Rocket mess heater

Hi guys,
The title is not a misprint lol it is the result of an overexcited builder.
So here it is. It is getting colder on the property and generator fuel is getting expensive to keep heaters going. I was finally on my way home for Thanksgiving so I stopped in Phoenix and bought some firebrick and pick up some free 55gal steel drums and 5gal steel buckets, in Las Cruces to pick up 14 free big wooden produce boxes 4ftX4ftX4ft to use for firewood and in El Paso to get some 4in vent pipe and elbows and some cinder blocks. ( I have already figured out the 4in is too small bit, but there are other problems). I built a box with the block 4ft wide 9ft long and 17in high. I built my firebox on one end wtih the tunnel being 10in long 4in wide and 4-1/2in high. I took one of the buckets and cut out a 4in hole in the center of the bottom and attached the riser tube allowing it to stick out of the top about 2 inches. Packed the bucket with our clay/sand mix soil (not the best but this is a temp. build to finish the winter.) covered the riser with a drum cut down to be 2 inches taller than the riser. The pipe in the box was ran down the length of the big box back up and then back down again before exiting up the wall and outside. (total pipe in the box with elbows was 22ft long, with another 6ft going up the wall into a double elbow and continuing up the outside wall another 6ft. We filled the big box with wet clay/sand mix and sealed the drum at the base with 75% mortar 25% soil mix. I fired it up and the draft was awesome. After burning it for an hour, we were boiling water on the drum but you could hold onto the pipe going up the wall with your bare hands and it was only "warm" to the touch. Outside, we had very little smoke and really most of that was steam. It sounds like it is working fine right? Only thing is, when the fire is allowed to get down to just coals, when you try to add more wood to kick the flames up again, it backdrafts like crazy and you have to open the windows and doors to let all the smoke (and the heat) out.  Remove all the coals and let it cool down a bit and it will once again work great.

So, What went wrong? any Ideas? Those who have read my posts before know that I like blunt truth over sugar coating so........

and here

I have a similar problem with mine, which is in its third winter of use. It works well until a buildup of ashes and coals starts to block the airflow just enough, then a strong gust of wind can cause the whole thing to change flow direction. My solution is to use a fan. I have a small fan that I place over the feed tube at the lowest speed.

I do use whatever wood I can get easily, and don't insist on well dried hardwood. That probably contributes to my issue, but that's not going to change. I also burn some wood & paper based trash as that is cheap and then I don't have to carry it to the dump/recycling center.

Four inch systems are known to be a bit "ticklish" by nature, to get them just right. However, assuming all things RMH equal and no built-in restriction issues, smoke back can be caused by several things. Once the burn completes and enters the coaling stage, is the coal bed large enough to be restricting the burn tunnel and thus the draft? Is the "cob" clay/sand bench dry? A dry and warmed up RMH is less prone to smoke back, but even so, coals restricting the burn tunnel is problematic nonetheless.

And probably what you don't want to hear, if I read correctly, the heat riser is packed with clay/sand mix? A high mass (un-insulated) heat riser can cause the stove to stall. I.e. When the heavy mass of the riser finally comes up to temperature (well or late into the burn) its inside to outside temperature differential is significantly diminished, then everything goes downhill from that point. When the stove cools down, it will restart and appear to run okay, until the heat riser saturates again.

Maybe check a farmers coop or garden supply for Perlite, and rebuild the heat riser with a clay stabilized Perlite mix.

Hi Joseph:  I agree with byron, 4" systems are problematical. Also  your not dry yet ... takes WEEKS to truly dry and totally heat up a mass.  Keep burning it will get better as it dries. Use a small fan to help stop backdraft. Next summer take it apart and make it a 6", use fireclay and perlite to make your riser , lots of rock in your mass, maybe use clay bricks instead of cinder block.  Your on the right road .. just a few corners to navigate!

