Need Help with a greenhouse rocket mass heater

This is my first rocket mass heater build, and it was a big one.

I have experimented with J tubes on the past with great success. but my heater is experiencing a low amount of draft and constant back burn. I believe there are numerous issues, but I need some guidance on what to try next.

my firebox is built out of half fire bricks. 4 1/2" x 9" x 1-1/4" on a base of vermiculite concrete. the dimensions of the burn tunnel are 6.5 inches wide 7.75 inches deep and the overall length of the base is 3 fire bricks so 27 inches long. that includes the opening of the feed shoot and heat riser

there's a 9-inch horizontal burn chamber.

i initially had a 6-inch double walled metal exhaust pipe with a 1-inch ceramic wool liner inside of a 7-inch pipe. cemented in place with castable refractory cement.  with this configuration, I had very poor draft. and a large amount of back burn.

today I replaced the pipe with a single layer of fire bricks. which created a 7.75" x 6.5" inside dimension heat riser I had not yet insulated the riser.

i am still experiencing a large amount of back burn and poor draft. it is sucking now, but only modestly.

My initial theory was the 6-inch pipe was just not enough flow for my inlet so I replaced that. Now I am trying to decide what to try to fix next.

My exhaust pipe and thermal mass are COLD. probably 30-40 degrees F so that I'm sure is a contributing factor but I'm not sure how much? also when I open the clean out port right next to the burn barrel I get a slightly better burn, but not great, still a lot of smoke and back burn.

My goal for this heater is to get as little immediate heat as possible with as much slow release heat as possible so I did insulate the inside of the top barrel with 1 inch of ceramic wool. Not having the gasses cooling on the top of the barrel I'm sure is a contributing factor. I don't have the heat riser very well insulated. I was planning to use ceramic wool or Vermiculite and refractory mortar, which would be better.

if my dimensions are off on the j tube should I consider lining the inside of the tube with ceramic wool? I already purchased 25 feet by 4 feet so i have plenty.

how large of a role does the heat after the mass going up the chimney play. my exhaust up the chimney even if I'm forcing air into the system with a fan only gets maybe 10-20 degrees over ambient. There is a large amount of empty space inside of the barrel is that causing problems?

Hi Eric; I'm just about to leave ,but I wanted to respond with my observations.
 Your transition area looks a little small as does the pipe leaving it.  Very important ! This could be your choke point.
An unburied pipe will condensate badly.
A cold wet core/mass will fight you.
Your dimensions don't sound quite right.
Lining / insulating the barrel is incorrect. You want that barrel to move the heat not hold it.
What is your chimney like?

Do you have a copy of the RMH builders guide?  Worth every penny.

I apologize if you already answered some of these questions, I haven't properly read your post.    

So the pipe leaving the manifold is 8 inch. my chimney is not completed yet. it's currently only 5 feet of 8-inch tube open on the top. but the mass is so cold by the time the gasses get the chimney they have reached ambient temperature.

I tried just using the bottom barrel, which is acting as my manifold and is uninsulated and I still was only getting a mediocre burn even when I was bypassing my thermal mass.

Welcome Eric,
I would like to second Thomas's recommendation on getting Ernie and Erica's book and I can confirm that it is worth it's weight. Every Penny.

As for your problems, I see several things that I believe are contributing to the back burn.
Dimensions are everything. Make sure you have as consistent a Cross Sectional Area (CSA) as possible through the entire system. There are a few places where you need extra room. One of those is the transition from the manifold into the pipe run. It looks like your 8" pipe is just punched through the side of a drum. This is a bottleneck in the airflow. I would look at getting something to transition from the manifold into the pipe that is at least 1.5 times your CSA. For what you described, 6.5x7.75=50.3ish CSA for the J tube, you should look at something in the 75+ CSA range. There are a number of ways to achieve this, this is how I did mine. Eric Hammond has another great option at the bottom end of the first page of his build. Also watch the clearance between the top of your heat riser and drum. For what you are working towards I would shoot for at least 4" of clearance. I believe on an 8" system 2.5" in the minimum advised clearance.

For your J tube, you are looking for a 1:1.5:3+ dimension ratio. So Feed height of 1, burn tunnel length of 1.5, and heat riser of at least 3. Based on your burn tunnel dimension of 3 bricks (27"), you should have a feed tube that is 2 bricks (18") tall and heat riser at least 6(12 on edge) bricks (54") tall.

I would try removing all the insulation from your drums. I know you are trying to get as much heat as you can into the ground, but in order to get an efficient flow the gasses must shed some heat in the bell area after leaving the heat riser. I would try removing all the insulation and see if that does it. If it helps, you know that was a factor. You could then try adding insulation back in from the bottom up in 6" or 12" rings until it causes problems, then remove a little and stick with that.

