RMH with copper coil

Hi, my name is Alecs, and last year my wife and me have successfully built a couple of 8 inch RMHs (by the Ianto Evans book) in an old wood frame + cob house. We also have 2 small rooms upstairs and an electric-powered boiler that we would want heated from one of the rockets.

Have done some research about where to place the heat exchanger (copper coil):
1. Wrapped on inside of barrel
http://www.rocketstoves.com/wisner.htm
https://www.youtube.com/watch?v=gq0d_hL5X9M
https://www.youtube.com/watch?v=HnbSCR6RTb0
2. Wrapped on outside of barrel as I've seen you mentioning and seen implemented:
https://permies.com/forums/posts/list/40/5937
https://permies.com/t/40107/rocket-stoves/hot-barrel
3. Wrapped on 1st 5 feet of exhaust as I've seen Ernie recommend here:
https://permies.com/t/12179/rocket-stoves/rocket-stove-water-heater#276477

- Would rather go #1 or #2 because it would just be easier to add to what we have now.
- 3/4 inch copper coil seems best, not to pricey and harder to flash to steam.
- I would be using a small recirculating pump backed up by and UPS if the power runs out (rooms being upstairs, the system will benefit from thermosyphoning as well)
- plan on installing 2 pressure release valves, manometer, thermometer. We feed the RMH at least every hour or so, I think there would not be a problem with it overheating before we can put out the fire in case of an extended power out.
- I've read Dave Lot's posts @ https://permies.com/t/40107/rocket-stoves/hot-barrel and I think I've also seen Erica Wisner mention that the system would benefit from running cold water around the barrel
This is contradictory to what Ernie is saying:
'Dont put a coil around the barrel put the coil around the exhaust and use the thermal siphon. dont put a jacket around the barrel it cools the system to fast and you get soot' @
- from my calculations the system would hold about 10 gallons of water

What I have not decided on and would like to hear your input is:
1. Go with #1 or #2 ? or as a last resort #3...
2. How much 3/4 copper to use in case #1 or case #2 ?
I have seen Ernie (and others) use 20 feet half inch for #1.
While for #2 60 feet, 40 feet.
3. How would any of the 3 cases affect the system ?
My understanding is that ( as Konstantin Kirsch best puts it @ https://permies.com/t/32099/rocket-stoves/doubling-power-RMH-cooling-feed)
'A RMH has minimum 2 pumps: First the heat riser pumpes the hot air up. Second the barrel pumpes the air down on heat exchange over the surface.
If you do have a vertical chimney than you have a third pump.'
So the system would benefit from either #1 or #2.

Hope to hear your thoughts,
Alecs.

1) - Ernie mentions that he gets some steam in this setup, and unless the system is totally open to atmosphere, (not a relief valve) it would be likely to have overpressure episodes if not burst piping from sudden steam.
- The video by kbbacon mentions that the heater it is installed in "never got above 200 degrees"; a well-functioning RMH will have heat inside the barrel of 400-600 degrees.
- the video by Rom1nb shows the water boiling, but in an open-topped tank so that it is relatively safe.

Any coil inside a well-functioning RMH MUST have powered circulation 100% of the time it is heating or there will be steam generated, which must be able to escape quickly and easily to avoid explosion. Even a minute of stagnation in a hot barrel is likely to cause an explosion, depending on the details of the setup.

2) - The yurt water-heating coil is open-ended so no pressure buildup is possible. Note the comment that that installation was done carefully and thoughtfully, which is NOT a common characteristic. If you don't really know what you are doing, you could easily make a horrible mistake.

- Dave Lot's system is again thoroughly safety-oriented and failsafe (note the drainback feature which empties the heating coil if the pump stops).

Any of these depend on either being non-pressurized or having all the safety bells and whistles, which makes them not suitable for a novice build in a circulating radiator system.

I would seriously go with a system like that described by Kirk Mobert (Donkey), of an open-topped water tank to be directly heated, with a circulating coil submerged in it to transfer the heat to your system.
http://donkey32.proboards.com/thread/1096/hot-water

Pressurized water will boil at a higher temperature than atmospheric water, so even if the tank boils, the coil will not flash to steam. With the coil held off the heated surface of the tank, a dry tank will not overheat the coil.

