Too much mass?

Hi all,

I built a RMH for my greenhouse, an 8" system with 9 meter horizontal flue run followed by 3 meter vertical chimney at the end. I used an old water heater tank steel drum to cover the insulated heat riser. I test burned the MRH and it worked amazingly well. I then started boxing the horizontal flue and added mass (concrete, mud and gravel). The total mass is around two cubic meters (calculated at about 4 tons). Ever since I added the mass and fired up the heater again it is as if the heater is struggling to form a decent draft. Nothing changed except for adding the mass. Why is this? Could I have added too much mass? The horizontal chimney at the end almost does not even heat up (exhaust gasses are very cool but still escapes). Is this due to the mass absorbing the heat so fast that a decent draft does not start? Do I have to wait for the mass to heat up before the stove will start performing as expected?

Thanks!

i have a similar system and adding the mass didn't affect performance, could it be that ash is accumulating somewhere? in my case ash accumulated on the heatriser top, i have a closed barrel but i'll have to cut it and make a lid so i can clean that area at least once a month.

Marco Ernst : Are you 'telling' us that your Rocket Mass Heater Has Lost Its Throaty Roar? If you still have that, I expect that your Thermal Mass just has to dry out ! Are
you 'telling' us your system only has the one Elbow and one Clean out ? Or maybe two clean outs ? IF that is your horizontal pipe configuration, how deep in the thermal
mass did you set your horizontal pipe, How high and how Wide is your bench ! A 3 ton bench would be a lot to heat ! Your system will definitely have a noticeable lag
between the heat energy entering the system and its starting to escape. For a solid mass with no air gaps we talk about an Inch an hour, Lots of large air gaps speeds up
the transfer of heat through the mass. Very small air gaps, like those between course gravel and pebbles adds transfer time !

If your system is seriously time lagged, the answer will be in keeping your Thermal Mass Battery topped off at all times, this may require having a fire to charge the
thermal bench when your greenhouse feels just fine, but remember your initial heat mostly goes into the thermal bench and not into heating the air. Also with a long
lag you will have no problem covering the early morning hours with heat when you need it most !

The more information you share, the more accurately we will be in helping YOU diagnose your situation !

For the Future Good of the Crafts ! Be safe, keep warm ! As always, your comments Questions are Solicited and are Welcome PYRO - LOGICALLY Big AL

Leonard Allen : This is just my humble opinion, but if that was your only problem and your cleanout in the base of your Rocket Mass Heater Worked well for you,
I would try to find a barrel with a removable top held with a clamping band, remembering to remove the original gasket an replace it With a rope 'fireproof' gasket ! My
barrel is slightly off set from the Heat Riser, and i can take my barrel top off and show the off center blast mark on the inside of my top and also shine a light down into
my Transitional Area, and ash pit !

For the Future Good of the craft ! Be safe keep warm ! As always, your comments and questions are solicited and are Welcome ! PYRO-LOGICALLY Big AL

Yes, I was a bit cryptic when explaining my setup. A little more detail. The horizontal flue runs from the drum straight for 9m with no bends. Then a 90 degree bend horizontally and immediately another 90 degree bend vertically where the 3 meter chimney follows. The area of the gap between the drum and the horizontal flue is about 1.5 times the area of the flue pipes (8"). So no bottleneck there. In fact, all the dimentions adhere to the specifications as set out in the RMH "bible" pdf .
I started burning the stove again and again and it looks as if you were right allen, the mass needs to dry out. The longer I burn the better the draft gets as the bench starts heating up. Yes it is a large bench. It is 9 meters by 0.7 meters by 0.42 meters. The flue as right at the bottom of the bench so it is covered on top with 220mm (8.6") of mass. The sides are covered by about 250mm a side of mass. I went overboard I think.

