RMH exhaust

Ok, I've searched over the Internet as well as bought the RMH bible (by the Wisner's) and I have yet to find an answer.

I have a 600 sf pole barn that I am finishing and will be using an 8" system. The pitch of the roof is 4/12 which makes the attic space 5' at the tallest point and the 1st floor is 10 ft tall. The hearth/bench will be 21" tall which makes the exhaust pipe 8'.25" tall. I will be extending the exhaust 3' above the peak. I will need a total of 16'.25" of exhaust pipe. Does every section of the pipe need to be double walled or can I buy double walled pipe only for the sections that go through a ceiling?

Not sure if it's useful to include that my roof is metal.

I've also included my sketches.

Hi Diane;  
You only need the insulated pipe to pass thru the roof and possibly from the attic floor up if your attic is to be completely unheated.
Two purposes for the insulated pipe) 1) to safely pass thru wooden framing 2) to keep your inner chimney warm so as to keep a good draft going.  

Single wall duct is okay for the bench. It is recommended that you use black stove pipe for the beginning of the run since this is the hottest part of the exhaust system. Switch to single wall black stove pipe at the beginning of the vertical rise of indoor chimney. When it hits the roof you can use a chimney support box and transition to Class A chimney pipe.

Also, your bench run is quite long at 40+ feet I think from your drawings. The bends are worth 5' of pipe. I think with the amount of stack you have you will be fine, but you might want a priming port at the base of your vertical exhaust for cold starts. You also have in your diagram a 2" gap between heat riser and barrel--you may be better off increasing this to around 3" to make sure the gases have adequate room to move. I'm no expert but have read a lot of peoples issues posted here and they always seem resolved when they increase the barrel height a bit. Anyone else having an opinion on the top gap advice would be appreciated for myself and others.

In case you want to cook on the top of the barrel, 3" would be fine. In many cases the top of the barrel buckles in, making the top gap suddenly cramped. When you don't want to cook on it, the top gap could be anything. Up to a yard is perfectly possible, 4" is a safe bet. This will provide room for the fast streaming gases to go through the 180 degrees transition without much friction.

My rule of thumb: minimum space for any 90 degree direction change 1.5 times minimum. For any 180 degree direction change 2 times minimum. The same goes for a manifold, by the way, people tend to make that too cramped.
Absolute minimum for an 8" system is 2", accompanied with the risk of buckling in and severely restriction of the system flow.

Personally, I once tried a top gap of 4' and the thing kept working like a charm. Couldn't get any higher since the ceiling wasn't any higher, a practical top limit probably doesn't exist.

Thanks Peter, that is sage advice. My first rocket could have benefited from reading this thread.

To each their own research, I tend to research everything I possibly can and over do everything.  I wanted a very reliable stove and found that all your external piping should be double walled, not to prevent fires, but to keep a good draft.  That's what I did.

Peter van den Berg wrote:

Personally, I once tried a top gap of 4' and the thing kept working like a charm. Couldn't get any higher since the ceiling wasn't any higher, a practical top limit probably doesn't exist.

Peter, you've forgotten, there is a practical top limit.  When you're reaching the maximum ISA!

You are right Max, although for an 8" J-tube system that would probably be 4 barrels on top of each other. That would require a high barn or something like that to reach that upper limit.

Sounds interesting to me, anyone who like to have a tower batchrocket like that?
Eight inch system, stacked barrels only. Calculating 1.5 m² for each barrel totals up to 6 barrels, 0.89 cm high each. A tower of 5.34 meter (17.5 ft) high, twice as much as my former workshop heater!

Would be really exiting to run such an appliance, don't you think?

Peter van den Berg wrote:You are right Max, although for an 8" J-tube system that would probably be 4 barrels on top of each other. That would require a high barn or something like that to reach that upper limit.

Sounds interesting to me, anyone who like to have a tower batchrocket like that?
Eight inch system, stacked barrels only. Calculating 1.5 m² for each barrel totals up to 6 barrels, 0.89 cm high each. A tower of 5.34 meter (17.5 ft) high, twice as much as my former workshop heater!

