How critical is the barrel to heat riser insulation tube gap.

I am slowly gathering materials to built a rmh in my workshop to help warm a cool area in my home. I stumbled across the rmh a few weeks ago and became hooked. I grabbed a 55 gallon drum from work the next day, then purchased the online rmh super efficient woodstoves book. Now my concern is the insulation tube for heat riser is a water heater tank 18 dia. and the barrel is approx. 22 dia. this leaves me with a 2 inch gap all the way around.

In the book it says to use a 1.5 inch gap assuming this is for an 8 inch system since this is the main system description. I wanted to do a 6 inch system simply for ease of obtaining 6inch pipe and also will have leftovers when i finish installing new stainless steel flex liner in my chimney for our woodstove. I dont have the money to buy alot of pipe and am not having luck finding used pieces for cheap. I have tons of questions but ill focus on this barrel heat riser for now. Thanks in advance

John

Hey John,
About two inches is going to be fine. Be sure to insulate very well.
The gap from the top of the insulated cylinder to the barrel's lid/upside-down-bottom interior surface helps a lot with performance. It should be about half the radius of your exit chimney - 1.5" on a 6" system or 2" on an 8" system. You can go a quarter inch either way, and you can also build up a little rim of mortar to adjust the effective height. We often taper this rim to help reduce fly-ash accumulation; a 'snowdrift' of ash up here can change your gap in a hurry.
The gap between the outside of that insulated cylinder (heat riser) and the barrel can be bigger; it just needs enough flow to shed heat (and hopefully slow down enough to dump fly ash here rather than down your narrower pipes).

Hope that helps. Let me know when you get to the manifold; it also needs enough volume and there are easier ways to do it than described in the book.

-EricaW

John Toyne : just my 2 cents on finding used stove pipe,(and brick) look for recent fire damaged,or urban renewal, or Turn-over properties undergoing rehab/demo , best signs are 1st floor windows covered by plywood open second floor windows ,very large demolition trailers,( the kind delivered by 10 wheel flatbeds) your local fire fighter is usually a skilled craftsman with a useful hobby or related 2nd job, and is very approachable if you tell them what you want. Anything in a demo trailer is usually fair game, though because of liability issues they may not want to 'see' you get the stuff out !

Look at local builders ads they should say they do H.V.A.C. work, approach the foreman at his truck not on the jobsite !

Look at 'to give away' ads in the paper, join your local freecycle, and check out Craigslist!

You are not just looking for the stove pipe from the furnace to the chimney which may not be 6'', but also the hot air runs and cold air returns, they will all work, their job is to make up the form around which you will pack your Cob, I don't see a lot of aluminum stove pipe around here except in trailers so I guess its local , Ernie Wisner says its O.K. to use, grab all the 'T's and elbows you can you will need more and lots of clean-outs and clean-out caps.

I would patch any holes and joints on the outside of the pipe, sealing with stove pipe tape or R.T.V. silicon sealent your choice, fasten your stove pipe together with rivets or short self-taping screws, some of the pipes need to be cleaned weekly, all of them yearly, so shorter is better ! Hope this helps ! Pyro-maticly Allen L.

John Toyne : add 1 more possible place to check for used stove pipe , the local 'habitat for humanity ' RE-store , though some places have restrictions on selling used pipe it will be worth your time to check out any local RE-store in your area, and the clerks can be a source of good information ! hope this helps Allen L.

Thank you guys for the fast reply, just needed to make sure that water heater tank would work since im at that point in my collection. I have a friend who is a concrete contractor that is currently working on a jobsite with large amounts of clay and is going to grab me a load in the next few days. He is also keeping his eye out for some perlite or vermiculite. My other concern is my bricks i can get some used red brick for next to free. I was going to use them since this is my first unit and I will not be hurt if it needs to be removed in the next few years. It is sort of my practice run for one I want to place in my sunroom which will be converted to our living room hopefully next summer. It is on a large concrete slab so can handle the heat.

So now I am just looking to get one together fast to heat my workshop which is in the basement under two bedrooms. The workshop was the original garage for the house but there is another garage built in front now. So if i can keep that area warm the floor in our bedrooms should in turn be warm also. And today we just got a good dumping of snow, kind or rare for this are now but still a good kick to get moving on this thing.

Thanks again the help is much appreciated.

J.T. : Counter-intuitively the softer the brick,weight for weight the better able it will be able to take the heat load/shock, it will however wear out faster, setting the wood pieces down into your feed tube is about the only thing you can do if all you end up with Is the soft ones, other wise, save harder ( like sized bricks ) for the bottom of the feed tube , but not the brick heat tunnel- if you have that choice !

While cost/cheapness is a very important reason for using Cob type construction there are many other good reasons for doing so. The mortar you use, irregardless of type is not as strong as your brick or able to handle high temps, as the brick is there to level, close-up gaps, keep the bricks from rocking/shifting,and dealing with small size differences, chips and the need for part bricks,you will want to use the sand/clay slip mortar which can handle the high temps better again stay in touch and let us follow you in your build ! Pyro-maticly yours, - Allen L.

