Rocket mass heater project in suburban home

Hello Rocketeers,

I've wanted to do a rocket mass heater for some time. Yet, I live in a suburban home and my brother (the owner) didn't want any modifications that might decrease the value of the house. All my other projects were OK (rainwater harvesting, landscape contour change, and permaculture lanscaping). My brother finally allowed me, as a Xmas present to do a rocket mass heater.

After some research I discovered that there is no local expertise on RMH in my area. I found other people who have been wanting to build them for a couple years now but wanted someone to do it first before they tried it. Thus, I took the initiative.

Here are some pictures of the ongoing project:



The house setup before modification. It was really sad to change something that looked so nice. Yet, the fireplace was a natural gas fireplace that wasted all the heat. During a power blackout when the central heater wouldn't work, this fireplace couldn't produce any heat to warm the house. Part of the RMH heater project was to remove all the natural gas plumbing and restore a traditional fireplace functionality. This would be, in addition, to the functionality of the RMH.



Did some prototype J tubes outside to get a design I like that would fit into the space for the modification. The one in the picture below had good draft without the barrel and after 30 minutes of intense fire did not have any problems. The J tube liked to operate at one speed more than others, HOT and FAST!



This image was from two weeks ago. I removed the existing fireplace bench. Removed some of the floorboards and got a good layout. I discovered a asbestos/paper subflooring that I spent nearly a week removing in hazmat gear. The paper in the asbestos/paper was glued to the floor and was was flammable and had to go. (Flame tests would leave this nasty, burnt paper/asbestos/glue mixture).



This is a view from a different angle. I made a masonry block around the J-tube to do some more testing before I set everything in brick. NOTE: The masonry test was filled in and nearly air tight, not like the one in the picture. The J-tube burned well, but not as fast and hot as when it didn't have all the flow restrictions attached. I had to modify the chimeny (not shown) for some additional length to reduce wind backdrafting. I had some smoke leaks that I had to patch. Yet, so far it is going well.

I'll post more pictures as the progress continues.

Brett Andrzejewski : You certainly have an interesting build, Are the bricks that you show that you used in the Picture of your outdoor Test fire Firebicks?
I am also concerned about the bricks you have placed under your %5 gall barrel as they look more like conventional cement/concrete pavers, And as
such they can not be counted on to not fail under the heat loads they are exposed to.Cinder block should never be used near the Rocket burner base
even for fill as this material will fail absolutely !

A good rule of thumb is as soon as you have the Heat Riser laid-up to immediately put a barrel on it and '' When the barrel goes on the vertical chimney
goes up!'' This part of the outdoor test allows you to know that the rocket burner base is assembled correctly, and then you can go onto test the Horizontal
chimney sections.

I can not tell from the picture of the 55 gallon drum whether or not you worked to remove the original paint/sealer on the drum or not ! However, could
you go to our sister site richsoil.com , find and click on the Rocket Mass Heater Heading and scroll down through the set of RMH videos?

I want you to see all of them however the ~8th~one shows a common method to burn the gunk off of and out of the Barrel, also important is to treat the
first 4' - 5' of the galvanized pipe, here you are seeking to get these fittings hot enough to change the way the Zinc galvanizing is bonded to the stove pipe,
there will be some out gassing of zinc during this process but when the stovepipe turns from shinny to flat grey all over you have 'cured you galvanized
stove pipe and will no longer have to worry about this, or you can replace the fittings for the first 4' - 5' with Black stove pipe for wood fires!

Though you specifically did not mention it, I am assuming that you are building over a concrete Slab Floor, how much insulation do you have between the
Rocket Burner base and the concrete?

One of the Areas where Erica Wisner warns us we can get into trouble is in creating something that a new owner of the house we are working on will not
understand. She often reminds us that our chimney which may never see temperatures as high as 200degrees F will to that later house owner be just a
wood stove chimney, and they will expect to hook up their Blast-o-heat 5000 into our chimney !