I did some temperature measurements. I have sensors touching the top of the bench in 3 places. Starting the fire the bench temp was closest to the heater 15 degrees C. After around 3 hours it went up to 22 degrees. It then slowly cools down through the night after the burn finished and reaches about 15 degrees again round about 6am. I will do some more tests and post some measurements later. The temperature last night in the clivia area (shadenet covered with greenhouse plastic, very poorly insolated) went down to 2.88 degrees with the outside temperature drommping to -1.5 degrees. I only need the Clivias to remain above 0 degrees or slightly below, not colder. So so far it seems to do the job.

For those interesed I also added antracite to the feeding hole behind the last logs. The antracite all burnd out during the night, might have helped lowing the cooling of the mass during the night as well but I will have to do more tests and measurements.

I still have a problem or maybe it is just the nature of the beast. The pdf states that an 8 inch system can burn logs up to the size of one's knee. I have smaal knees and still I can not burn logs that size in my heater. For some reason the logs dont burn fast enough or maybe too fast forming "chimneys" between the logs followed by flames running up the logs instead of burning sideways into the burntunnel. Using smaller logs is fine (or better). The wood I am burning is wattle, very hard and difficult to chop. Takes a lot of effort to get it going as well.

Marco, two sugestions, insulate your vertical chimney. No mass, but rockwool, so it heats up. And may be make it longer, 6m instead of three.

And for the starting up, i put sticks the size of my fingers aproximately. And pour some burning alchool/ meths/E85 on the wood. Do this only when cold. Never ever on a hot stove. Hope this helps.

Marco : Feeding the Dragon in your R.M.H. just keeps getting easier with each feeding, Though they cant be quite finicky eaters when cold and you are trying to wake them
Up!

Small dry wood is like warm fresh baked bread smeared with lots of butter is to you or I ! (soft wood is like cake, but only for starting) After your R.M.H.s Burn Tunnel is
glowing red hot and there is a nice bed of coals in the bottom of the Feed Tube, you can try a special treat for your Dragon. Try a chuck of wood slightly bigger than your
knee, knock off four slabs from the chunk leaving you with a Oblong chunk with little or no bark ! ( Bark is the tree's way of protecting itself against Fire and Insects ! )

Fed to your dragon by itself or with a couple of other smaller chunks, should work just fine, depending on YOUR Dragons temperament ! Always load your chunks into the
feed tube bigger end down, this should keep the chucks from hanging up on each other, this will always end up causing your to notice the "two or more pieces creating a
chimney'' that you have noticed, it should actually be there most of the time but always burning in the right direction - sideways !

You will also notice that when you are trying to burn less than full loads, that restricting the flow of air at the Feed Tubes Surface with a flat slab of stone/brick/Paver,
will channel the air better around the wood fuel and Stimulate your Dragons Appetite !

Everyones R.M.H.s Dragon has its own temperament that you will quickly learn, simply from listening to the loudness of its Roar ! The fact that this is a cliché, does not
change the fact that this is true !

For the Future/Good of the Craft ! Be safe keep warm ! As always your comments and questions are solicited and Welcome ! PYRO-MAGICALLY Big AL !

Thanks for the advice all. Tonight is feeding time again I will try insulating the chimney as well and maybe add another meter of hight. Will make the first load tonight only thinner logs before going for a few thicker ones. I will also start looking at a gravity feed system since the greenhouse is way outside and it is really hard to go outside to feed when I am all snug in bed!! Was thinking of a expanded metal tube as not to form a chimney but still keep the wood in place. Will keep posting what the dragon does. It is like a living thing that fire!

allen lumley wrote:

You will also notice that when you are trying to burn less than full loads, that restricting the flow of air at the Feed Tubes Surface with a flat slab of stone/brick/Paver,
will channel the air better around the wood fuel and Stimulate your Dragons Appetite !

Everyones R.M.H.s Dragon has its own temperament that you will quickly learn, simply from listening to the loudness of its Roar ! The fact that this is a cliché, does not
change the fact that this is true !

For the Future/Good of the Craft ! Be safe keep warm ! As always your comments and questions are solicited and Welcome ! PYRO-MAGICALLY Big AL !

that is true, it works much better when covering part of the feed tube's surface, about half of it or even more too