Thank you for the replies (also got a PM from allen lumley).

On further investigation decided it is safest to go with an open system (using an open expansion tank like so - http://www.deppmann.com/2011/10/30th-4/).
So, in an open system which one of the 3 flavors would you choose for a 'standard' 8 inch system (55 gal drum, 23 feet long horizontal pipe, ~20 feet long flue) ?
I am worried about the impact off adding either #1 or #2 - gases getting to cold, creosote build up, inefficient system - Dave Lot says #2 helps the stove 'really take off' while Ernie's opinion is that 'it cools the system too fast and you get soot'.
Ideally the coil would heat my 10 gallons of water to a temp of ~170, I have seen 160F in a 5 gallon system (https://permies.com/t/40107/rocket-stoves/hot-barrel#343291) so that should be possible.
I'm guessing #3 is to weak/slow for that.

Few clarifications:
a) Electric-powered boiler can heat water through a resistor and one(or two) water coil(s): http://www.panosol.ro/images/Boiler%20bivalent.png
b) I have experience in plumbing, have also used pressurized stove water heaters (https://kotly.com.pl/produkt-boiler-viadrus-u22-s-hercules-33-kw-3084.html?l=en), setting it up once I have the plan should not be a problem.

I need to eventually answer the initial 3 questions:
1. Go with #1 or #2 ? or as a last resort #3...
2. How much 3/4 copper to use in case #1 or case #2 ?
3. How would any of the 3 cases affect the system ?

The next time I heat water like this rather than use any internal coil I'm going to use a pressure cooker on top of the heat source that will thermosiphon to a storage tank. I don't know if it would work for your application but I will present it for your consideration.
You would have to modify an old pressure cooker with a downpipe as in the drawing. It's possible that the downpipe would have to be made with a funnel bottom (think coffee percolator) and be used as the "up" pipe.
I'm pretty sure this would have no problems heating 10 gallons.

Roy Hinkley : There is a Difference between having a system that Will Work, and one that will work safely ! As drawn you show a closed system.

Also you show the use of a pressure cooker as the primary heat collector ! Any Closed system holding a liquid has the potential to B.L.E.V.E.

The Term stands For Boiling Liquid Expanding Vapor Explosion. In a small way this is what happens when an aerosol can is 'disposed of ' in a fire.

Please! Ask a local Fire Fighter, Paid or Volunteer - Nearly everyone has heard a BLEVE at a fire if only from exploding paint cans and everyone

has a story of an Exploding Propane Tank !



Your system needs to be vented to the air - This means all of the time, and not vented by a pressure or temperature relief valve which can and

has failed !

Also the purpose of that flattened funnel is to assure the creation of a hot spot to allow the creation of steam bubbles to circulate the hot water

through the grounds To 'Brew ' your coffee. Bubbles we don't want or need !

Assuming a barrel top temperature of 800ºƒ the difference between that temp and the Boiling temp of water 212ºƒ means that you would have

to regulate the Amount of Heat Energy only by the Amount of Wood you are feeding at the Feed Tube ! And never be wrong !

A heavily Insulated tank supplied by small diameter tubing will equalize and stop Thermo-syphoning and You will have the BLEVE conditions.


How are you going to deal with draining your system, After you have your water hot- but its deep into the Negative Numbers Outside and you

need additional heating! I would not recommend the addition of valves to shut off the flow in a Thermo-syphon system, This is not a solution

where any valve can be accidentally closed,or left closed and forgotten. you can only be wrong once ! Also remember that the common temps

found at the top of the barrel is hot enough to melt common solders , Perhaps there are brazes that could work for this application. !


This is written more negatively than I intended, But please remember that this post will be sen 100s of times and while you deserve credit for

thinking outside the box and your Safety is important, we also need to think of Future Fellow Members who will reed these threads !