In this case I am afraid that your entire Natural Gas fireplace was never intended to be modified ! I hope that I am wrong! Some careful explorations must
be made, hopefully you can find a way to positively identify what is behind the Brick Facing there in the corner! In the corner one of the walls appears to be
a 1/2 wall and so you can readily check on the other side of the 1/2 wall for 2x4 studs hid behind sheetrock and other Flammable exposures. However on
the wall that makes up the surround for the Natural Gas Fireplace, and especially where the two walls join to form the corner I expect that there are likely
conventional 2x4s behind the Brick Front. Minimum safe distance to a reflective heat shield should be 18 '' with a 4 -6'' gap behind that ! This would include
the First 4 feet of the horizontal piping in the thermal mass bench !

You speak about ''restoring a Traditional Fireplace functionality'' and then ''This would be in addition to the functionality of the RMH'' This confuses me a little
you do understand that would require two separate chimneys Right ? If this was only ever a Natural Gas fire place you build is just that much more
complicated !

A wood burning fire place requires a properly rated chimney damper, and the Rocket Mass Heater can not have one !

At this point more than anything I hope I am wrong, and the whole system was made up with a second chimney in place ! For the craft ! Big AL !

Thanks for the critique on my build. After reading it I may have to do some modifications to my build. I'll try and answer your questions.

The bricks I used for the outdoor fire test are fire bricks. After burning some test fires for up to an hour I used pliers to take apart the J-tube. I measured the temperature of the outside of the brick with an optical pyrometer and they were about 140 F. The inside of the brick was off the scale and almost red hot.

Yes, the bricks under the barrel are typical cement pavers. I am thinking I should replace them with firebricks.

Yes, the original paint from the barrel was removed. The barrel was original green in color and had a special paint mixed with some reinforcing fibers. After burning a pocket rocket loaded with wood and chimney, I still had to angle grind the paint off with reinforcing fibers.

Thanks for the observation on the 'curing' of the galvanized piping. I'll cure the first 5' when I do my modifications.

I am building over a concrete slab. Because of my previous test fires (brick outside was only 140 F) I did not put insulation under the J-tube.

###
Previous fireplace was originally a wood burning fireplace. Fireplace brick is 2.5 inches with a ~1/2" steel fire shield. Under the fireplace was the ash dump. No wood that I could find, the chimney is at least 3 bricks thick in every direction. From my experience in the attic the chimney does not go through attic, ie external to house frame.

The previous owners put in the natural gas fireplace. Natural gas piping put in, ash dump sealed off, fire brick was then covered with sand, insulation and some fake firewood.

###

The 2 chimney thing:

The original chimney damper is still in place. It is heavily rusted and I can kind of move it open and closed.

The RMH will have it's own separate chimney. It goes beneath the fireplace and out through the ash dump to its own external chimney.

I don't have a picture yet, but there are currently two chimneys on the house.

[edit] spelling corrections

Oh, I forgot to say the J-tube is surrounded by 1/2" insulation on every side but the bottom (the concrete slab).

Error in my previous post

I checked my notes and realized the underside of J-tube firebrick after one hour of burn was 400 F. The exterior of the chimney brick was 140 F.

Hey Brett, I live in Edgewood and have an operating system with many experiments under my belt. Working with water heating currently. PM me if you would like to discuss the project.

Kevin

Brett Andrzejewski : Several people have proposed using the old Ash Pit/External Clean Out as a way to take their RMHs hot exhaust gases from indoors to out !
I am sure if a Constant Cross Sectional Area is Created, with sweep ing curves wherever possible this should be very Doable, This should have been Elevated To
a Forum Thread of its Own ! If you are this deep into re-building your Fireplace you could look into adopting The Rumford Style, the beauty of this is you can dry
stack your bricks to reconfigure The existing fireplace !

This is another one of Ernie Wisners specialties, and I am sure you can get some help there !

