My attempt at a rmh

So I decided to build a rmh with the intention of using it to heat water the heat my greenhouse shed etc ran into I few problems with putting the copper coil on the inside as was making it very smoky so going to try on the outside and the insulating it but can anyone help me with why the flame is fluttering won’t let me upload a video sorry
Thanks all

It's hard to say, but usually a flutter in the flame suggests that there isn't a powerful enough draw from the chimney end, or up the stack.

If your exhaust is cold and you have a handy cleanout, open said cleanout and put a candle in there, or if you're not that patient, you can do the old hairspray flamethrower trick. The idea is to heat the end of the chimney, and then close the cleanout so that the only place the hot air escaping the chimney can come from is your burn chamber. This should generate more heat, which will be pulled further towards the end of the system, and your RMH should soon be roaring.

Unless you have insufficient draught due to too much turbulence, not enough capacity, too many turns, or other things that can negatively affect draw.

I would be extremely careful about heating water in any situation where it could become trapped in a part of the system and unintentionally pressurize, like inside copper coils. I hope you didn't mean that the coils were inside the burn chamber, as that environment is too hot for most metal over time, let alone copper. There is a wealth of information on how to do what you're proposing, and also lots of information about what happens when you try what you're trying without all the information and the boom-squishy things that can happen.

Please be safe. Keep us posted, and good luck.

-CK

Hi Mike,  Welcome to Permies.  I'm glad your experimenting with building a rocket mass heater but there are a lot of details that are needed in order to help you out better. Could you upload more pictures, in particular the overview of your system? In order to get a video here (at least the way I have), is to upload your video to a hosting site like Youtube or Vimeo etc. and then click on the appropriate button and add your link. Use the preview button to ensure it looks like it worked. Leave no spaces or it won't show. Hope this helps.

Brilliant thanks for the warm welcome and the quick response at the moment the outlet of the rmh is averaging around 180c with the chimney exit about 120 when it’s in situ the flu pipe will be in a brick chimney will this help?
Also what should the distance be from the top of the riser to the top of the bottle I’m thinking this might be causing the turbulence
And finally would wrapping the pipe around the outside affect the Preformace of the rmh in any way and thank you for the concern regarding heating water in this way and have researched it a lot so the system will be a open recirculating system with pressure release valves also

Given that it appears that you only want to heat water, not have a solid mass, I think any coil around the "barrel" is going to miss a large fraction of the heat harvesting. What I would suggest is a larger barrel with a hole cut in its bottom and welded or sealed around the lower edge of your existing barrel. Fill the outer barrel with water up to above the top of the inner barrel, and you will have a "ring" of hot water that you can pipe directly to your spaces for radiation, or submerge a coil in to heat the circulating water.

I note that you have copied the typical youtube rocket stoves made of metal. These are inherently less efficient than a properly built rocket mass heater which has to be made of masonry or other non-metallic refractory material, and the combustion core highly insulated. If you insulate your metal rocket, the heat will destroy the metal probably in a year or so. If you don't insulate it, you will get a less than optimal burn. The lower heat range of a metal core may contribute to the lower than desired draft which is a factor in pulsing. An uninsulated riser inside the barrel will let heat radiate freely and equalize the temperatures in riser and barrel, which negates the "push" you get from the barrel cooling the exhaust gases and helping with the draft.


Another thing - I understand that corrugated exhaust pipe as you show for a chimney may have significantly more friction than smooth stovepipe, and contribute to lower draft and thus pulsing.

Thanks for all the information like I said this rmh was just a trial using whatever parts I could find or scavage just wanted to really see if my fuel (Woodchips) would run it what design ideas would you have to heat the water with a fire brick one please as I’ll move forward with type 2

Since the water should never be in contact with the combustion core (so as not to steal heat from the fire and cool it and make it less efficient), the same setup I suggested before would work with an insulated firebrick core.

You didn't mention before that you plan to use exclusively woodchips as fuel. This makes your feed tube sensible, and also changes the parameters for how your firebox should be designed. Other people on Permies have experimented with woodchip feeders, and you might benefit from researching their efforts. An exposed metal feed which would tend to shed excess heat easily may be good, while a firebrick burn tunnel would let the heat build up for efficient combustion. I think you would want a smaller air supply opening. Reading about batch box primary and secondary air supply at batchrocket.eu should help you figure out a good system, even though it is geared to burning solid wood.

I’ll second what Glenn mentions: try reducing the air intake. I made a similar stove. I placed a piece of sheet metal with a 2” hole in front of the air intake on mine and it helped.
Maybe insulate the exhaust flue??
How are the wood chips working.?

Jeremy Baker wrote:I’ll second what Glenn mentions: try reducing the air intake. I made a similar stove. I placed a piece of sheet metal with a 2” hole in front of the air intake on mine and it helped.
Maybe insulate the exhaust flue??
How are the wood chips working.?

As long as they are dry they work very well just as good as pellets at the moment I’m drying them in the oven but obviously that’s not practical so I am currently looking at ways to dry them (solar dehydration, air dry in the summer with a black tarp on in the greenhouse) but hopefully going to try and use the rmh to dry them maybe forced air or as I’m hopefully going to be using a gravity fed hopper system to feed it the feed tube when full takes about 30mins to burn so I’m going to try and use a radiator in the hopper system to do the final drying as they get fed into the rmh

Glenn Herbert wrote:Since the water should never be in contact with the combustion core (so as not to steal heat from the fire and cool it and make it less efficient), the same setup I suggested before would work with an insulated firebrick core.

You didn't mention before that you plan to use exclusively woodchips as fuel. This makes your feed tube sensible, and also changes the parameters for how your firebox should be designed. Other people on Permies have experimented with woodchip feeders, and you might benefit from researching their efforts. An exposed metal feed which would tend to shed excess heat easily may be good, while a firebrick burn tunnel would let the heat build up for efficient combustion. I think you would want a smaller air supply opening. Reading about batch box primary and secondary air supply at batchrocket.eu should help you figure out a good system, even though it is geared to burning solid wood.

Many thanks for the link to batchrocket although very long the amount of detail and information is a eye opener and has given me ideas to try on my prototype especially with making the ventury as well as a secondary air intake
But the whole project still depends on heating water to run 4-5 radiators

While I’m here another question before I make some mods to it tomorrow what should the gap be between the riser and the top of the bottle please the riser is 100mm2
Many thanks