Hello all
this is my first venture into building a RMH I've been reading Iantos book on the subject and I have a few questions and I don't know where else to turn, I hope someone can help.
I'm looking at exhausts at the moment and finding it HARD here in Ireland to source metal ducting so I've turned to ebay I'm looking into purchasing this will it suffice given the heat?
Also I have an existing 5'' box stove exit flue which I am hoping to hook this 8'' ducting to will this still draw the exit fumes?
Kind regards and thank  you  in advance.
Chris

In short, no.

What you want to use this for? If the heat riser, read the metal thread. Ianto's book is good. But not perfect.

Hey there!
thanks for the reply This isn't for the heat riser I'm doing that out of bricks, this is for the exhaust that travels through the cob mass on the way out.
Chris

Christopher Churchill wrote:Hey there!
thanks for the reply This isn't for the heat riser I'm doing that out of bricks, this is for the exhaust that travels through the cob mass on the way out.
Chris

Christopher. First of all, what will be your system size?

Then, you know, that system size changes are usually considered as no good, especially if you "choke"  the heater. Coming, lets say, from 8 inches burner, to 5 inches exhaust.

There's two diameter or CSA changes which can be beneficent in a normal heater. The plenum, which people don't think well enough ahead. And often chokes the heater. And bench elbows.  There is lots of friction involved in theses. So making the elbows larger eases the flow.

There is two other situations it can be good too. In a short heater, where you have lots of elbows to fit the space. It might be good to make the pipe after the burner bigger,  and use less elbows.

Let say 6 inches burner, linked to 10 inches pipe.

So the gases  slow right down, and have more heat exchange time with the bench, and more exchange surface than a smaller tube with lots of elbows, which increases drag.

Then at the end of the bench, you go back to system size on the vertical chimney, so gases gain speed again. To avoid system stalling.

This, taken to an extreme, you turn whatever mass for your heater , into a bell. Where some of the gases can stall completely, and you still have flow.

http://batchrocket.eu/en/building#belltheory

http://www.stove.ru/index.php?lng=1&rs=16

By the way, you know that you need a chimney?

Hey there
my overall system size is going to be 8'' the only time I'm reducing the exhaust pipe is after it exits the mass and joins the Chimney, would that work? it will be going from 8'' -> 6''

Christopher Churchill wrote:Hey there
my overall system size is going to be 8'' the only time I'm reducing the exhaust pipe is after it exits the mass and joins the Chimney, would that work? it will be going from 8'' -> 6''

Reducing from eight down to six is risky. I don't think it will function well. Except if your chimney is amazingly drawing.

This Chimney goes above the roof of the house it's connected to an existing box stove, what would make it draw well?

I HAVE to use the existing chimmney.

Christopher Churchill wrote:I HAVE to use the existing chimmney.

And the chimney is 6 inches?

Make a 6 batch, instead of an 8 J ! 

OK, fair enough
Yes the chimney is 6' the ianto book is a 8' design do you know where I can get a diagram for a 6' build?

How much surface and volume do you have to heat?

For the batch, here!

http://batchrocket.eu/en/

ANy reason not to build a 6'' rocket? How does the batch differ from the RMH?
about 40 square meters.

Christopher Churchill wrote:ANy reason not to build a 6'' rocket? How does the batch differ from the RMH?
about 40 square meters.

Christopher. An 8 inch j would be monstruous for 40m2.

Even a 6 batch. The batch differs un the way that it needs less tending than a j, ans tends to smoke back less. Aldo a smidge more powerfull.

When you say 40m2, how is it insulated?

I screwed up calculations it's around 100->120m2 srry...

How's the insulation?

Because i heat about 113m2 of badly insulated workshop, in the french alps. 5000ft elevation.  With a 220mm batch rocket. (9inches)   And find this a bit small.

Tho, i have 6.3 m under the ridge. The thing weights about 4 tons.

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

It's a house the insulation is quite good, I'm in ireland and temps don't get below freezing THAT often.

So to get back to the point at hand, building a 8 inch RMH and using a 6inch chimmney won't work?

It might, it might not. I'm a strong advocate of, trial and error. But if this for heating your house. Go batch.  They're cleaner, don't smoke back as much. You can watch the fire, you need less tending. They're safer imho. Since there's a door. And a six inch batch is as powerful as à 8 inch J. In the end, you'll have less hassle.

