Helena Davies

We're on our third RMH test-setup and experience the following:

- Really good draw when fire is lit (smoke from cigarette blown above hole gets sucked in) with great horizontal burn.
- After 20 minutes or so the fire starts creeping up the sticks in the feed hole and a little while later the fire is burning upwards out of the feed hole (and still a bit horizontally as well).

What would cause this?

Here is our setup:

- We've built a prototype outside pretty much according to the book.
- First we built the feed tube, burn chamber and initial section of the heatriser from firebricks.
- Then we cobbed all that in.
- Then we placed our insulated heatriser on top.
- On lighting that setup we got a really good horizontal burn. Very encouraging.
- Then we added the barrel over the heatriser. Initially with a 7cm clearance over the top the rockety sound became much less, but we increased the clearance at the top to 15cm and the rockety sound came back. So we left it at that height (we're not concerned about cooking on the top).
- Then we made a transition chamber with walls from old brick and cob and connected the tubes that would eventually be cobbed in.
- This morning we lit it. Everything burned great for about 20 minutes and then fire starts creeping back the feed tube and we start to get smoke coming out of it.
- I opened the inspection hole to the transition chamber and it was really steamy in there. When I stick in my hand it felt like a sauna.
- Presumably this is from the cob drying out. But would that stop the draw so much?

Is what we're getting normal/expected or are we going wrong somewhere?

Thank you all!

Helena

Hi everyone,

I've been rereading Ianto and Leslie's book and noticed in all their diagrams they show insulation on the heat-riser side all the way down to the floor. At the moment we've only insulated the actual heatriser core (marked in green in the attached diagram). Ours is one 8" pipe sitting in a 19" pipe and the area between filled with a perlite/clay mix and capped with a a sand/clay mix (see attached photo). Is insulation further down essential? How could we go about that?

Sorry to keep coming with questions here and a big thank you!

Helena

allen lumley wrote:Helena Davies : You need to finish burning off the paint on the outside of the barrel before you take it inside ! Go to our sister site richsoil.com, and click on ' Rocket Stoves '
what you want to look for in this series of videos is the 7th-8th/ one that shows how to burn off the barrel prior to using it inside your house Best Big AL

Thank you Al
Believe it or not, this one has already been burned once. It was blue initially. The white seems to be a second layer... But we also have a newer barrel with less rust that we'll probably use. Just need to cut the top off. And we'll burn it off of course

Best wishes

Helena

Attached another couple of pictures looking into the transition area. I've measured it at it's widest points as 40 x 40 cm (15.5 x 15.5") and 40 cm deep. Seems quite big and I'm thinking too big perhaps? Would this cause the hot gases to accumulate and prevent a good draw and causing smokeback?

In the first view you can also see we've closed off the flow from the barrel at the top. That's because we're offsetting the barrel at that point with the aim of directing the heat into the room. Maybe this is also a consideration?

Thanks again so much for all help

Here is an image of our setup as of today. The rocket-heater part is at an angle because inside it needs to be like that - unmovable parts of the house in the way



This is with the added height to the heatriser mentioned above. Yesterday the heatriser was lower and we didn't get enough draw. The smoke coming from the exhaust yesterday was quite thick and creamy white in color. Today with this extra height we have a better draw and the smoke coming out at the end is more transparent.

The main problem we're having at the moment is smokeback. Once burning well, I've tried filling up the feed-tube completely with thin (2 fingers thick), dry firewood in an attempt to create a faster airflow, but it didn't help: flames are burning horizontally into the burn-chamber, but also coming up out of the feed tube (together with black or grey smoke). We'd need to resolve this before we can move inside.

Some questions/considerations

We're in a windy place. Yesterday was almost without wind, but today it's quite windy. Would that have an effect?

We haven't yet cobbed in the whole burn-unit, just closed the gaps with cob.

Our transition area is quite large now (especially with the extra height). Would it be possible that it's too large?

We had the pipes built to order, which was quite expensive (standard stove pipe is only 5" here) and we were hoping to get away without a chimney. Does that sound feasible?

I'm hoping for some pointers in the right direction...

Thanks again so much for all help. Wouldn't know how to do this without this great forum!

Helena

Thank you Prescott. Yes, we found the same as you did. With only the heatriser in place we had a nice rockety effect. Then adding the barrel dampened that almost completely. But then closing the gaps and connecting the barrel to the exhaust pipes improved the rocket effect again.

Prescott H. Paine wrote:Are you cobbing your core? They draw so much better all sealed up!

That's good to hear. We noticed an improvement already when we sealed the openings between the bricks. Would you actually cob it all up in the test-setup?

Thanks again,

Helena

Thank you for your reply Allen. I appreciate all your help a lot! We've been reading around the forum and tweaking and testing more. Still no satisfactory results, but we're full of hope that it's going to work.

allen lumley wrote:Helena Davies : Measuring from the floor of the Burn Tunnel, up The Heat Riser's Height should be ~ 3Xs ~ the Height of the Feed Tube !