One other thing, the bricks you are using are not ideal for this use. You can actually melt them if you get the system running good and hot. They will work initially, just be aware they have a short life expectancy in a RMH. See if you can source some good clay fire bricks.

I hope this helps. Keep us posted, post pictures, and ask questions.

One other thing, what are you using for fuel? It needs to be really dry.

Eric one question, how long is that pipe? I'm running 40 feet before going out of my wall at 6 inches. If you get to long I know you can lose a lot of draft.

Thanks for your sharing.
Jerry

I can't see your pictures for some reason, and the advice you've gotten from those who can see them seems sound.  Especially if you have insulated your bell/ barrel,  the cold downdraft created after the riser is what pushes exhaust through the mass, especially at startup.

What I found after dismantling an 8 inch riser lined with1" CF, was not a riser diameter of 6" like I thought it  should be,   the CF had swelled up enough to shrink the dia to 4&1/2 to 5 inches.

I've had good results sucking the air through from the exhaust side with a fan (like hot air going up a chimney does), rather than trying to push it through from the intake.  Pushing the air might work if the fan and firebox were sealed together in an airtight connection, but the partial vacuum created by sucking the air also eliminates the possibility of pushing exhaust out into the room.

I've found smoke back is usually worse when the firebox opening is larger than any other point downstream. just covering part of the fire box opening may help (after all the other details have been attended to.)

Thanks for everyone's advice. I purchased Erica and Ernie's book, and I agree it was worth every penny, it would have saved me hundreds of dollars. I was excited to return from vacation on Sunday to work on the stove. I was planning on increasing the exhaust manifold opening to a 10-inch pipe that reduces to an 8-inch pipe as Caleb recommended and i was going to increase by heat riser to 54 inches. I was also going to cob the inside of my combustion channels to size them down to 7x7 and try to protect the 2000 degree hard bricks.

However, this is was I found when I returned, see below.

We just had 9 inches of really dense snow melt with an additional 1 inch of rain. the soil still seemed to try in the center of the greenhouse but it appeared like water was seeping in from outside the greenhouse. after a half hour of sucking up water, it appeared like it was going to continually seep in. for that reason, and also to better use the radiant heat coming off the barrel, and also to shorten my exhaust pipe which was a 55-foot straight run of 8-inch pipe which is longer than all recommendations. I decided to move the combustion unit 10 feet farther into the greenhouse.  

while digging the new pit it became apparent it wasn't only seepage coming from the wet ground outside. but in fact, the water table had risen. since Im deep enough to reach the water table under moderately wet conditions I decided I need to raise the combustion chamber.  I put down 4 inches of pea gravel to act as drainage under the system which I'm planning on running to a sub pump pit. I'm hoping I will not need to use a pump, but it will be available if we have more flooding. I put down 12x12in stone pavers to help stabilize the pea gravel bed and then added about 4 inches of vermiculite. i may try to fix the vermiculite with clay or a may create a pad of refractory mortar to float on the vermiculite bed, then do a layer of clay fixed vermiculite.

I also noticed that all of my firebricks had cracked after the first firing, and didn't think it was that bad but as I tried removing the bricks to move the firebox it turns out almost every brick had cracks.

I am not trying to find new refractory bricks in having a hard time finding a supplier near Chicago.

I need some advice, I'm tempted to try a cob core but I have seen a lot of casted cob cores seem to have a lot of cracking. I am looking at online suppliers of soft insulating fire bricks but some require you to order a whole pallet.

Does anyone know any suppliers in northern Illinois?

has anyone had luck with a cob core?

Do I want to use soft bricks or hard bricks, or a mix, hard in the feed chamber soft in the horizontal burn and riser?
What is the minimum heat rating on the bricks you would recommend?

Would it be easier to use the order ceramic board? would that burn out faster?

are there any good plans that would use 1-inch ceramic wool since I already purchased it? I'm having trouble figuring out how I would secure it.

my plan is to use an 8-inch duct vent for the chimney with a 10-inch pipe surrounding it with the gap filled with vermiculite has anyone tried anything similar?

if I can't find fire bricks I was thinking of building a plywood form of the channels that would be 7x7 and using a 2-4 inch layer of cob than brown construction paper, fiber blanket, paper, cob paper blanket, paper cob. knowing that the plywood and possible paper would burn out. Thoughts?

Caleb Mayfield wrote:One other thing, what are you using for fuel? It needs to be really dry.

I was burning 3-year seasoned logs that were really dry.

