Hello all.  I've had a rocket mass heater for just under 3 years now and love it.  I have it in my greenhouse, which is not ideal from many perspectives I understand.  It has worked well for me.  For a one or two hour burn, I can keep the greenhouse above freezing for the night.  Life has a way of changing, and mine has in the last 2 years.  My ex would make me happy to be in the greenhouse for hours.  This is not the case any longer, thankfully.  

Please dont flame me too much on this, but I understand SOME ribbing....  

I love the rocket mass heater, and understand the use of it and I know that most people that have them use them in the space where they habitate so they are present all the time to babysit the RMH ( to poke, prod, and shuffle and shake the wood).  How much experimentation has gone on for long term fueling?  In other words, I remember seeing something about a pellet feeder system for the RMH, but as I havent been "on" this site for a while I dont know how that turned out.  Has anyone done other testing on systems? 

One of my main questions is whether anyone has tried a propane driven burner at the end of the burn chamber to heat the mass of the heater.  Something to provide heat input to the RMH without babysitting the burn the whole time.  Seems to me that perhaps a burner placed at the bottom of the insulated riser would heat the mass without the need for constant prodding and, while not the solution for everyone, be a good solution for not being present for hours at a time but still utilizing the mass of the heater. 

forgive me please if this has been covered in depth or if I'm missing something.  I'm not on here much and dont have the time any more to read for hours and hours at at time. 

Thank you in advance for your replies. 

Matthew

The main advance for minimizing the amount of tending required has been Peter van den Berg's development of the batch box in place of the J-tube. It lets you load, start and forget it for an hour while it burns, and it gives about twice the heat rate as the same size J-tube. A one-hour batch burn is equivalent to tending a J-tube for two hours. You can find lots of information at batchrocket.eu.

Hi Matthew;  Yes, there was a pellet fed RMH . I believe there still is. However as I recall he had a pellet jam up and a fire....He got that feed issue resolved but still.... one fire is all it takes ..... There have also been waste oil burners mentioned ... but only once ...(i.e. flaming units of death)
As glenn said the batch box design is the longest burning RMH currently being produced.  There is even one being offered on line (ebay) for sale from a permies member.
I have not heard of anybody using a propane burner thru a J or L tube RMH  but if you can afford the propane I see no reason you can't do that. Expensive , yes,  but installed correctly (safely) it should heat your mass. I highly doubt that you would reach anywhere near rmh temperature with propane ... but it would store heat in your mass.   Good Luck   let us know if you try propane.

If you are going to use propane why use any kind of "mass" heater? 

The mass doesn't magically amplify the heat, it merely stores it temporarily and releases it slowly.  Mass works great when you have a hot fire that runs intermittently, you store some of the surplus heat while the fire is burning and release it slowly when the fire is out.

If you are going to use propane it would be simpler, and perhaps more efficient, to just use a smaller flame/catalyst and run it continuously.  Or better yet hook it to a thermostat and run it automatically as needed.  You can buy greenhouse heaters that are designed to do this.

Peter, the reason id like to continue to run my RMH is simple.  Its already there, and I only use it occasionally.  Like, I only have to run the RMH maybe 10 - 20 nights a year due to the semi underground nature of my greenhouse.  3 watt Christmas lights do the trick for me until the outside temp reaches 15 to 18 degrees or less, and here in the mountains thats NOT that often.  For an only occasional burn it seems feasible.  I like the storage of heat part, but i have a hard time now staying "around" for 2 hours plus on cold days as I'm inside cooking supper or spending time with family. 

I understand that with my usage, a different heat source might have been a better choice originally, but again, things have changed and as with this whole greenhouse, everything is an experiment and only after using it for a while could i come close to seeing how well it would work and the potential problems associated with the system.  I believe that it is a great system but somehow think that a different heat source with the same heat storage capability might be better for me, if possible.  

matthew sorrells wrote:Peter, the reason id like to continue to run my RMH is simple.   

The don't use propane.  Converting a RMH to propane will NOT be simple.  You can't just toss a bundle of propane in and light it. ;-)

You'll need to plumb the propane, add a burner, shutoff valve, etc.  By the time you finish you'll have spent more time and effort than it would cost to simply buy something like a Mr Heater portable buddy, and your converted RMH won't work as efficiently.

Peter,
ive not heard of a "mr heater portable buddy" .  I've been out of this stuff for more than a year.  I think if I could warm the mass with something that doesnt need constant attention it would suit my needs better.  I know of no other heater that stores heat for long periods of time, but the method in which you apply heat leaves something to be desired. 

I temporarily considered disassembling a kerosene heater to try to use the wicking burner in the end of the burn tunnel to apply heat, but I couldnt figure out how to get it run and how Id keep the mass of kerosene safe while supplying the wick. 

It can be done.  Do I have the knowledge?  heck no.  But as a collective I think yall might. 

propane line ran through the cob to a burner in the end of the burn chamber.....    maybe tied to a thermostat or a timer or even a simple grill propane tank and a lighter to light and let run for an hour or more then shut it off.  Keep the gas cool in the cob until the chamber?  I dont know.  Seems to me the wood makes the most heat with the draft, but a maybe less efficient but easier method of fueling to heat the mass might work too.  I'm not trying to make it difficult, and I know that i could just put a big greenhouse heater in there, but to run a heater all the time with no heat storage seems ludicrous after seeing how warmth can be stored. 

Thanks for the thoughts so far.

Matthew.