Im excited, I just found an add for insulating firebricks. The guy had 25 boxs with 25 in each box. He's asking 20 dollars a box for them i was shocked, the only problem is a 3 hour drive each way to get them. He posted he buys scrap vehicles to and I just so happen to have a scrap vehicle sitting in back yard. I am just waiting on his reply for a trade and what he would offer. Even if he says no I think im gonna take the trip and grab 6 boxes, twice what i need. But if this rmh works well im want to build another in my sunroom.

John Toyne : I just went online to check on firebrick w/in 100mi., no luck, but just to share , fire/ kiln brick will still vary by manufactures

Standard names are Split 1 1/8 - 3 3/4 - 8''
Standard 2 3/4 - 3 3/4- 8''
2.5 Thick Single 2 1/2 - 3 3/4- 8''
Fire Clay 2 1/4 - 4 1/2- 8"

There are also Chamfered Strechers, Tapered, Keys,Wedges, and Arches, all have 1 side 8'' true. You can call ahead and pre-pick your boxes if its a mixed bag - goodluck Pyro-AL

I seen that you mentioned heat/ac ducts could be used but the ones you mostly see around here are galvanized, just looking for thoughts on using galvanized pipe for inside mass. I've read about a few different opinions on this subject. This would most likely be the easiest/cheapest pipe for me to obtain.

John Toyne :When you are burning the paint off of the outside of the barrel, and the gunk out of the barrel, with the Pocket Rocket you are going to make to do it , you can rotate your galvanized stove pipe 2-3 lengths at a time, to make your exhaust chimney(s) , this will kill to birds with one stone ! O.K. ? Pyro AL

Just to add in quickly, I have a 6 inch stainless steet T that is for a woodstove connection I planned on using for the first section outside of the barrel, with one leg of the T being the 1st cleanout. I then planned to buy one 6ft or so length of stove pipe then finish the run inside the bench in galvanized duct. Just before pipe exits the bench I wanted to try and connect a 6 inch stainless flex pipe I have left over from my chimney liner (approx. 12 ft.) to run up the wall and out basement window to a small length of vertical pipe to a final elbow to prevent rain from getting in.

I cant see galvanized being a problem at this point but not sure. I just figure the temps at this point should be well below 900* at which point the zinc will evaporate. Then again I'd rather be safe then sorry, but its possible I cant afford to be safe???

You must have been typing along with me, this is good enough to just burn the galvanizing off before hand? I am ok with that. But I got a little carried away yesterday with the cutting disc on the grinder and have already removed the top of the barrel just after I cut the water heater tank to size. Just after I cut the lid off I thought that was my chance to make a rocket stove. I still plan on making a small one I've got my eye on a nice 5 gal. pail at work, just waiting for someone to use the rest of the oil in it.

J.T. : I have NO experience with flex stove pipe, if i had some to use up i would probably try to use it to make nice sweeping 90 degree bends but thats just me !

Can you give me an Idea of costs and how you connect other lengths to it ! Stainless must be expensive ! I just came from my nearest 'Habitat for Humanity' And only found large Stone tiles that i plan for Christmas gifts, but boy they had pallets of nails ! PYRO- AL

J.T. : I don't think it is really needed for the stove pipe, but better safe than - -!, It is, however a good idea to burn the barrel, now we need to put our heads together and come up with a new top !
A 5 gall pocket rocket is what we use (with a windscreen of plastic, or piled snow ) when we go ice fishing, usually it lasts 4-5 outings PYRO _ AL

Yes the stainless flex is expensive. It cost me 300 for 25 feet with the T connector flashing/flue cover and storm cap. It only comes in 25 ft lengths with the kit. I only need about 12 ft. for my chimney, so I will have about half of it left over. I have thought of using it inside bench just like you mentioned but I just figured it might be safer to use stainless in places where it is exposed??? But this is only 12 ft of length I think I will be running at least twice that inside the bench. I need to get some pipe and see how easy this is to connect with the flex because your right this would help make some great shapes in the bench, maybe a body formed chair similar to a hot tub chair. Oh the possibilities are endless. I think I just need a simple design and just stick to it. I dont want to get carried away for my first build.

J.T. : You are giving yourself good advice - listen to it ! AL

I'm excited, rmh is getting closer. Today i got a great deal on 150 brand new fire bricks. I figured I will get enough to make a second rmh, if the first goes well.

My current list of materials

- 55 gal. drum
- 17 gal. drum
- old water heater tank 4 inches smaller diameter then the 55 gal drum
- stainless 6 inch T
- small length of ducting (for now)
- firebrick K23 9" x 4.5" x 2.5"
- 12' of stainless flex line.