Hi Eric;
Bummer about your water rising.
When looking for fire bricks you want to locate a masonry supply house. I have no doubt that Chicago has many.  They will sell rock & regular clay brick (so does home depot) but they will also sell firebrick full and split ,home depot does not . Your going to want fire clay, it is at the same place.  
Yes, on increasing the manifold exit also on the 54" riser.
Soft insulating brick is hard to find, the suppliers near me want you to order a pallet or almost one before they will get it in.
Soft brick is desirable for the burn tunnel and riser as it allows the heat to go to your mass rather than having to heat heavy firebrick up to temp. before getting to the mass.
2300 F,  or more  on any product is plenty.
I am currently building my first core with ceramic board so I can't speak from experience... but All the master builders are switching to C.F. board for cores and C.F. blanket for a 5 minute riser. You still want split   firebricks for your feed tube.  They say C.F. board gets harder with firing.
your "plan" on 8" surrounded by 10" with perlite (not vermiculite) in between... is this a riser?  If so you might want a C.F. riser instead.

Is it possible the bricks got water into them and they fractured as ice formed inside?

Thepictures didn't look like there was ice in any amount, and ground water/seepage would prevent freezing over the short term. If the bricks were concrete-based instead of clay, that would certainly do it.

As far as the barrel, the whole purpose of that is fast heat release which you don't want; I would advise a masonry bell, a box which would absorb heat and retain it for slow release. The heat riser needs to be very well insulated, whether it is split firebrick wrapped in ceramic fiber blanket, a 5-minute riser of c.f. blanket inside a piece of stovepipe, or something else. With a good sized bell that allows free gas flow inside, the exit to the duct is less critical; a 10" x 8" reducer should be fine.

I saw what looked like the long duct just buried in a trench; is there insulation under that duct? If not, you are trying to heat the whole earth which is futile, especially if there is groundwater which will efficiently carry away any heat put into the ground.

thomas rubino wrote:Hi Eric;
your "plan" on 8" surrounded by 10" with perlite (not vermiculite) in between... is this a riser?  If so you might want a C.F. riser instead.

This was for insulating the chimney and my plan was to use vermiculite temperatures shouldn't be hotter than 250 F. and its primarily to hold the airgap centered

Graham Chiu wrote:Is it possible the bricks got water into them and they fractured as ice formed inside?

I agree with Glen its unlikely due to freeze, its possible it was different rates of expansion between the firebrick and the cinderblock. also, those bricks were only rated for 2000F and I had a very hot fire with the fan-forced air. they felt very crumbly and brittle when I was removing them, so I think the most likely cause was overheating.

Glenn Herbert wrote:

I saw what looked like the long duct just buried in a trench; is there insulation under that duct? If not, you are trying to heat the whole earth which is futile, especially if there is groundwater which will efficiently carry away any heat put into the ground.

there is no insulation under the pipe other than an air gap. I was assuming that heat rises. and my plan was to try to heat as much earth as possible. I was planning on burning as long of fires as possible and infrequently as possible. I do have below grade polystyrene left over from building my walk-in cooler I could create and insulated channel. but it would i think it would create a huge amount of more excavation. how much mass would I need between the pipe and insulation to keep the insulation from melting?

I'm confident the water table is typically a lot lower than it is now so I'm not too concerned about water moving the heat away, and my other thought is we have about a 4-foot frost line in my area, so there is a limit to how far heat/cold will conduct. I am also planning 3  or 4 100 foot earth tubes. which will help me to pull heat back out of the earth.

I think I'm going to change my plan for using 1/4 inch 2600 CF board wrapped with 1 inch CF blanket.

Has anyone used CF Blanket Rigidizer? Does it lower the R-Value? any other issues with the rigidizer?

It's true that hot fluids (air, water) tend to rise by convection, but radiation and conduction work equally in all directions. The gases in the duct will be flowing too quickly to seriously stratify and will be warm or hot throughout, so you will end up with half of your heat going down. If your soil is fairly sandy, it may have enough heat-retarding quality that you build up a reservoir of warmth over the season, but if the water table can rise, even briefly, it will remove all of that stored heat.

The duct after the first five or ten feet will be cool enough to not endanger styrofoam, and I think an inch or so of dirt or sand separation would be sufficient, increasing to 2-3 inches near the source. Of course the core is a different case, and you need noncombustible insulation under that.

I might make a V- or U-shaped channel to place the insulation in, followed by the appropriate amount of dirt/sand and then the duct.

Thanks, Glenn,

Your first post didn't sell me on the idea of insulating the bottom of the pipe but your second post did. I did think about the issue of conduction. but I didn't think about the radiant heat and the issue of and the brief rising of the water table carrying away heat.

In hind site, I wish I had just built a bench/raised bed in the middle of the greenhouse. I was attempting to merge some of the concepts of earth tubes and RMH's but i think the high water table is an issue I didn't fully anticipate even though I knew our water table is high.

Thanks again, Glenn!