I can't weld and the batch seems to be a lot more complicated than the RMH?

Hey there again, I've commited a grave error I belive I've ordered all the ducting and had it delivered only to find out that the existing chimney is actually 5.1'' in diameter NOT 6'' but the HVAC ducting I've bought is 6''  for the exhaust inside the MASS.
Will it work?
Chris

Can you send it back at no cost?

Well, i can tell you that i had a working 139mm j going through a wall with 111mm pipe. But for only 80 cm if length. After, it was back to normal chimney. And i ran a 6  batch through the same configuration. Tho neither ever ran verry well.

Nobody will ever tell you it'll work fine. As if it fails miserably,  this could kill you and others.

No I can't send it back, did they not run well due to the decrease in size?
I've been reading the CSA it very important within the J but I don't see anything about it being important all the way through to the exit chimney. Also wouldn't the decrease in size of the Chimney increase the speed the fumes went through increasing t
he draught?
Chris 

As i said, the only way to know would be to build it.

You want approval, which i can't give. I said, my two attempts with a smaller chimney didn't work well. Except when very cold. 

Do you realize the difference between 5.1 and 6?


5.1 is approximately 13cm So the full speed column of gases is 7cm. The boundary layer, iirc goes, at atmospheric pressure, in air; from complete stall against the wall of the pipe, to full speed, in about 3cm. Each side.

6 is 15,24 cm; so you have about 9.24 cm of full speed gases.  So this is 67,05 square centimeters of full speed gas column.

While this is 38,48 cm² for the 5.1 inch pipe. Nearly the half of the column of full speed gases compared to the 6 incher.

CSA is 182,41 cm² for the 6 incher

and  132,73 cm² for 13 cm

What i can tell you, knowing those numbers,

is that the drag in the 5.1 inch pipe will increase, so whatever gain you might have gotten with the speeding up of gases will be lost. This, plus the bench pipes drag. You're in for either a very short run of horizontal tubes. Or a stalling and smoking back heater.


Sell your tubes before you do anything with them. Keep enough to link the bench and chimney. And, at least, do a half barrel bell.

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build?sort=3&page=1

With a bell, you would extract more heat, and get a chance of running denser gases in the chimney.

I would do a 6 inch batch and bell myself.

But all of this is just my opinion.

OK after reading a few more threads here, I'm going to run the chimney next to the barrel to increase the draft. And after reading this: https://permies.com/t/1451/rocket-mass-heater-exaust-simpler
I'm not to worried anymore I'm going to increase the manifold and the space between the barrel and the top of the heat riser which "should" increase the flow, from my reading the draft is the important thing here but the PUSH coming from the barrel.
Thanks for all your advice.
Chris

The barrel push may be able to overcome a lack of positive draft from a good chimney, but it may not be able to push hard enough to get flow through a too-small chimney.

What style of system have you settled on? With less demanding heat requirements, you might be able to make a 5 1/2" batch box that is not hampered by the chimney size, yet puts out enough heat for your house. Even a 6" batch box -might- work okay with your existing chimney, though I wouldn't try it myself as a permanent installation. I have a 7 1/2" J-tube combustion zone in a good-sized bell (essentially zero friction), and have a temporary 6" stovepipe and insulated chimney out the sidewall and up because that was what I could do with the time and resources available, and it works fine in nearly all situations.

I've settled on a 6inch J tube, I have a insulated 5' chimney going through the wall and up outside it has a VERY good draft at the moment. I'm going to hook it up and see if it works if it doesn't I'll be punching a hole in the wall and putting the 6' exahust straight through the wall.

Just as an update: I've gone ahead and used the 6'' and it works amazingly so far! I haven't put in the cob on top of the hvac yet but I fired it yeserday and all looks good
Chris

Gudrun then!

If anyone is interested!

Christopher.

One trick, put another T at the end of of your first bench, instead of the elbow. Better cleaning options having two tees. I would put a Wye, where your sprayer is on the first photo, even with a bit more of dead end tube. On the second bench, the T at the end of it, facing the wall, i don't see this as a good cleanout either, better make it face the photographer. We've had this type of problem arising few times in the past, on this forum.

I've have had to ship all of these parts in and they take weeks so I'm not changing them now
That being said the cleanout at the end of second bench will have the wall in front of it removed.
So no problem there
Also this is at the end of last week so we have the benches cobbed in now or the foot and a half is anyway.