Oliver's 1st ever Rocket Mass Heater

Good morning everyone!!!

This is my second post here on permies and I am happy to say this post is to share with everyone my First Ever Rocket Mass Heater! (and to get some help). I just finished up the heater inside of my hoop house. It has not been working properly all the time but 90% there I believe. Had a few fires, adjusted a few things and now I need to figure out the next adjustments.

DESCRIPTION:
burn pit = 6"x8"x10"H (brick)
burn tunnel = 6.5" diameter x1.5' long (cob & brick)
Riser = 46"H, 7" D inside, 9" D outside, 1.25" insulation (clay dirt & perlite) 2" gap to inside of 55gal barrel
55 gal drums = 48" H cover only a foot up with dirt
Connection to exhaust = 12" D tapered to 7"Dx27'L exhaust (buried 1.5' under ground)
smoke stack = 6'H exterior of hoop house

FIRST FIRE IN THE HEATER:
My first fire was OK. This newbie got the flow going then the fire going BUT got smoke back as the fire climbed the logs. The heat riser sizzled and popped. Second fire was VERY difficult the next day.

1st ADJUSTMENTS:
1) Today I removed the top 55gal drum and found the perlite/clay insulation in the heat riser had closed most of the gap between it and the 55gal drum (so i shaved off the expanded clay/perlite to recover the 2" gap)
2) Restricted the feed tube air flow with a paving stone to the width of the sticks I put in and...VOILA no smoke back.


The fire is burning better now with 1 complaint. It still does not seem Rockety enough and I think it might be the heat riser diameter...Oh wise homosapiens help please.
***It has been 8 degrees F colder at night since I installed the rocket heater even with it off...I'm a bit confused***

Thank you all for the wisdom and help you have given to show me this awesome new experience! The rocket stove will be perfect for my beer brewing.

I have a video with descriptions of everything. I would love to share them...would anyone walk me through how?

Also, to edit this post, my cob was wet when I ran my first fire and I believe that after a few days of two fires a day it has dried out. I have not yet put a thermometer on the 55gal drum but will tonight. It is operating a little bit better now but still not rockety enough. I will try making my chimney 14' to see if this helps. Secondly I will try insulating my 55gal drums with cob and see if that helps as well...? I will update soon.

(too many questions but my last one is does anyone know how long it should take to heat up the 1.5' of dirt above the exhaust?)

This thread was helpful:
https://permies.com/t/32493/rocket-stoves/RMH-smoke

Is the pipe you are using for a flue smooth on the inside? For best results, it should be.

Hello Oliver,

Welcome to Permies!

I'll try and answer some of your questions, hopefully some other experts will assist too.

Heat riser to barrel gap: I increased my gap from 2" to 4" and got better draw.
If you read my construction posts I am dealing with back smoke issues when windy. You may want to consider a exhaust cap.
Do you have back flow when the rocket mass heater is not running or after it has cooled down? Take an stick of incense and see if you are still getting flow out of the system in the morning. You may need to cover the burn chamber with an air tight object. The RMH will still draw air through the system even after the fire it out. The RMH may be pulling cold outside air into the greenhouse through leaks, thus lowering your temperature.

I did a quick back of the envelope calculation, making some assumptions like dry dirt, for heating the Earth around your ducting. You would need to burn about 10 lbs of wood to heat all the thermal mass to a nice room temperature. Once you've heated all the mass the first time, it will take much less to maintain the temperature. If your ground is moist it will take more wood.

If you cob is still wet, your entire system will take longer to heat up. The RMH will get better when the cob starts to dry (2 to 3 weeks).

Okie Dokie...put a chimney riser on and that helped a bit with the draw as well as restricting the feed tube. I may need to put some sort of cap on the chimney as it can get windy here. I am going to cut an inch or two off the riser to see if that amplifies the flow.

I'll update in a few hours to let you all know if it worked.

Oliver Stanton wrote: I am going to cut an inch or two off the riser to see if that amplifies the flow.

I'll update in a few hours to let you all know if it worked.

I hope you get this before you've performed the cut. Please consider the following: I was thinking that you could raise the barrels up two inches. The reasoning is the taller your heat riser the stronger the draw. Thus, if you cut the heat riser an inch or two you will decrease the draw. At the same time the 2 inches between heat riser and barrel is a potential flow restriction point. Thus, my thinking in raising the barrels up two inches and not cutting the riser.

Sorry for the confusion.

oopsy...too late. But it does have a better draw. I cut two inches off the riser and also restricted the feed tube to about a third. ALSO, the more fires I have in it seems to be drying up the cob more. We will continue to see but all suggestions are certainly welcome. Thanks again and in advance to everyone!

Question: correct me if I am wrong but will covering more of my 55 gal drum send more heat below ground to warm up my 1.5' of dirt thermal mass? And covering less of the barrel will cause more heat to enter the room immediately, correct?

