Rocket Mass Heater not working - not enough draw, please help

The fire feed is 6.5x6.5ish
the lower burn tunnel is 7"high and 6.5"w
the heat riser is an insulated 8" stove pipe, its square on the outside and round on the inside
the barrel is standard 23"inch diameter
The exhaust entrance is 8" x 4" (cob and brick)
The exhaust pipe is 5" round and goes 10 feet then back 10 feet and then out to a 6" round exit of 5feet.

So yes everything is messed up, it does exhaust well but the draw is too weak and all the smoke is flavouring the kale in the hoophouse.
Its strating to drop in southern ontario, i need to get the hoophouse sealed up and this stove drawing properly before it gets to cold for testing out the RMH.

Every is already cobbed in, I do not want to change the fire hole or the tunnel, I could change the exhaust outlet...
The 20' of 5" exhaust pipe is now burried under a raised bed filled with soil, so it would be nice to keep that...I do not have any other piping...

I've played around with the barrel height with not much success.

So what to do?
Reduce the 8" heat riser? make the exhaust outlet larger than 4" x 8"
The cob is still wet, let me know!!

Organdy Hatfield wrote:make the exhaust outlet larger than 4" x 8"

this.

Thanks Tel,

Its worth a try, it does seem like a possible bottle-neck ...trusting that you might have some direct experience or theory behind your suggestion, seems good enough to try. However, it seems like the 4 x 8" would be large enough, because it gets bigger below that and then into the 5" duct..

I thought also the square shape of the heat riser, so I could round it out with wire mess and cob. Or put in a 6" inch stove pipe inside the 8" and cob it in place, it already has exterior insulation wrapped in metal..

Also, I see everyone using skinny barrels, is this because you get a better draw? or is it because the standard size barrels are too big and ugly?? One drawing I saw said that there should be 1-1/2" between the barrel and the heat riser walls, not just on the top.

Ciao

Organdy I was also stuck with an extra large drum, all the drums I saw in the videos where turned into plastic drums in canada and the metal ones are hard to find.
I ran into a similar problem with my rmh dehydrating room, in a race against time i quit fiddling with it and put an in-line blower on the exhaust to boost the draw.
Im sure it can be fixed and increasing the exhaust could be the solution, but it's up to you how much fiddling you want to do once your design starts to backfire, I designed 3 different stove unit's from scratch just working out the kinks.
In the end i turned it into a pellet burning rmh, I had a really strong blow after i decided to go pellet's but a cheap one worked fine for all the wood burning stove versions I made prior. It's one of those 50 bux and I can move on to something else or an undetermined amount of test's to get your stove working to a caliber equal to the original design. I wouldn't build it the same way if I was off the grid, but I'm not so I quit suffering and got it working with appropriate means which were available to me ie: an extension cord and a power outlet.
The flaw in mine was excessive elbows in the pipe so I could include an under deck floor heating function. Home depot doesn't seem to beef with me returning fan's that obviously smell like smoke but where purchased same day. What drove me nuts was each burn test had to be cooled down before i could adjust and burn again.
Up to you, seem's you got just 1 issue left before moving on, I know what im suggesting will squash it but you won't get any bragging rights on the forums.

it doesn't really make intuitive sense immediately, but that outlet should be rather larger than one might expect. have a look here for a brief discussion.

By your photo it looks like you exist outside your hoop house at ground level.
The chimney is what makes the draw/draft!! The taller and hotter the more draft.
Can you run the chimney up inside the enclosure, to keep it hotter, and exit through the roof?
Add more height to it?
Steve

So I have taken one brick out of the 4" x 8" exhaust outlet at the bottom of the barrel to make it roughly a 8" x8"x8" cube where the 5" duct connects...It didnt do anything, still getting a week draw and smoke is coming out into the hoop house

The only reason I made horizontal exhaust piping is because that is what everyone does to create a heated bench, in my case, a heated raised garden bed.
It just doesnt make sense, it should be working...

I cannot exhaust vertically because it is a plastic hoop house, however, I could remove the 20feet of 5inch duct and go straight out side and then go straight up vertically 10 or 20 feet with a brace. but this ruins my heated raised bed option.

So any more suggestions would be greatly appreciated.
thanks

try keeping the heat loop in the bed , but add that outside chimney, (got more 5 in pipe?)

I added an extra ten feet of outside vertical pipe, straight up, in addition to the 20' of 5" inside the hoop house and it didn't change anything. i thought it would increase the draw, but it doesnt, there must be a bottleneck somewhere.
when i take the barrel off, the draw is great up the heat riser, once I put the barrel back down, the draw weakens considerably. I've tried an 1-1/2" and 2" height for the heat riser to the top of the barrel and no difference, maybe I need to try 3 to 4"? have to mix up some more cob for that, but I see to doubt that could have that much more of an effect...
Mabye its the 5" inch duct? it seems strange that it would be...it must have something to do with the barrel placement and where the duct meets the exhast...
thanks for the help.

Perhaps you don't have enough clearance from the top of the riser to the barrell. You may need a taller riser as well. I have a 6" RMH that started to burn better once I added an extension to the riser and built up under the barrell. I think with an 8" horizontal burn tunnel and heat riser you need a taller heat riser. We'll how good of a burn do you get? Do you block of some of the air feed? What is the ratio of size between the horizontal burn and the heat riser?


I'm guessing the wisdom of doing a dry run outside is showing?