For the Good of The Crafts ! Big AL

I would never suggest a coil inside the barrel of an RMH, not if it is expected to function well which means reaching several hundred degrees in the barrel interior. Even at too slow circulation, never mind stopped, there is the potential for heat absorption faster than the water can flow out of the heated space which will result in steam and probably an explosion.

So if a coil is the oply option you will entertain, go with a coil around the barrel. The larger the tubing, the easier to avoid sudden flashing to steam. Keep the coil relatively short (use two coils in parallel if you need more heat extraction) and arrange the coil(s) and reservoir for gravity circulation - short piping runs will allow this to be more effective. I can't give exact numbers for tubing lengths.

Whoa wait a sec.
The pressure cooker thing uses an open to air storage tank. This is just a crude drawing so you get the idea. Yes you need reasonably sized inlet and outlet pipes.
You're only using a pressure cooker because you can get a perfect seal so there's no water leaks. At no time are you actually going to build pressure.
Use flexible lines and the whole thing can be removed from the heat source unlike copper coils or just spaced away from the heat to control temperature.

While #3 is the safest, will it provide enough power ?
I'm leaning towards Dave Lot's system (https://permies.com/t/40107/rocket-stoves/hot-barrel#353442) that I like - it's already implemented and tested for output (and safety).
He's been heating 73 lbs. of water (~9 gallons) to max 140F in 2-3 hour burns. From the pictures seems to be ~42 feet of 3/4 copper wrapped on the outside of the barrel.

Do not have time too review right now - will get back too you.

Hello. Hope I can help ya.

Here is my rant :

When I came across the problem of how too heat water, I did not want any pressure developing, and at some point, when I install a storage tank, it will have a 1/2 inch copper water pipe - starting at the top of the tank - going up too the roof, and outside thru the wall. So, not only will it vent out any steam, but it will vent the steam outdoors, so I will not get any mold or mildew inside the structure. The tank will only be 4/5 ths full of water, that way any steam or explosive water heating (turning on pumps on a hot stove) will be expelled inside the tank, and you will not get any liquid water in the vent tube - it HAS to be kept clear....

I also had too answer the question of how long of tubing. . .
I wanted good coverage of the barrel, and each round of the barrel is about 3 feet or so, so I winged it and I believe I picked up a 50 foot role of flexible 3/4 inch tubing. It covers about 1/2 to 2/3 of the barrel (loose fit) and it's big enough too have low water flow resistance - filling up or draining out. I have already tried to build a solar water heater out of 1/2 inch piping, and you get very little heat transfer (not enough liquid in there) and very poor water flow - 1/2 inch pipe would be too small and offer more resistance. You could go with 1", but that is costly stuff.

The next question that came up was internal tubing or external tubing. . while the external tubing is a rather loose fit, the internal tubing idea - with the tubing inside the barrel - I feel - would cause too much turbulence in the air stream and possibly block the stove from working to it's full potential. It would also be a nightmare too clean out if needed.

Yes, I do have a drain back system, and I am planning on having the larger storage tank sitting on the floor beside the stove - with the above mentioned vent pipe going thru the wall - so the stove heats up, water gets pumped around - steam goes thru the wall - if anything shuts off or you forget to plug in the pumps - the copper tubing is dry - and it radiates the heat into the room.

If you are going to be constantly using this set up for heating freezing water, I would heat up the stove first, then turn on the pump - so you have the draft going. You can't boost the draft if it's not warm yet.... and yes, I have found with a block of ice in the water pail - the thing just ROCKETS !

I am planning on using this set up here - as a snow melting system - since it only improves the draw of the stove - the colder it gets . . . As for Ernie's advice of soot and build up, I do have too agree, that with a full water jacket replacing the barrel, it would absorb a huge amount of heat.... but with my tubing, it is barely touching the stove (that stuff is hard to bend ! ) and only absorbing the heat the barrel is loosing anyways . . It does make sense, that if you cool the gases too much, you will get build up inside the stove, but on the other hand, if the stove is rocketing better and faster - hotter - would that compensate ? I will have to watch the RMH videos again - where does the re burn / after burn take place ? If all the gases are burnt in the riser and at the top of the barrel - can you extract the heat from the middle / bottom of the barrel ?

