rocket mass heater install in double wide manufactured home wood floor crawl space existing chimney

Ok it was suggested to me to document my progress here so here we go... bare with me as time is something i don't seem to have a lot of so pardon my crappy typing skills and lack of grammar and spelling. BTW "Grammarly" app is an awesome tool to cover the tracks of those who don't use proper English or spelling. So we just bought a double wide manufactured home 1700 square feet. i am planning to install a rocket mass heater in it near the kids bedrooms and kitchen in the living room. there is an existing chimney in that room that i will be exhausting through. i am planning on running a six-inch system. the chimney liner is 8 or ten inches so i will run the 6 inch pipe through the center of that chimney liner and pour perlite in the space between the two pipes to insulate the exhaust and prevent in competing air flow issues. for the floor i was planning on fastening three 2x8x12s together with 3-inch screws and four lag bolts and then sticking those perpendicular to the floor joists the span of the heater right down the center under the heater and bracing the support beam with cement block. will place solid blocks as footer dug down maybe a foot below the surface of the crawl space. there will be four of these support pillars spaced evenly apart. i am using the diagram out of the book for a six-inch system. the bench will be different though. the exhaust will be shorter than what's recommended to be on the safe side of back draft perimeters. i am using fireclay slip for the mortar joints of the fire box. using clay perlite mix for expansion joint around the firebox and then cob on top of that. the bench will be cob also. the whole unit will be lifted off of the floor to give a four-inch air gap using cheap bricks i bought from home depot spaced about 12 inches apart. next level from the floor will be four inches of perlite clay mix on top of 7/16 cement board used for tile installation. i was going to wrap the cement board with aluminum foil also to maybe give a lil extra help. this was a really important part of the planning for me. i want to be very sure that heat will not be reaching our wooden floor in most extreme conditions. that is all i can think right now and im getting ready to head out the door to go work on this right now as it is getting increasingly cold and our fireplace really SUCKS. please let me know any concerns if you would like to offer any advice i would be most grateful as this is my first build. im apprehesive to even say its my first build because i can imagine the ridicule i may recieve for doing it in my home. but i bought i fire extinguisher and carbon monoxide detecter and will be running under very close watch and will shut it down at the first site of danger. we are not living at this place yet either but will be soon. i will be happy to talk to anyone even on the phone if you would like. 5134649814 this is number i just set up specifically to recieve calls for this purpose. it will forward to my cell phone. please feel free to call any time before 11 oclock pm eastern time

Are you hoping to heat much of your house with this or just supplement the living room and maybe kitchen?  If you are in the mid/northeast a 6" system is only good for one room more or less. As you say the existing chimney is 8 or 10 inches inside I would suggest building an 8" system. It doesn't have to be much more massive than a 6" system. I think you could perfectly well use the existing liner. If the interior of the liner is more than 8" across, the duct and perlite would be good. 8 x 8 or 8 x 12 are common tile flue liner sizes, but that is nominal and the internal clearance may be 6 7/8" square or 6 7/8" x 10 7/8" or something like that. Is the chimney internal or external? Internal is much better. How tall is it from the first floor level?

The support you describe sounds good aside from the possible footing depth. How deep are the footings for the rest of the house? You should match those if possible. Is the crawl space insulated at all? Does it freeze inside?
Code requires masonry support all the way from the ground, but as long as you do not have inspectors breathing down your neck you should be okay.

Aluminum foil would be good facing the airspace below the cement board but it will do nothing on top of it. Its only function is for the shiny surface facing air to reflect heat back into itself and keep it from radiating. Foil on the floor would only be useful until dust settles on it as it is the shiny surface that does the work.

Give us some more details of your proposed build and we can give more accurate and complete advice.

Yes my plans were to heat the majority of the house with it. I didn't realise the six inch system wasn't enough. I'm really glad you told me. I was planning six because I had salvaged some six inch galvanized pipe that was about to be trash. But I would much rather spend the money to get 8 inch if it makes that great of a difference.