Thanks Byron and Thomas for the info. I kinda jumped the gun on this build and should have done a bit more research. I didn't find out until after the fact that 4in pipe was not the best option but after all the work everyone did in digging and packing I was reluctant to walk in and say "my bad" lol.  I will be home for Christmas and will remove the riser and replace the mix with clay and perlite. What ratio would you suggest? There is a 4in ring between the riser pipe and the outside of the steel bucket. Would filling this with the perlite mix be enough or should I add a thermal blanket around the bucket as well? Also, Thomas, you suggested rock in the mass instead of just the "cob"?  Ratio? And we packed the cinder blocks so tight with clay that they are almost like solid blocks, was this a bad idea? This first attempt was just to get heat in the building for the winter, so I am not considering a complete rebuild this summer, but will make any changes you guys think i need to make for now. I should have the house built by the time winter rolls around again and I will have received and read Ernie and Erica's Book The Rocket Mass Heater Builders Guide. For the building, it was we need heat NOW. but for the house I want everything as close to perfect as I can get it. You advise here is much appreciated and I look forward to hearing more of your thoughts on the matter.

Hey Joseph, yeah, better not tell the cob crew gals about ripping up all the 4 inch duct pipe. You're liable to get slapped silly.

4 inch ring? I assume you're referring to the 4 inches of space between the riser "pipe" and the 5 gallon bucket. That will be plenty of insulation thickness for standalone clay stabilized Perlite, no other insulation wrap needed.

Matt Walker made a nice video on a DIY Perlite heat riser. Expect the inner most metal tube/pipe to burn out. A cardboard tube will also work at that place, it's just a form:

Hi Joseph; I see that byron has pointed you towards matts video, that is where I learned to built my first core and all my risers.  So for your mass you want the most dense heat holding material you can cheaply source.  Rock is better than clay and just about everybody has rock , water is best but problematical, heavy cast iron , solid steel plate  as long as there are no air space's then heavy dense is better. Your mud filled cinder blocks will work it,s just that cob is work to make ... you only have to pick up a rock... As long as they can not crush your pipe then the bigger the rock the better, your cob is to fill in around your pipes and then around those rocks or steel. More large rock means less cob to mix ! Think of it as cob lasagna ... little mud some rock little more mud some more rock. Then you can cap it with a cob mixture. I faced mine with a clay brick surround but the choices there are endless what ever you would like be it cob or stone , brick , sheet rock or even sheet metal could be the final covering.

Hi Everyone.

Sorry it took so long for an update but things have been crazy to say the least lol.

So I got home for the holidays and we had power issues to deal with so the generator was running 24/7 to keep everyone warm at night. My sister and her new husband were staying in the camper because the 4" RMH was not coming close to dealing with overnight lows and Norman and Marquita only had and electric heater in their building. I finally got Ernie and Erica's Book and man did that shed some light on a few things. I had no idea that the measurements needed to be so exact. The first thing I did was pull the drum off and remove the way too short riser and replaced it with a riser sized for the 4" pipe. I used firebrick just like Ernie and Erica. From the bottom of the burn chamber I went up to 48" and glued everything together with fire cement, and this made a huge difference when the new drum was put in and we test fired it. Armed with my new found knowledge I decided to build a RMH for Normans building. I went with 6" pipe and and decided to cast the riser with clay and perlite as you guys suggested. Following Ernie and Erica's instructions and measurements produced such a nice unit that I decided to tear apart the 4" system and rebuild it as well. Since I am still here to write
this, you can infer that everyone was on board with the idea and nobody shot me lol. After seeing the unit in Normans building, I was even allowed to make the mess and leave it for them to clean up. Trust me, this is a huge deal as I had clay, rock and block scattered everywhere. Half the bench has been rebuilt in my place so far and with just that little bit we had 80 deg inside with 25 deg outside when everyone went to bed and woke up to a nice 59 deg in the morning. (we insulated the building as well so that made a big difference itself but I am convinced the biggest change came from the 6in upgrade) With those results I cant wait to see it finished. Thank you guys for your advise. I am uploading a few pics of the one in my building.