I am very puzzled by your reports that the Brick (?) Stops (?) in the Attic area, under those conditions I would expect that the Chimney damper would have been
wired shut and some kind of a cap placed over the unfinished chimney, under the conditions you describe The Water vapor could leak past the Damper, rise up to
the underside of the Roof, condense against the Roofing nails penetrating your roof and Rain in your Attic Area !

Up here in the fridge-ed North, I have seen bathroom vents that led only into the Attic, Frost icicles that thawed and rained down on and through the insulation,
leaving rust marks on the 2nd floor ceilings !

I just re-read the part about the chimney not passing through the Attic, And am really confused, unless a tree fell on your house in a previous lifetime and the then
present owner took the insurance money and went cheap, Or ran out of money during the build, I am totally at a loss to understand what you are telling me!

I think I would actually hire a contractor to come in and tell me exactly what I Had, and His Idea of how to proceed !

Remember that your original goal was to Create a Working Rocket Stove, you should be able to have that -and a 5 year plan to finish up with a working Fireplace !

You were thinking in terms of a 1/2 inch of insulation, I was thinking of 4 inches of Perlite and Clay Slip, so get a second opinion !

For the Good of the Craft ! Big AL !

I have seen many unused chimneys that have been demolished to just below roof line. It's cheaper to do this and fix the hole than to rebuild a weathered chimney. Sometimes the fireplace is permanently blocked and sometimes it just has some insulation blocking it. This creates a dangerous situation. A future owner or tenant might put it to use during a power outage and burn the house down.

Brett, you have a PM

More pictures!

Below is a picture where you can see the ducting going under the fireplace through the ash dump. I had to drill and chisel my way through a lot of brick. It was hard work and my drill was not happy with me. I've covered the ducting with some dirt in the ash dump to prevent the cold draft from coming in. I plan to seal it with a more permanent solution when I am done testing. You can also see the remainder of the natural gas piping on the right. I either need to cap that off or removal all the piping and valving inside the walls.



The picture below is the ducting coming out of the ash dump. I was very fortunate in the the diameter of the ash dump is 8 inches and the alignment was nearly straight. I only had to drill a small amount of brick and angle grind a small amount of metal to get a straight shot. I does slope with a small downward angle.



Lastly, the picture below shows the edge of the house (edge of the house is where the attic ends and chimney begins), the fireplace chimney, and the RMH chimney. I've been thinking about the exterior chimney it being metal. The metal will conduct all the heat out so rapidly that the chimney will contain cold air. The cold air being more dense is probably pushing against the hot air from the RMH stack. I am currently thinking of insulating it so the air stays a little warmer, thus adding a little more stack effect from 2nd chimney. I also need a more permanent solution for securing the chimney. It is being held in place by some barrels and logs during the testing. Oh, you can also see my attempt at a omni-directional chimney diffuser (~$10).



Allen, I was wondering about a Rumford fireplace but had not thought it all the way through. Thanks for the input!

Edited by moderator to fix image links

Brett Andrzejewski : This is a very good Well thought out project, and should be a warming and cheerful addition to your home, please keep us updated on how
this works in your future, If you can do it this may become very popular !

Winston Churchill once said of Anglo-American relationships that England and America were Two Great Nations Divided by a common tongue, everything in the
pictures showed everything you described to us, so that now everything I misinterpreted seems silly, and once again the old adage about a picture and a 1,000
words holds true again !

Again please keeps informed on how this works for you ! For the Craft Big Al !

Quick update (without pictures):

I took apart the setup to add some additional insulation below and around the J-tube.
Broke the J-tube while doing it, had to rebuild it. It was surprisingly heavy, a hernia waiting to happen.
I am working on cutting firebricks to raise the barrel.
Need to seal the fire barrel riser, cure the metal ducting and more testing.

[edit] clarification on terms

If you are putting your burn tunnel anywhere near wood (like on top of your joists), keep in mind that it can get 800-1,000°F. It takes quite a bit of insulation to overcome those temperatures. You may want to incorporate an air space with bricks for support into your design. We have a refractory material which is more insulative than vermiculite or perlite and we could make a pad for you, as well.

