RMH smokes back after a couple of minutes

This is my second time I build a 8" RMH.  The first I build 8 years ago, and is still working as it should.
Now, the new one (in another house in the center of France)  gives me quite a headache.  I know with new installations, it is not easily started until the bench is dry.

So I tried to fire some starter blocks right below the vertical tube (via the cleaning exit on the left of the stove).  Same preheating inside the heater. Draft is very good.  

Then I fire some wood and it starts up normally, the same rocket sound as my previous RMH. But after a couple of minutes, the sound halts, and I get a complete smoke back.  And I can't stop it so I have to put  a wet towel over the fuel feed to stop the burning.
And I suspect it is probably because the exit chimney never gets warmed up.  I have the initial draft  because I preheat that exit chimney but after a while it cools down, before the smoke could have warmed it up.

It is installed in an old house, single glass windows.  

What I've tried :  

taping bubble foil over all windows, just to see if  that draft was the cause. Nope.

Increased the fuel inlet, because it initially was only 36 cm

Added a metal inlet so the wood doesn't block the oxygen intake.  That inlet does not obstructs the burn tunnel. That gives a cleaner burn in the first RMH

Put starter blocks in every one of the cleaning exits. In the hope the chanel in the bench doesn't cools down the exit gasses too fast. The flames get sucked inside to the direction of the exit chimney.

The only solution I can now think of is the bench is too long.  It goes around a pilar, and maybe I shouldn't have done this. I did however used the calculation from Ernie en Erika's book (Start with 50 feet, substract 5 feet per turn).

Does anyone has other suggestions that I can investigate without  having to break up my bench?

Hi Pascal;
Looking over your build dimensions and photos, everything looks good except the wrap around the pillar.
I think it is just a bit too far.
Every build is different, and the chimney draft also will differ from location to location.

Here is an idea for you to think about.
Can you get a 4" bypass pipe from the barrel or perhaps the brick section it sits on to the vertical exit chimney?
If so, this will supply the hot air needed to get your dragon roaring.
Once your system is dry and up to operating temperatures, you may not need to use the bypass at all.

Or tear it apart and reroute, avoiding the pillar.

Hello Pascal,
I'll start by saying I'm not super experienced with RMH, I've made one and it has its issues that I'll be fixing this summer.
Looking at your photos (and knowing what's wrong with mine) I'd say you're loosing too much heat from the exhaust flues, which then affects the draft negatively (smoking back). Your bench is long, and it's probably still quite cool since I'm seeing moist cob. The oildrum as a bell radiates heat faster than a mass bell would. And your tubes are slightly textured, which also can slow down the flow a little bit.

My suggestions are:
1) Wait! Wait for the cob of the mass benches to dry... Can you heat the space in another way, just for now? As the mass dries, it will stop cooling the exhaust tubes.

Then test. Is the draft OK? Then you don't have to do anything else, it's working!

2) You might consider building mass around the barrel (bell), slowing down the heat radiation. My RMH has too much mass on the bell and a bit too short exhaust tubes, the draft is super great but we're loosing a lot of heat to the birds outside... I'm going to rebuild 2/3 of my bell, making the wall a bit thinner, and if I can find a way to lengthen the exhaust tubes, I'll be doing that.

You can see my build here if you like.

And last but not least...where are you located? I'm also in central France, in an old house with single glazing windows... Southern tip of Cher (18).

Good luck!

Pascal Dutilleul wrote:Does anyone has other suggestions that I can investigate without  having to break up my bench?

What have local outdoor temperatures been like when doing your test fires?

Does the chimney need extending a bit perhaps?

Is it insulated?

Hi Pascal - I love the look of your heater! I hope we can sort out the teething troubles with you.
Building on Nina's suggestions it may be that it will all be fine once everything has dried out. Maybe it would be worth finding a bit more insulating blanket to wrap around the barrel? That would make the whole heater warmer and maybe keep the flue drawng for a bit longer. Once it is all dry you may not need it, or could cob it up as Nina suggests.

Hi everybody,

Thanks for all your kind suggestions.

As I  understood it correctly, I have a couple of options:

I need to have patience until the bench is dry like Nina and Nancy suggested.

Additionally adding mass or insulate the barrel seems also a valid option.

Or I could force things a bit, like Thomas suggested, by adding a small bypass.

@Coydon I tried at several outdoor temperatures:  9 °C and 13 °C. In windy and wind still conditions

@Burra They chimney is insulated by  putting the 8" tube inside a 10" tube and adding clay stabilized perlite between.  Only the lower 60cm in the attic is not insulated.  But I've added my leftover pieces ceramic blanket around it. But that didn't helped.

As I currently have a week of vacation it is time to finish the bench. I need to add a couple of centimeters cob on top to level it. And I wanted to be sure the system works with the detour around the pillar.

Because I don't have any ceramic blanket left, I'll add more mass around the barrel. And additionally, I'll take the leftover blanket I used on the lower part of the chimney on the attic.

But even if this doesn't work with the wet bench, I am more or less confident it will work when the bench is dry (I hope )

@Nina We bought  a house in 36160 Lignerolles. So we are practically neighbors  

Pascal Dutilleul wrote:@Nina We bought  a house in 36160 Lignerolles. So we are practically neighbors  

Aha! So we'll meet! Permies is such a great place to be

Hi Pascal;
With a piped mass, I promise you will be grateful if you add a bypass.
Yes, your stove needs to be dry and warmed up, and hopefully will perform perfectly when it is.
Weather conditions change, and windy days can be a problem.
Shoulder seasons, when it's not really cold but you want a fire and your mass is not heated up yet for the year.

On my newest build, I used a standard 4" stove pipe chimney damper.
I wired superwool on it to seal the damper.
It works great.

thomas rubino wrote:
With a piped mass, I promise you will be grateful if you add a bypass.

I indeed already bought a pipe  of around 4" (in fact it is 125 mm which is a little larger).
Unfortunately they had nothing like a damper at the store.   I guess I have to figure out an easy way to install that bypass as a proof of concept and see if it works.

Fortunately I have a cleanout running to the back of the heater, so I could somehow connect the bypass here.

Pascal Dutilleul wrote:@Coydon I tried at several outdoor temperatures:  9 °C and 13 °C. In windy and wind still conditions

I'm from a colder climate and have a short pipe run so no personal experience, but from warnings I've heard it sounds like classic cold plug performance. Once the bench dries and holds heat better between firings, I'd expect the behaviour to diminish.