Oliver-
To answer the last ? first. Yes covering more of the barrel will increase the heat into the exhaust duct.
Next I think you may experience the same problems I have with a duct buried in the ground. Which may be amplified by the fact that I think that your duct is simply covered by dirt.
It may be an overstatement to say you're "trying to heat the planet" as someone said of my system, but it is an inefficient design.
You might ck out my system and the comments on the in ground exhaust duct here RMH autopsy and redesign.
I think ideally if the mass is below grade and an exhaust duct is to be used in the ground/floor it needs to be insulated from the surrounding earth. If I was starting from scratch I would dig the trench, line it with plastic for a moisture barrier, then maybe put down a thin layer of cob vermiculite or perlite mix to insulate it from the surrounding earth, then fill in around the exhaust duct with the same cob that would be used in a bench (cob/urbanite/rocks/gravel mix) then maybe a thin layer of gravel/pebbles/?? to top it off for a walking surface.
I'm sorry to say that your (and mine ) duct in dirt will result in very poor heating of the space above.

So...next time I build a RMH for my greenhouse or house house I would be better off:

1) having my thermal mass above ground
2) having it below ground but with a different substrate to cover it
3) (as #2 said and pulling off another thread) surround the exhaust pipe with a different thermal substance such as barrels of water (as in the greenhouse thread with 6-8 barrels lining the exhaust of the RMH)

Are most RMH designs built with mostly cob surrounding the exhaust tubing (thus most are benches?)? Or the exhaust is surrounded with sand as in paul wheaton's portable, move it & reassemble in an hour, design?


okok...I'm done

Oliver Stanton : You Did Not give us a location, or Climate zone #, A well made Rocket Mass Heater RMH, that has its top sealed or generously protected with plastic
can survive with little or No maintenance Most Places in the U.S., in the humid N.E. and N.W. coasts moisture can be a problem, A lot of Greenhouse operators are
finding that their rocket helps keep the Air Drier, -Reducing problems with High moisture problems related to Rusts, And Fungi and Bacterial Crop damage ! Having said
that, A raised Grow platform is easy on the Back, and allows you to just throw a row cover over your Seedlings which are getting Heat at the roots, and are above the
coldest part of the greenhouse -the Floor ! Any mixture that 'works' for you is fine !

There is still a lot of Diversity in G'house builds, and as water is such a good storage medium, it will always find a place in most G'houses, Aquaponics is a great Natural
ty-in to any Greenhouse !

Loose fill RMHs like Paul Ws* are a minority, They are probably a very practical way to go to learn what will work for you. This makes allowances for the less amount of
heat that can be stored within the Thermal Mass and the slightly slower heat Transfer through the Mass To radiate Heat into the Structure. Remember That ~40%~ of
the heat of a Rocket Stove comes off of the Barrel !

*The Touted ''move it in an hour'' is truly possible IF you can throw enough Men at the Job, inside most green houses that would be nearly impossible as would any Quick
changes in the Arrangement of the Traditional Cob Thermal Mass ! ( This is just 1 mans opinion, and I only help out at a Couple of G'houses ! ) Big AL

I'm thinking of digging up the RMH my greenhouse and putting it above ground. Then I can put my compost directly on top and use it more so as a localized warming as opposed to warming the whole hoop house.

Would decomposing compost be a good mass? What are the best type of masses to use whether it be in a greenhouse or a house house or green eggs and ham.

I still believe the best type of mass is water (or possibly, but impractically oil). The first problem is how to most efficiently transfer the heat from the rocket heater to the water. Then how much water would it take to extract the heat and maintain exhaust flow and be the equivalent of 2-3000 pounds of cob typically used in a bench type thermal mass. Then how to make it aesthetically pleasing in a home application. Most any design would not be as simple as using cob, nor as cheap and easy depending on your skillset and resources.
Since the main point behind a RMH (after efficiency) is simplicity and low cost, deviations from the basic design are not yet ready for prime time.

Oliver Stanton : While I have discovered Former Hot spots in a small compost pile, where grass clippings looked like they
were burned ( A sure sign of not turning the pile often enough ! ) I would not count on a lot of heat from any pile
that was not at least 60'' X 60'' X 60'', most Gardeners would say much bigger !!!

If your budget will allow, the best way to extract the heat from a Compost pile will be by flowing water through piping to
extract the heat ! Best Big AL

What are the best type of masses to use whether it be in a greenhouse or a house house or green eggs and ham.

The most common options are neatly summarized in a chart at our blog. See the Reference Tab and choose Thermal Mass. Unfortunately, I don't think we covered compost pile in our list of options. However, it would be subject to combustion at a pretty low temperature, so it would not really be a good choice.

I know this is an old thread, but I just wanted to add something. I have been converting my hundred year old brick greenhouse to a John Hait design. It is not a true PAHS building because I just cannot put in the length of pipe or the large enough umbrella. I am creating an umbrella and my first set of earth tubes are 5' underground. I would have made them 10' if I did not have to dig the thing by hand. They tell me that it takes 6 months for heat to go through the earth (faster if wet) which is why at 20' down you get the average temperature. I am insulating the perimeter foundation of my greenhouse, on the narrow sides I can only go down but in the front and back I will go down and then extend the umbrella with plastic and insulation to get as much earth to be a large heat battery. I think if you insulate your perimeter and put in some form of umbrella your pipes can be underground. There is not the quick heat coming out of a bench (relatively quick) but it will rise, and the lateral movement will hit the insulation and eventually come up too. I'm using the solar heat from the sunny days to heat mine but heat does come into the greenhouse from my pipes and even with it not completely insulated the very thick brick walls help somewhat as well. My citrus made it though last winter. Although it was not that strong of a winter. I hope to get the umbrella finished by next summer and then... pineapple!

Jason