The Feed Tube is less critical in that regard.

When you give your measurements, you ought to include the CSA of the critical elements: Feed Tube, Burn Chamber, Fire Riser, the ducting, and the amount of space provided as the transition between the barrel and the ducting (the manifold, in other words). The Wisner's have observed that the manifold is often made too small. And Peter has observed it needs to be much larger than many of us expect. Something like 4 times the CSA, if I recall correctly, but Peter is the guy to ask about that, as he's given more detail on how to calculate that than anyone else I have read.

Ok, thanks for the replies. I seems I misunderstood a couple of things..

First, the burn chamber. I just realised that in hurry yesterday I posted another "mockup" pic. The burn chamber was later shortened and is about 3-4 bricks now.
I also thought that if we built the heat riser out of bricks there would be no need for additional insulation.

All the ducts are the same diameter. What I could do, is reduce the diameter of the burn chamber.

There is no special connection from the barrel to the exit flue. There is just a hole in the bottom of the barrel that fits on top of an L shaped pipe connector and then goes into the thermal mass. I could possibly dismiss the L connector, and build a wider one out of bricks, to prevent a bottleneck.

Another big mistake, but fixable, is that, in all the hurry to get it finished, we did not notice that we don't have the J shape. I just realised that we don't have a feed tube! We just left an opening on top of the burn chamber (similar to pic no2). We'll try to fix that today and I'll get back to you with new pics, measurements and the results.

Some rookie mistakes there, cause this was not really well planned but done in a hurry. We'll probably dismantle it after spring, and build a new one before next winter..

Stevan Covic wrote:I also thought that if we built the heat riser out of bricks there would be no need for additional insulation.

That's a big mistake, the regular hard firebricks are heat sinks. Unless... you used the whitish soft firebrick for the core, that would be alright. So you left out one of the really neccessary things! Also, an overly long riser built out of hard firebrick isn't helping either, you would just introduce more mass in the area where you need the highest temperatures you can get.

Stevan Covic wrote:All the ducts are the same diameter. What I could do, is reduce the diameter of the burn chamber.

Waitaminute! Are you saying there's a steel duct inside the tunnel and riser? Take that out, especially the elbows will be the cause of a negative effect. You are killing the much needed turbulence by building it like this. There should be no steel in the combustion area at all, every metal we know of will fail sooner or later, rather sooner. Please, buy the book "Rocket Mass Heaters third edition", everything you need to know is in there.

Stevan Covic wrote:There is no special connection from the barrel to the exit flue. There is just a hole in the bottom of the barrel that fits on top of an L shaped pipe connector and then goes into the thermal mass. I could possibly dismiss the L connector, and build a wider one out of bricks, to prevent a bottleneck.

That's another mistake, all the gases are gathered at that place, the exhaust opening need to be wider than the ducts you are using. Also, when you are adding insulation around the riser and burn tunnel only the top of the feed tube doesn't necessarily need this. And by insulating you are almost certainly restrict the exhaust opening. This device isn't to be built in a hurry, you are bound to build it the wrong way so it's no surprise it isn't working as expected.

Sorry if I am sounding like a school teacher, a proper running rocket heater isn't an plain toy. It looks deceitfully simple but it isn't, more a wolf in sheep's clothing when built the right way...

Peter; This

a proper running rocket heater isn't an plain toy. It looks deceitfully simple but it isn't, more a wolf in sheep's clothing when built the right way

is the best statement on RMH that I have seen yet !
Stevan; Listen to everything that peter has suggested to you, he is an innovator of the highest quality when it comes to building & fine tuning a rmh ! The only suggestion I will make is to look at casting a heat riser with fireclay & perlite as described by matt walker (broaudio) on youtube, hopefully those materials would be available in your location .

Hey there, thank you all for the constructive criticism. I did read the book a while ago, however this build came suddenly and I am in the middle of a bunch of other projects, so that is why we kind of just slapped it all together.
Just got back from the farm and although I didn't get to take any pics, I did get the measurements- feed tube (will be) 40cm, burn tunnel 90cm, heat riser 120cm

We will also discard the L tube that connects the barrel with the thermal mass, and build a wider chamber (manifold) as you've suggested.

I will also try to source a smaller barrel that I can place over the heat riser and then fill the gap with some kind of insulation(clay+perlite probably).

Thomas I can get perlite, and I'll ask around about the fireclay. What we use here for all types of furnaces is called "yellow earth". Essentially it's a kind of clay used for cob, for building or plastering houses.