Steam preventing good burn?

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

Bump,
any of you rocket people have an answer?

Helena Davies : I think that we need an update, I can live with the large gap at the top of the barrel if we can make the rest of it work ! What is the gap between the side walls
of the outside of your Heat Riser and the 55gal barrel, have you used a level to ensure that the barrel and the heat riser are both vertically plumb? You can do a quick test by
filling the indented top with water, even if the barrel top has a slight bow or dish you can tell if there is a high side! If one is just a little out and the other has a corresponding
lean, that is alright, but if they are not simpatico` they will pinch off enough at top and bottom to cause a serious starvation, or lack of flow of your Hot Exhaust Gases !

With good results for 20 min and then a reversal of flow, have you tried using a couple of bricks to partially close off the feed tube opening and direct the flow of air more central
to the wood being burned ! If this works great, if not, or it makes it worse I would wonder if you need more insulation, the whole area around the feed tube should be coolish,
This is due to the amount of cool air that the furnace is using !

You know that you are not going to be able to run a horizontal only chimney right ! A 15 ' vertical chimney can sometimes allow you to actually add 10 ' more horizontal chimney,
I Would add on more vertical chimney and just see what happens then, and keep an eye on which way the wind blows ! As a diagnostic person I am a little out of my depth here
but at least you don't need to worry about me proscribing Leeches ! for the Craft ! BIG AL