Stephen Brightwood

It seems many people are getting away with much shorter stacks and it certainly isn't laboured that strongly in the "literature". However 12' takes it over the ridge since it is a flat so i hopefully will be OK for the future.

Stephen

The feed tube has a Peter channel on it already and it helps a bit - not on the original photo. As can be seen the manifold is much bigger than on most builds which generally constrict considerably at the barrel support, where as the manifold in mine is open all around the base of the barrel. offering minimal resistance and plenty of thermal mass to dump its heat and sink for exit to the horizontal flue.
The internal dimension of the heat riser is 6" and the flue out is 6" - its an optical illusion that suggests they are different as the feed out hole is low down. The heat riser is made of pumice and is around 2.5" inches across so it meets the spec for insulation. The internal surface of the heat riser is white after burning, and it is only the outer surface which has any blacking. From my reading few people get away without a bit of soot in the cool areas so I am not really concerned about inadequate burn temperatures. The pumice in the J-tube heats up quicker and stays hotter than firebricks and the sacrificial stainless flue liner in the burn tunnel glows red hot after about 20mins.


I decided to take your advice regarding trying a flue extension to the outside section - so I added another six foot. The outside flue is now 12'. I did another test burn and had a three hour burn with no significant smoke back. I had it burning with a lid on the feed tube for most of the time - and without it there was still a bit of smoking - with it on it burnt for extended periods without smoking. This is significant as previously with a lid on after a short time the feed pot got so hot that it smoked and the reverse flue action took over. I assume that two things have changed - the flue extension has tipped it over to been adequate draw and the thermal mass is behaving better as it has been burnt for two days running. Previously I was getting steam out of the top of the external flue, now there is no visible steam which means it is condensing before it gets to the flue exit. Effectively its a 100% clean burn at this stage.

So thanks for the perseverance and I can now proceed to apply the final plaster coat and get a proper lid for the feed pot.

Stephen

The barrel to heat riser spacing has been tried at a few heights so its not that. The barrel to flu transition is the same size as the flu so if it is the bottle neck then the flu is also a bottle neck - at what point would the flu not be a bottle neck in this case ? The collection manifold is simply enormous compared to most designs.
I can accept that the outside flu could be higher and a i added a few more feet last night. When I get a bit more flu i will add it on.
The J-tube is made up of pumice which is highly insulating. The whole J-Tube assembly is better insulated than almost all of the designs I have seen using firebricks. This is not the issue. In fact the probable issue is that the fire chamber is to well insulated causing a very hot burn and strong convection currents above the fire.

However neither of these two issues really explains why its starts smoking back as it gets to its hottest, for that a counter flu explanation seems best. I am going to go ahead and reduce the height of my feed pot today as I feel certain that this is the primary problem.

Stephen

Did some modifications which helped, but left it for a week whilst I got on with some other things. Went back to it today and it was intermittently smoking. I partly attribute this to the heat mass been cold - but not all of the problem can be attributed to that.
After raising the heat riser by 10" and adding a bit of an extension to the external flu I am still getting smoke back from the feed pot when things start cooking.
I have come to the conclusion that the feed pot is at least a significant part of the problem. The burn tube and feed pot are insulated so they act as a chimney which creates a counter draft.
Tomorrow i am going to decisively chop the feed pot down to about 2" from its present 7"

The plenum/manifold below the pot is very large compared to most and is plastered on all surfaces so it is smoother than it looks. i do not believe it is the issue here.
I can add another few feet to the outside flue - but there are no vertical vents for it to compete against so again I think it wouldn't make much of a difference.
I believe you are spot on in saying the additional heat riser should improve the overall draw. All i now have to do is work out how to raise the lip of the manifold adequately to take it.

Stephen

Things to note from the photo's.
The tram lines across the thermal mass show clearly where the burner is still driving off moisture.
Generally when I take the burner apart the internals to the top of the burn tube are white, this time they are part blackened which indicates a drop in burn efficiency.
The bottom assembly within the manifold shows two cracks. I am hoping that the cracks will stabilize and help alleviate thermal stress. The actual surface of the material is standing up to the punishment extremely well. I have considered adding a piece of lose fitting stainless pipe to the inside to take some of the abuse and it can be replaceable.
It all looks very messy at the moment as the final details have not been set and it will receive a final unifying coat of plaster and paint eventually.

I have come to the definite conclusion that 26" of heat riser from top of burn tunnel to top of riser is inadequate and I will be adding a further 10" section. If anyone has any final opinion on whether they consider this to be enough I am all ears. Will be going for a 4-6" gap between the gas bottle and the heat riser.

Stephen

And the thermal mass and outside flue

Some pictures

As a little experiment I added a 4inch computer fan driven from a 9V battery to the stack to see what effect it would have. This is a very lazy fan but it massively improves the burn.

I am almost tempted to cut my losses and just go with a serious 12V fan.

Stephen

I will try to take some photo's tomorrow.

What did I learn today - that raising the barrel by just four inches improves the burn by a noticeable amount. Still not there yet as the same progression develops after about 2 hours but its slower and the smoking is less.
The top plate is cooler and is now unable to boil water so this seems to confirm that the burn improves with a bigger gap but that the 2" specification is for a cooking hotplate. The thermal mass is still driving off its moisture content so its not doing quite as well as it should overall - but I fear any improvement in this regard will be insufficient in off itself.

So what I am thinking is that I will get a new section of heat riser (10") and then raise the barrel by 14". This is a manageable modification and I feel it will cure the issues. If not I will remove the feed pot and that should remove the offending counter chimney.

Stephen