The smartest thing I have ever heard someone say - I will say to you now :

I do not know.

O.k... now that - that is out of the way . .

You ask - should I go with 1.2 or 3.

I would look at the negatives .. and take the best option :

#1. inside - how would you clean it. This is a wood burning device - your going too have too clean it out eventually. A full water jacket would extract too much heat and possibly effect the operation of the stove.
#2. outside the barrel does not look pretty, and you do not get the super hot - heating affect as you would right in the fire, but it's clean. - you also have the option - which I have not tried yet - of wrapping a piece of sheet steel around the outside of the tubing making a super heated air space for the tubing. The air would still be able to rise and exit the top of the spacing, it would just concentrate the heat around the tubing instead of radiating past it. Again, how would it effect the draw of the stove ? Without the heat radiating from the barrel would it slow the draw ? I am going to have too try this one and find out . . .
3. I am not sure about trying too heat water too far from the stove. Yes - there is a definite point too having all kinds of nasty build up inside the stove if you cool it too much / too fast - like replacing the barrel with a full water jacket - but I know from taking temp readings from my stove, that while the top of the barrel is 1,000 degrees - it quickly falls off to around 400 at the very bottom of the barrel, and then at the very end of the line the exit temp is barely 180 (for my stove - without water running).... so the further you get away from the heat source, the less latent heat you will be able to collect . . make sense ? I might add as a side note . . . I am not sure I posted this in my thread, but I believe the chimney temps were hotter when pumping ice water thru the system - better rocket effect - faster moving/hotter air inside the mass . . . I will have too check on this for you this winter . .

The next item is temperature. In my example, I have blocks of ice floating in the water pail. You will get a totally different outcome by raising that temperature. Higher temperature in the coil would suggest less radiant heat absorption - so more heat would go into the stove / less soot build up. 140 degree F water is a far cry from 32 degree water . . What would the return temp be for your system ? - That is what would be going thru the coil. So 100 degree water going thru the loop may not have very much effect on the draw of the stove . . again - I do not know .

The question of pressurized water is always there, and I know from experience (and also having an open ended pipe dropping into an open pail) that if you turn on the pumps on an already hot stove, you do get explosive ejections from the end of the pipe as the first few liters of water flash too steam - something else too keep in mind. - but then having a storage tank that is 4/5 ths full (lots of expansion room in there) - with an exhaust outside should compensate for that . . I am thinking - have the return from the stove (with the explosive steam) enter into a 3/4 inch "T" the bottom of the "T" dropping directly into the top of the air space at the top of the tank, while the top of the "T" will have a 2' piece of copper 3/4 inch that then reduces too 1/2 inch. That way, any high pressure bursts will have the room too compress the air in the top of the tank, but also have space to expand into the upward part of the air vent - without actually entering the 1/2 inch portion of the vent. Just an idea...

And finally, in your last post you quote the temp of 140 F - you are right, that is the max. Even after a 3 hour burn, the make shift pails with no lids on em (wide open) lose too much heat . . so with the setup that I have here, 140 F is as high as it will go. The hotter the water - the less effect it had on the stove, it was just circulating sustaining that temp . .
I did want too try wrapping the copper tubing with a sheet metal jacket - figuring that if I can get this setup to 150 or 160 - heck - even melting the pails - then I know I will have a system that will be worthy of investing in - i.e. buying the low temperature radiator / tubing for in the house.

But then that's just me..

This is just my opinion, hope it helps.

Let us know what experiments and projects you come up with....

and of course the warning of pressurized water :

I love that picture

Oh, and just in case your wondering, this is what I meant by a "low temperature" rad . . .

I have one on each floor hooked into the return pipes of my heat pump/ radiant floor heating. The temp in the tanks varies from 115 - 95 F . . even down too 95 F it still blows nice and warm . . I have had to replace the fan once so far , and it is a standard Home Hardware - box fan . . 25 $ . . Picked it up when I picked up my radiant floor tubing . .