Would be really exiting to run such an appliance, don't you think?

There's one along those lines. Tho, we never heard of the first burn yet!

http://donkey32.proboards.com/thread/1890/inch-batch-bell-rocket-heater?page=5

So I went to a local building supply place and got a quote. I told him I want a straight through the roof chimney and showed him my sketches. I told him it was 8" pipe. He laughed at me and said it would be expensive. He gave me the quote and said he threw in a few more pieces just in case. That extra piece (an elbow...not sure where I would use an elbow for a complete vertical chimney with no turns) was over 300 dollars. In the end the whole system was $1,600. Is this typical for a RMH? Part of me wants to take it through the wall because of the cost but heard I won't get a good draw. Its getting cold here and I have no heat. I'm really worried I won't have enough to do my RMH. The black stove pipe is reasonable but the double walled stuff is expensive. Any ideas?

If it were me I would do single wall chimney all the way to the ceiling, use a chimney thimble through the ceiling, then single wall to the roof, a support box and switch to double wall class A chimney. They come in 2, 3 or 5 ft lengths I believe. A five foot piece is probably $300. There is no reason you need insulated chimney in the attic really.

Guys, you can also insulate your black stovepipes yourselves too. Rockwool and chicken wire, or metal around it. Tho, Whenever you are passing a floor, or roof, you need to be code compliant. So, no wood within the right distance, as your local code asks, and  insulation if possible.  

And the seller wants to shove an elbow down your throat, that you don't need, as if it was a present, tell him to go and get stuffed! If you don't need it.

Ianto Evans' RMH in the library (Myrtle) at the Cob Cottage Company has the exhaust going out of the wall around 6 feet above ground, and has an elbow on the end to point it down to avoid the rain, and help prevent a breeze from causing a back draft. It looked like galvanized ducting, certainly not insulated double walled, but it goes through a cob wall and has a good overhang above.

While a tall chimney that extends above the roof ridge will help get a really good draw, it's not always necessary and if expense is an issue, you can always try it through the wall as high up as you can go. If you have problems with that, you can make adjustments later as well.

Mark Tudor wrote:Ianto Evans' RMH in the library (Myrtle) at the Cob Cottage Company has the exhaust going out of the wall around 6 feet above ground, and has an elbow on the end to point it down to avoid the rain, and help prevent a breeze from causing a back draft. It looked like galvanized ducting, certainly not insulated double walled, but it goes through a cob wall and has a good overhang above.

While a tall chimney that extends above the roof ridge will help get a really good draw, it's not always necessary and if expense is an issue, you can always try it through the wall as high up as you can go. If you have problems with that, you can make adjustments later as well.

Yes, but we all know that the ideal conditions at cob cottage are rarely replicable. And, i think it's Leslie Jackson, who was saying, even there in some conditions it was better not to light the RMH.

Have you tried it?  Did it work every time?

But anyway, you do what you want. I would rather not smoke myself in, be safe, and have a great working stove. Than save a few of my monetary unit scrounging on materials.

Because, what you are advocating, could kill people.  Imagine, a family living in a tiny house, or not so tiny either. A gust of wind puts out the fire, then it smokes the inside of the house, and nobody wakes up. It is not very likely, but nonetheless it could happen.

To me, the pipe through wall system, is way too "on the edge" to be used sensely.

 Because each structure has different air flow, the answer to your question is an unknown. It's always a good idea to use an insulated vertical exhaust, it helps the system draw, especially when the mass is cold. The question to your question is what are your exhaust gas temps after they have passed thru the mass? Do you have a lot of heat left over, like 200f? Also think about the next owner of that structure, is he/she going to hook up a wood stove to the pipe going thru the roof, and take out the rmh, because they don't know what it is?

Hi Diane.

I would always go for an insulated chimney if you can. As has been mentioned, it factors in a degree of safety in most types of weather conditions. You may well get days where the stove will not be safe to light with an uninsulated flue system.

Over here in the UK, Insulated SS pipe is available at a reasonable price Insulated Flue Systems - I realise it's the 'other side of the pond...' but there's probably an equivalent product available near you or via an on-line supplier.