Another update:

I got the J-tube, ducting, and barrel all sealed into place for another couple tests. I got a better burn this time, more rocket. RMH barrel and ducting got hotter than previous test runs. I currently have doors open during the testing to allow a air flow through the house.

I discovered some pin hole cracks that formed at the base of the barrel when the mortar dried. Need to fix them.
I discovered a leak (yellow condensate liquid that smells of smoke) at my second clean-out port. Need to seal it. That same leak was also leaking condensate vapor during the test burns.
I can also smell combustion products in the house, but believe these will go away after the leak fixes.

I also need a way to make an air tight cover (or nearly air tight cover) for the burn chamber. I am thinking that when the central HVAC system kicks in it will back flow the combustion gases back into the house.

I also need some J-tube ash cleaning tools.

OH! I test burned some dried Christmas tree twigs with pine needles when the J-tube and heat riser was hot. The RMH really roared when I put those in. NOTE: I did have to spend a bit of time cutting them very straight.

Cindy:

Thanks for the information. I will keep the refractory insulation in mind. At the moment I have the J-tube elevated on refractory firebrick that I cut into 2x2 inch squares under the J-tube. The insulation beneath the J-tube is the high grade Kaowool.

Oh, that Kaowool is ichy stuff and I am guessing not great stuff to inhale. I was wearing a dust mask when I was cutting and manipulating.

Update and back smoke (backward flow) problems

I was doing more testing today and have been getting backward flow problems. There is a steady breeze of 10 mph with gusts to 20+ mph.

The 10 mph breezes are not a problem. It is when the gusts of 20 mph hit, i get backflow and smoke in the house. I have a homemade air diffuser on top of my chimney, seen in the previous posts.

Any suggestions?

Thanks in advance

Insulate your outside tube. Whay haven't you fited it in the chimney by the way?

And fit a chimney cap like this http://www.luxurymetals.com/chimneycaps.html#anchor_637

Brett Andrzejewski wrote:Update and back smoke (backward flow) problems

I was doing more testing today and have been getting backward flow problems. There is a steady breeze of 10 mph with gusts to 20+ mph.

The 10 mph breezes are not a problem. It is when the gusts of 20 mph hit, i get backflow and smoke in the house. I have a homemade air diffuser on top of my chimney, seen in the previous posts.

Any suggestions?

Thanks in advance

FWIW, I'm looking forward to your progress on this project.
I bought a cap like this one for my traditional wood stove. It looks like that's the one you're using. On mine, the harder the wind blows, the harder it sucks air up the chimney. In the picture, it looks funny, like it's broken or installed tilted or something.

Cam Mitchell wrote:
In the picture, it looks funny, like it's broken or installed tilted or something.

It's handmade, yes, it is built tilted slightly. I didn't want to spend the $150 bucks.

Brett-
I like your idea of adapting the existing fireplace. That brick wall will add to the thermal mass and radiation.
I reread this thread twice to see if I missed it, but I have 1 observation of a possible problem area.
It looks like your exhaust duct exit from the barrel is a likely problem. Could you elaborate on it's specifics? It looks like you just have a hole in the bottom of the barrel where the duct connects. If so this is a bottleneck for exhaust flow and will cause weak draw.
Your concerns about HVAC air flow are warranted. You likely need to have some type of separate air intake for the RMH feed tube. Since you're on a slab it will have to come from outside or maybe the old chimney?

A second observation is that your barrel looks really close to the interior wall. When the barrel gets as hot as it should it might get that wall fairly warm if not down right hot. Even though it's brick if there's something in the wall (wiring. plumbing) or on the other side, you should keep an eye on the walls temp.

Lastly a simple 180deg pipe (2 90deg elbows) for an exhaust stack cap will work fairly well and significantly reduce (not eliminate) cold air coming down into mass after the fire goes out.