The chimney liner is metal and round. Not sure what you mean about it being inside or out. I don't believe it is exposed on the out side though it is on an exterior wall.

The crawl space has a cement block retaining wall installed all the way around the perimeter. The insulation is in "the belly" of the manufactured home. Which means it's attached to the under side of the home with a poly membrane. There is no added insulation to keep the crawl space from freezing. I don't know if it does freeze under there we just bought the place about a month ago. I kind of doubt it does since it is sealed off from the out side so well.

Not sure what other details would be useful to you. Ummmm let's see. Ah ha...The manifold design I am attempting is a barrel  stacked on top of another barrel that has been cut to fit just over the 48" riser. Will cut it to fit with a 2 inch gap between heat riser and barrel. Do you think that is a good gap between heat riser and barrel. The barrels will be connected together with the clamp that keeps the lid on. I am hoping to seal that with a fire place gasket. Would it be ok to use construction adhesive to hold that gasket in place when I put the barrels together. I figure it doesn't matter if it will continue to hold with those temps. I just want it to hold long enough to put the barrels together. For the manifold I was going to use one of the connections they use for the return air in HVAC ( rectangular on one side and fits pipe on other side, has a 90° turn also). is there a better connection you would suggest for an amateur. I am unskilled in the use of cob and masonry. I am a dryer vent technician so I feel very comfortable working with duct and have alot of tools for that material. But I am willing to do something different if it is suggested. Still hoping to have some great revelation for the manifold to deal with fly ash and CSA. Thank you for your help and time

So I was wondering if this was stupid. I think it was a good idea but obviously I'm asking because I rather hear  from someone with more experience than me that it will be ok. I have two clean outs in this system and I pointed them both up because the mass though short and small compared to many will be a pebble style. I thought that would make an easier access and like the idea of having a couple hot spots to set water. Also was thinking of even connecting the two tees. The thought is to capture the hot gases and radiate heat from this inline loop. And as the gases cool they drop and be whooshed out by the secondary pump and the heat riser thermal syphon pushing it out. The primary reason  I believe this will work is because my exhaust is so short and straight there will still be plenty of thrust in the system to move the cooled gases from the inline loop out of the chimney and terminate the home safely. Please let me know what you guys think. I'm kinda riding on the example of the way the Chinese heated there stone floors as shown in the cob mass heater DVD

As short as your mass is, you would be much better served by a bell than a duct. The space formed by a 55 gallon barrel cut in half lengthwise and placed flat side down will give enough volume for the hot gases to linger and give up most of their heat. Connect the manifold exit to the bottom end of the half-barrel, and the chimney exit from the bottom of the other end of the half-barrel. The hot gases will rise and give up their heat, and only the coolest will sink and exit to the chimney.

You appear to have a good spacing under your core and mass to protect the floor from the heat.

Glenn Herbert wrote:As short as your mass is, you would be much better served by a bell than a duct. The space formed by a 55 gallon barrel cut in half lengthwise and placed flat side down will give enough volume for the hot gases to linger and give up most of their heat. Connect the manifold exit to the bottom end of the half-barrel, and the chimney exit from the bottom of the other end of the half-barrel. The hot gases will rise and give up their heat, and only the coolest will sink and exit to the chimney.

You appear to have a good spacing under your core and mass to protect the floor from the heat.

Awesome!! Thanks for the quick response. I'm pressed on time which is why I'm going to continue with my original plan. Right now we are heating with kerosene 😕 I'm glad you think the space and insulation are sufficient. That was a big priority in my plans. What spacing between riser and barrel will get the best radiant heat from the barrel? I was considering the Peter batch box design and how the riser is nearly a barrel length shy from the top(technically bottom) of the top barrel. He said it was to give quicker heat into the area which is what I am looking for.  I can't go that big because of the ceilings but was thinking of raising the barrel a few more inches or even a foot more than the recommended 2 inch gap. Thanks for all your help

Given a size of exposed barrel, it won't make much difference to the overall heat radiation as long as the riser to barrel top gap is big enough for full airflow. The bigger the gap, the lower the barrel top temperature will be and the more evenly it will radiate. If you want to cook on it, keep the gap near the minimum (2" or so, depending on your system size).