Hoser,

The horizontal burn is made from brick, it is roughly 6" wide x 7" high, maybe to small for the 8" heat riser?
Do you mean the length of the horizontal burn tunnel? I think it is pretty standard from what I have seen, the height is 2 bricks stacked and the length is 5 or 6 bricks. ill have to measure it,

The burn is on and off, its good and then it stops pulling air and drawing and smoke comes straight up into the hoop house. The flames start coming out of the burn tunnel
When i pull off the barrel, the draw and burn is great!

I have never tried covering the air feed, I did put a brick over partially on top to try stopping the smoke from coming out but it didn't work, should i cover it completely?

The taller heat riser is going to be a challenge, also there is not that much more space to go vertically in the hoop house before the barrel gets too close to the poly side wall.
I could raise the barrel up higher, but it seems that that would create more stagnation and less draw..no?

Thanks

Going back and rereading things... try covering the feed. The consensus seems to be some where around 1/4 the opening left open, creating a stronger draw and hotter burn.

With it getting colder at night already and the amount of exhaust you have running low horizontal out and then rising after it has been through the cob run, the exhaust maybe too cool to keep the draft strong. If this is the case whether you want to change it or not , you either need to change the exhaust as suggested or put a blower in the chimney run.

From the top of your riser to the barrel with an 8" heat riser needs around 4-5cm of clearance (maybe 3-5cm) if there is less than that...that may be the problem.

Also the same sized pipe should be used throughout so the fact that you have the exhaust at 5" vs 8" leading up to that certainly may be the issue.

So from what I see. The chimney exhaust may cool too much to create the needed draw. The 5" running may be too small and you may need to remake the run with bigger exhaust( ie 8")...try disconnecting it at the barrel. The barrel may sit too close to the top of your heat riser. In which case building up under the barrel to raise the barrel may help. Simply covering the feed tunnel may help it burn hotter helping the cool chimney problem. Doing this may make other issues more obvious like making the smoke worse...hard to say.

i will try covering up the burn feed intake to a 1/4 of the size....

I have already tried various heights between heat riser and barrel, focusing on 1-1/2" to 2", without much success, but may have to re-adjust after adjusting the exhaust... Also have a question about the amount of space around the heat riser and barrel on the sides, in the picture you can see where the barrel has imprinted the cob, its pretty even all the way around the heat riser, with more space on the exhaust side...are there measurements to follow here? I could reduce this space by cobbing over the square heat riser making it round...

I will try disconnecting the 5" to see if there is a good draw out from that point at the bottom of the barrel exhaust area.

I could also put a 6" pipe in side the 8" heat riser and insulate around it and cap it at bottom so that it turns the 8" into a 6" super insulated, then the 5" exhaust would be more efficient in theory...

we will see how it goes..
thanks.

Organdy Hatfield wrote:
I will try disconnecting the 5" to see if there is a good draw out from that point at the bottom of the barrel exhaust area.

That is the first place I would look. I'm willing to bet your problem is there.

You also go from a 5" exhaust through the cob then bump back up to a 6"...that can kill the draft too.

Stove is working!! We will survive the winter!
the 20feet of 5" stove pipe was the problem. I disconnected it, and connect 6" pipe and now it works great!
Thanks everyone for the discussion. Will post an updated picture.

Good job.

Needs at least 8 inch exhaust or bigger ,go 10 inch if you use the mass or size down the inside to same or smaller tunnel, space at to of barrel needs to be a hair more area than the tunnel, square inch figure it .

exhaust must be at least as big as tunnel that simple, don't know why these folks that know won't tell ya and than they want to sell a video that may not either.


If you want confirmation in your area go to an old havc heating and air man they know lots more about the tech on this than any rocket stove forum yet has to offer.

than you know it will work before you start.

This is where bigger is better on the exhaust if you running a long run bigger is a must.

Really makes me mad sometimes when people want money for info that was free to them in the first place , and trying to keep this maybe [b]2 thousand year old tech a secret.[/b]

The reason toilets get smelly is that they don't have bigger diameter exhaust ducting.

Now i have an amateur question?
What do you mean by the phrase ''exhaust outlet''?
Is this the transition area to the horizontal exhaust pipes that go through the bench?
The area with the bricks and cob down the barrel that connects with the bench?
Th ebarrel stays intact right?Like you dont cut a hole to it.

When I built my rocket stove, I was under the impression that everything was too be the same size ..  . from the start of the burn tunnel - right to the end of the chimney . . .  or else you will run into problems . . .so, that is what I did.   I figured out that a 5.5 x 5.5 inch tunnel is equal too a 6' duct (I think that is what it is . . . don't remember off hand )  So that is what I used.

Check out my little dragon @

https://permies.com/t/40107/hot-barrel

You can hear it from across the room, so ya know it's burning good !

Just a thought.

Dave Lot wrote:When I built my rocket stove, I was under the impression that everything was too be the same size  . . . .   I figured out that a 5.5 x 5.5 inch tunnel is equal too a 6' duct .

Sorry guys . . . 5.5 x 5.5 inch square is roughly equal to a 6 INCH duct - pushed the wrong button . . . so the entire system should be that size all the way thru .. .

And actually, Peter van den Berg's recent research has found that because of the extra drag in the corners of a square cross section, a 6" square is functionally equal to a 6" diameter duct. It doesn't hurt to make the core 5.5" square, but you can make it 6" square and get more power out of the system (because more wood can fit and burn at one time).

try preheating the updraft exhaust or chimney to create updraft,
or likewise consider if it would be practical to make a taller chimney,
please refer to this Youtube video from the CAMPING TOGETHER channel at 3 min 22 seconds...
as it refers to preheating for updraft with a pellet stove
maybe it could help you for the visual to see if it would be practical. And as always be safe friend.