Thanks John,

The start of the exhaust from the barrel is a 8" T ducting piece. One side of the T points up toward the barrel, the other side of the T points down into the concrete slab. The ducting run is connected to the other T.

I have run tests with doors open and doors closed and didn't see a difference in draw.

So far after the runs I've done the wall temps never got hotter than 120 F.

Thanks for the tip on the 180 turn as a cap.

Brett Anndrzejewski : Are you saying that you have set the barrel off of being centered directly over the Heat Riser? Any attempt to make an oval shape out
of the leg of the 8 inch "T" where it attaches to the bottom of the barrel will reduce your Constant Cross sectional Area Exactly where you ant to go big 2 Xs,
even 3Xs the Cross Sectional Area of the Burn Tunnel ! Big AL!

Late Note : You may want to check out how John Adamz made his transitional area, check out the (3rd page?) of "RMH autopsy and redesign modifications'' in the
Rocket Stoves forums, I bumped it up to make it easier to find ! Good Luck a. l.

Allen- Thanks for the bump. The redone transition area is midway down page 1. As an update I changed the duct opening exiting the transition area from a sharp corner to a curved/rounded one around the circumference of the duct. Small difference, but it did help a bit.
Brett- You should really do away with that T at the barrel and making a larger transition area to the horizontal duct. Just make sort of a venturi shape 2-3X the CSA of the burn core starting from the bottom of the barrel going down to just slightly bigger than the 8" duct at the bottom of the pit. Also make that area go down below the level of the duct 2-4" to reduce fly ash getting in the duct. You can easily access it from the cleanout you have already. Keep the edges/corners smooth and rounded and you will be pleasantly surprised with the significant increase in flow.

!!! ITS ALIVE !!!

My RMH roars now! Wow! What a roar!

I removed the barrel, made the T ducting where the ash pit is more aerodynamic and larger. Still back flow issues on windy days.

I added another 5' section of chimney ducting. Now, the system draws cold, warm or hot! Now, when the wind blows it only makes the RMH draw faster.

Here is a picture taken from the top of the roof. You can see the other house vents and ports in comparison. The RMH chimney is in the back. The RMH chimney is now about 1 foot shorter than the tallest house vent.



Thanks for all your help!

Congrats,
I was amazed at how much of a difference it made to my system by adding a 3ft double wall section (just what I had left laying around) to the bottom of my 10ft chimney stack.
Do you have a pic of the redone area were the "T" was? Have you covered the duct with cob/?? yet? How high are your barrel temps getting now?

Thanks,

By adding the additional 5 feet must have taken the chimney above the house's wind flow profile. I too was amazed. I did a little fire dance!

I have not redone the chimney with the T yet. I have the parts but just haven't put it in place. I plan to do that after I finish up the bench. At the moment the ducting is covered with wet mortar. I am getting it level for the next step of placing tile over the ducting. Trying to make it look like a traditional suburban fireplace bench.

I only did small test burns (a hand full of small wood pieces, ~ 5 minutes burn) but my barrel temps were staying about the same as my other small test burns (~250 F). My ducting temperature was 20 F warmer (120 F) compared to my other test burns. Temperature measured at first clean out port.

I haven't had the time to do a long burn because of other tasks, meetings. Perhaps this weekend I can do some longer burn tests.

[edit, grammar]

Update:

I did some longer burns today. I was observing only steam (condensate) out of the chimney. My barrels temperatures maxed about about 290 Fahrenheit. I believe this is because my heat riser is a little taller than most and my gap between barrel and heat riser is 4".

My ash seems to be pretty fine and very little to no charcoal is in the burn tube after cleaning.

My bench is getting warm (130 Fahrenheit near barrel, and 90 Fahrenheit near U-turn) , although it is still drying and curing from all the mortar I put on.

290 might be a bit low, but maybe not with a 4" gap. Is there a reason you made it so big? Sounds like things might work out with the current configuration after things dry up.
Does it seem like it's still drawing pretty good?