But if you make more exposed barrel surface, you will get more instant radiation.

The extra loop should increase the heat transfer considerably, given your main duct length. A pebble style heater can easily be modified to a bell next summer when you have time. Looking at the pictures, it seems that you might almost be able to fit two half-barrels in the length available, counting the depth of the fireplace. Cut the whole end off one half, and you can telescope the assembly to fit the space exactly.

So I have somewhat settled on my plans for the core. I have been somewhat back and forth on some ideas and was unsure which way I was going to build this which is why I had left alot of details out concerning my plans.

What I'm thinking of doing for the insulating factor of the core is I'm just going to dump sand around since it's too cold to make cob now. And when I get to the manifold I will form some cob to make that transition with a good seal.

I'm a bit nervous about working with cob since I haven't a lot of experience so I'm going to keep a close eye on the manifold or maybe just pour some concrete around it so I know it will stay sealed. I will run it full blast and take a temp reading of the manifold before I pour concrete around it to make sure the cob is thick enough that it doesn't get to hot for the concrete.

Do you know what temps concrete will do well under?

The manifold area is unlikely to get hot enough to damage concrete, especially if there is an inch or two of cob first. Three or four inches of cob would make a better seal.

If you do put concrete around it, I think a half inch thickness with fiberglass screen embedded in it would give all the strength you could want, while being practical to remove if you decide to change it.

Aaron Dailey wrote: I'm a bit nervous about working with cob since I haven't a lot of experience so I'm going to keep a close eye on the manifold or maybe just pour some concrete around it so I know it will stay sealed. I will run it full blast and take a temp reading of the manifold before I pour concrete around it to make sure the cob is thick enough that it doesn't get to hot for the concrete.

Well, you can mix up batches of "cob" or clay based mortar in the same container (wheel barrow or mortar tub) that you would use for mixing concrete. I did mine indoors in a mortar tub, first dry mixing fire clay and masons sand, then adding just enough water to bring it to a really stiff mortar consistency. All done by hand, wearing thick rubber cloves, since masons sand is similar to sand paper on the skin:)

The nice thing about clay base mortar / cob, it can be re-hydrated for rework and be reused, at anytime, now or years from now.

Is this the rock wool insulation that is suggested to be used around the heat riser? https://m.lowes.com/pd/Roxul-Wood-Stud-R15-59-7-sq-ft-Unfaced-Rock-Wool-Batt-Insulation-with-Sound-Barrier-15-25-in-W-x-47-in-L/3388304

Yes it is.

what do you think would be better use for long term heater: a perlite clay mix cast riser using a 8 inch duct for the inner diameter and putting a 7 inch and 6 inch pipe together to get a 13 inch duct for the outer diameter compared to half fire bricks set on there side and wrapped with the rock wool i linked?

For the heat riser, the perlite-clay has less mass internally and possibly more insulation, so it would heat up faster and tend to burn hotter. If your system has poor draft that needs the push from the core's internal heat, though, the greater mass of firebrick might maintain the heat longer in the coaling phase and let the last coals finish burning better. I have seen no coals at all left in my entirely cast core; I do have good natural draft.

ok so its time for an update. im pretty excited to share this!! the exhaust of my rocket heater (no mass yet lol still working on that)  has been extended from six feet to 30 feet not counting the turns theoretical length. im pushing the limits when you add the turns but the way it is designed i believe increases my limits a bit and am curious to hear what other peoples thoughts are on it. so i have counted three to five pumps total throughout the system depending on whether you count where it wraps the fire box as one big one or one for each side of the fire box. plus there is another extra one at the end of the exhaust because it is almost on top of the exhaust coming right out of the manifold and then as usual there is the thermal siphon in the barrel. which when explaining it a friend suggested that the thermal syphon was two pumps in one which i though was a good point but i still only count it as one.

please feel free to make recommendations and share your thoughts enjoy

there is one down fall to this design that stands out to me right off the bat is that to feed the heater you will have to lean over the mass to see in side which i really do not like but i think i will figure something out when it comes time

I don't think the duct passes by the barrel count as pumps; there is no rise for any reheating to power. The place where the first duct run is near the chimney exit would serve, once the system warms up, as it would be warming the rising gases.