The height from the bottom of the burn tunnel to the top of the heat-riser was 95.5cm (37.5"), but we've just increased that to 107cm (42") to create more draw in an attempt to make our system more rockety.
The feed tube was 27cm (10.5") high, but we added a course of bricks around it in an attempt to stop smokeback. Now it's 38.5cm (15") high.

Also, the burntunnel is 20cm wide x 15.5cm high (8" x 6"). So its CSA is 310 square cm (48" square). We have a 20cm (8") diameter heatriser with a CSA of 314 square cm (50" square).

allen lumley wrote:What is the gap between the top of the Heat Riser and the inside of the barrel top !

That's 7cm (2.75").

allen lumley wrote:There is a cheat that is going more popular with instructors at R.M.H. workshops ! You wet the bricks exactly as you would when doing your
final indoor build ( so the brick doesn't dry out the mortar before it can set!) And then instead of laying the bricks up dry dip them in clay slip
this will seal all the little holes that get created with every piece of wood that you drop in, so that your Rocket has good draw

Thank you Allen for this tip. I'll try it out. Until now we've been sealing the holes with cob from the outside, which seems to help, but we still get the occasional puff of smoke coming through.

Question 1: Would you say it's essential to seal the test-setup as well as we can to asses the draw?
Question 2: How long do you need to wet the bricks? Just dip them in water or soak them for a while?

allen lumley wrote:Don't move things inside until you are happy with the performance w/ pipe out doors !

Yes, that makes perfect sense. I've read adding the thermal mass will change things again, but we're not planning on testing that outdoors of course. But apart from that would we need to build everything outside as we would inside? So including all the cob around the feed-tube and burn-chamber? Or do you think it would be sufficient to simply close all the holes with cob/clay-slip?

I'll post some more results shortly. Again, many thanks for your help

Hello, we've continued our story under a new topic. See here: https://permies.com/t/28339/rocket-stoves/test-set-results-pictures-expect#221882

Hello rocket stove fans! We’ve made some good progress and we’re now at a point where we have all the materials, burned the barrel, prepped the pipes, made the heatriser, built a couple of test-setups outside and fired it all up. Exciting stuff!
Some questions arise and we’d really appreciate any guidance. We are not sure how advanced our test set up should already be, or whether the things we are seeing now already mean there is no use to further make the test set like the real situation (like cobbing certain parts in/closing parts off, etc) I have attached 3 pictures showing the set up.

- First image: For reference our bricks have the following dimensions when laid flat: 230mm long (9”) x 110mm wide (4.5”) x 60mm high (2.5”). Feed tube is on the left. Burn chamber spans 5 bricks, so 300mm (12.5”). The heat-riser will be placed on the right. (We have a custom-built heat-riser. We have a 55 gallon drum (200 litres), the heat riser has been filled with a perlite clayslip mixture.)
- Second image: another view of the brick set up

- third image: test set-up with barrel over heat riser - after the fire has been burning for 4 hours without the barrel (just heat riser, to dry it out) -still burning sideways but not so enthusiastically, and we have raised the barrel with a few extra bricks to get better results.

We made a first test fire yesterday with just the bricks and un-insulated heat riser, and it burned sideways. Then we insulated the heat riser and made a test fire with just the heat riser in place, but the fire would not go sideways unless we primed it by dropping a piece of burning newspaper into the heat riser. We kept it burning sideways like that for a while, but we got a lot of smoke back. When we would lower the barrel over it the fire would stop burning sideways. We think this is because the heat riser is still completely wet (we made it in the morning and started testing after lunch) and probably cold. So we decided to keep it burning for a few hours. Now in the evening, the heat riser starts to dry out and is warm/hot along the sides. if we lower the barrel over the heat riser it continues to burn sideways, although less vigorous, and smoke back happens occasionally, although mainly when there are strong gusts of wind (it is a windy day).

A few thoughts/questions after all the enjoyable experimenting and also worrying amount of smoke back we saw...

Question 1: There was hardly any smokeback during the first test. But that was a very nice day with hardly any wind. The second test was on a windier day... Does this explain the amount of smokeback we see and would that not happen inside?
Question 2: During the first test the heatriser was without insulation. The second test was with an insulated heatriser, but it was still wet (and cold). This will probably have an effect? Can we only judge performance when the heat riser has dried out completely?
Question 3: This is a test-set up without mortar and with gaps. The barrel isn’t cobbed in yet (simply paced on some bricks). When we mortar everything in, can we expect better results?
Question 4: we have not even connected the pipes yet... do we also first need to do that as well or do we first need to make sure we get better results without the pipes?

So, we don’t see perfect results, but the setup is only a test-setup... Not sure what to expect? Should we tweak things until we get better test-results before we start doing the real build or is this all a perfectly normal part of the process? Do we need to build a more advanced prototype before we can judge things?

We’ve double checked all the dimensions against the information in the book and those look fine... Without the barrel we hear a nice rockety sound, which apparently is a good sign...
Any feedback/encouragement would be very welcome at this stage Thank you in advance!!

Hello Allen,

Just a quick update - we ordered the metal work, but it won't be ready til the end of next week.. so not much happening hope you are well!