Matt;  I can envision a propane weed burner laying in the bottom of your feed tube. You would have to remove enough bricks on the face to allow the burner to lay flat and to get enough air  thru the face and down the feed tube to burn cleanly.  Most weed burners are 3' long I would cut that down to 16" or less . Peter is suggesting a standard propane space heater... if it ran all night it would certainly use more gas than a wide open weed burner running for an hour or two that you are wanting. Your idea is certainly not even close to burning wood. But in your circumstances it sounds like a viable temporary plan. You have a rmh with mass, you can't be there to feed it ...  you really don't need serious overnight heat (like I do in northern Montana) This is a temp fix that will put heat in your mass,  Go for it!

Matthew;   In thinking your idea over... I see the BIG fault with my weed burner suggestion.  You would need to test that ,by watching it....a lot. Imagine that torch blowing out but continue to flow propane into your mass !!!   BIG BOMB potential.   If you sat with it no problem.... but if you sat with it you could still burn wood... Suddenly attempting an unsupervised propane supplied mass heating might not be such a grand idea after all.  Salvaging  a propane refrigerator for the burner and related thermo couple safety shutoff might be a much safer way of getting propane heat into your mass , without bomb potential!

I was really just considering it for a couple hours at a time when temperatures required it, but good thinking.  maybe an old fridge out of a camper?

I think I would look for a larger old servel propane fridge.  I would think a bigger fridge would have a hotter burner, but i'm guessing.

matthew sorrells wrote:Peter,
ive not heard of a "mr heater portable buddy" .  I've been out of this stuff for more than a year.  I think if I could warm the mass with something that doesnt need constant attention it would suit my needs better.  I know of no other heater that stores heat for long periods of time, but the method in which you apply heat leaves something to be desired. 

There is nothing magical about mass.
Mass is great if you have a short, hot fire and want to stretch out the release of heat.

Mass does nothing for you if you have a continuous heat source.  If you want slow steady heat from propane, use a CONSTANT slow steady burn....with NO MASS.

Your rocket mass heater HAS to ventilate some of your heat into the outdoors, it NEEDS to do this to create a draft/draw/whatever you want to call it.  That heat flowing out is wasted energy.

With a catalytic propane heater you do NOT need any ventilation.  They typically produce little to no Carbon Monoxide and what little they might produce will be consumed by the plants.  The main outputs from a catalytic heater are CO2 and water, both of which will be good for your plants, well as long as the humidity doesn't get too high.

So, once again, if you want to heat with propane do NOT use the RMH, use a catalytic heater instead.  

If you really want to use the RMH, then don't waste your time and money on propane.  Propane and a RMH are not a good fit for each other.

Bigelow Brook Farm has a commercial greenhouse that is heated with a pellet fed rocket mass heater: 

You might find some ideas you can incorporate into your own location.

Matthew, I hope this doesn't come off too harsh, but if you have a greenhouse with growing plants inside that need frost protection, and you don't have an automated (thermostat) heating system that kicks in to prevent a freeze-up, you may wake up disappointed.
If your heating needs are so infrequent, and so low that a string of Christmas lights can handle it, then an electric heater on a low thermostat setting covers it neatly. Maybe it's a piece of electric baseboard, a space heater, heat mats, roof de-icing cables (some of these even have built-in t-stats).
Heck, even a thermostat plug and the Christmas lights? That would alert you (lighted greenhouse at night) that it was too cold, so maybe you go out and light a fire...

If you want to go out and run the RMH, great! It'll be cozy in the GH, and the electric won't kick in... Want to stay in for family time, no problemo! the electric has your back.

Row cover or plastic over your plants in the ground gives you a zone and a half of protection, or heated benches that are covered would also reduce the heating load.

As far as the other suggestions:
Maybe you could get away with stacking functions and the waste heat from your beer fridge keeps everything at the right temps... and maybe that's enough reason to visit the GH more often!
There is NO reason to store heat from burnt propane, just burn it when you need the heat...

Ive done about 5 pellet burning videos in my original compact RMH hybrid.  Pellets work very well but you have to buy them, unlike burning free dead hardwood. 
Here's the latest video.   This bag of pellets was only 5$ and its good stuff.
Video link

As was suggested at the top, convert it to a batch box. One or two loads of wood should see you good for 24 hours. Light it and leave, no tending required.

I agree with the crowd suggesting a batch-box rebuild. Honestly, it's designed to overcome exactly the problems you're experiencing, Matthew.

I like the idea of a pellet-powered RMH with a constant feed, but having seen these systems for household wood stoves, I will caution that you need to ensure that there's no possible way that the flame can catch the pellets in the hopper. Otherwise, bye bye greenhouse.

The best way I have seen to do this is to have the hopper containing the pellets above the feed level and offset, so it's not sitting directly above where the pellets go into the burn tunnel. The pellets at the bottom of the hopper are then pushed sideways with a worm screw feeder, as is used for moving granular bulk items through tubes, that takes isolated batches of pellets and drops them a batch at a time, as needed, into the burn tunnel. In this way, there's no continuous path of pellets for the fire to follow back to the hopper.

Honestly, except for perhaps some sort of elevated grating onto which the pellets can drop, to allow for airflow all around the deposited pile, this is probably the easiest way to adapt your RMH for what you're looking to accomplish, if you don't want to do the batch-box rebuild.

Though I wonder if it would work to cast a mass of wood pellets in a mould the shape and size of which would produce a stick-sized and shaped pellet log that would take up maybe a quarter of the burn tunnel, such that you could fill it with four such composite pellet logs. If the binder used was chosen to be flammable, perhaps enhancing the caloric value of the logs, and formulated to burn slowly, you'd have a longer burn time, more heat per burn, and more stored heat in the mass.

But honestly, I would look at the steps necessary to convert your RMH to a batch-box style. Either that, or for the amount of time you're talking about, get a space heater. If it's one with integrated oil rads, there's your heat storage. Or get a more conventional propane heater, and make sure that the heat from it is hitting the mass of the RMH. That mass will still store heat, even if it's coming from outside the system.

-CK