I don't think you need to worry about the duct in front of the feed - it will probably not get so warm as to be uncomfortable. I find that the feed set back 17" from the front is workable. I often do lean a hand on the surface near the feed.

ok so i have an update and a question. the heater has been working pretty awesome at heating the house and i still havent even built the mass. so that being said i am very happy with it. we did have some smoke back trouble after extending the pipe. i found that extending the feed by one layer of bricks and then capping the top off with two bricks kinda acting like a cover and then leaving the brick out in the front of the feed as the air intake solved this for the most part except when we would load too much fuel  in the feed it would block the burn tunnel and we would get smoke back. i also found a couple places where the pipes were pitched backwards or near level and i adjusted that yesterday so that there is more of a rise toward termination. seems to be working better just from that small change. i still will keep the "cap" on at night just to be sure there is no potential smoke back as it has been so effective at reducing that issue. i know this isnt ideal and wouldnt recommend to anyone that they build a rmh with smoke back without knowing the danger and understanding the risk.

whenever there was smoke back it is usually due to human error but the room for error is much greater because of this design. i think the main issue is i pushed the limits on how long the exhaust should be. i could easily shorten it but am curious as to whether just sealing the core with cobbish (right now the core is still basically just fire brick and the clay slip has broken so the most of the joints are taking in air) and adding the mass will help with the issue of smoke back. Does anyone else think it will help? i wonder if the mass will actually have a negative effect and it get worse. anyways if it does i will just have to shorten it (dont really want to because i want maximum heat exchange).

my question is i have yet to tape all the joints mainly because when the exhaust was shorter i didnt think it was necessary because the joints would be air intakes(which isnt ideal but not hazardous to health) rather than places i would be concerned for air leaking out. i could even take a cleanout cap off and watch the air rush past the opening though it would cause smoke back at the feed. when i extended the exhaust i did become more concerned about this issue of potential leakage from the seams but am wondering if that is a real concern now.

i wanted to know if it was a real problem so i kept most of the joists untaped and have kept a co2 detector in different places around the heater and the detecter has never detected a single ppm. is there something i am missing? have never seen any smoke come out of these opening either. is there other gasses that people are concerned about and thats why they tape all the seems? i guess im just unsure as to why people tape all the seems when i cant find any evidence of it being a problem with my system that isnt really that wooshy.

thank you for any advice guys
aaron

Aaron Dailey wrote:
i wanted to know if it was a real problem so i kept most of the joists untaped and have kept a co2 detector in different places around the heater and the detecter has never detected a single ppm.

Just a small detail: was this detector one that sniffs CO, being carbon monoxide, or CO², being carbon dioxide? The first one is odorless, colorless and highly poisonous, the other is one of the normal burn products aside water and heat and is what trees absorb to perform the neat trick of photosynthesis.

Peter van den Berg,

so honored to speak to you. thank you for sharing so much of your experience publicly. if it wasnt for your forum and ernie and erica"s book i wouldnt have been able to build this heater. i checked out the detection device and it says it is a carbon monoxide alarm. sorry for the confusion and thank you for your time.

Thought the explanation of the cap was a lil confusing and hard to explain so here's a pic 😁

And another hopefully better one. I have played around a bit with the configuration and this seems to work the best. It works really well surprisingly though I'm not entirely sure why. I thought I would have a dueling chimney problem but it doesn't even though I'm beyond the 1/3 length of the heat riser length by four inches at least