I originally had 2" gap but was getting poor draw. I changed the design to have 4" gap and got better draw. This was all before adding the additional chimney length. I'm sure if I went back to 2" gap with the current chimney length I would be getting higher barrel temps and a good draw. Yet, I don't feel like tearing the RMH apart yet another time.

The draw has been consistent regardless of wind conditions and RMH temperatures. I get mild draw when cold, and massive draw when hot.

Hi Brett, once you do a longer burn and get the heat riser HOT, your top center barrel temps will be hot. Real hot!. I'm guessing you'll be hitting average 6-800f. In order to get those temps though, you have to have a FIRE! Not twigs and pine needles.....real, serious burnin' stuff. Your Dragon is all growed up....you have to feed it a real meal. Those temps can be deceiving cuz when you aren't looking they will change. The second your fire slacks off the temps will drop dramatically.

Glad things are working out.
Kevin

kevin Davis wrote:Your Dragon is all growed up....you have to feed it a real meal.

I feed my Dragon 2 real meals. I loaded the dragon up with branches and twigs so that it had big stuff and small stuff to burn. I feed it for 2 hours like this and was getting barrel temps of ~380 Fahrenheit.

I then gave my Dragon a propane meal with a 500,000 BTU/hr weed burner torch for 10 minutes. The J-tube was getting orange hot my barrel temperatures averaged around 460 to 480 Fahrenheit.

In both cases the masonry above the ducting near the barrel was doing about 160 Fahrenheit. Farther from the barrel it was doing about 90 Fahrenheit.

Update:

I noticed some variance in my handheld IR temperature gun. Some places on my barrel are pretty shiny and reflective while others are darker. My temperatures are about +/- 50 Fahrenheit depending on the spot I focused the temperature gun on.

John Adamz wrote:290 might be a bit low, but maybe not with a 4" gap. Is there a reason you made it so big? Sounds like things might work out with the current configuration after things dry up.
Does it seem like it's still drawing pretty good?

is a 2" gap better than 4"? if so whats at play there? noob thinking would be bigger gap better flow? is there a venture effect going on in 2" gap?

ronald bush wrote: is a 2" gap better than 4"? if so whats at play there? noob thinking would be bigger gap better flow? is there a venture effect going on in 2" gap?

I think I have several things going on in my system (different than a typical build). The 4 inch gap allows for better air flow, less resistance. At the same time it decreases the barrel temperature because the combustion gases are farther away.

The other thing I have going on is my taller heat riser. I believe my heat riser is 4.5 inches taller than most. I wanted a extra draw just to be on the safe side. The taller riser also decreases my barrel temperature.

I've been noticing that my ducting temperature as the ash pit and first length ducting will steady state at about 160 Fahrenheit.

oh, there is no Venturi at the barrel gap.

I think you guys could use some clarification of the riser to barrel gap calculation.
It's a bit hard to visualize but it can be thought of as an "opening" with the dimensions of width-riser to barrel gap, and length the distance around the inside perimeter of the riser. This "opening" should never be smaller in area than the CSA of the burn tube. An 8" round riser has a CSA of ~50sq" and a circumference(perimeter) of 25". So 2" (barrel gap) x25" (perimeter) =50sq". Thus a barrel gap of 2" + is generally used.
The gap of the riser to the barrel side is also similarly significant in sizing.

that does clarify that part, thanks! so,the barrel is where the area can start to get a tad bigger. after it leaves the top of the barrel gap(2"gap) does the vollume of that area matter, as long as its more? then it goes out the barrel with an opening twice the size of the burn tube?

this is fascinating subject to me for some reason! i wish i could install one in my home but dont think they would approve it here. i can build a heated bench outside though! for fast heat im leaning towards the half barrel bench idea on here, as long as it dont hinder the burning or exhaust flow at all.


thank you for chiming in John

Thanks for the clarification John,

It all makes sense in my mind, yet my thought process and written description may not work for others.

When the flue gas leaves the heat riser you have to take into the consideration the area. In my case I have a full 8 inch diameter union + extra ash pit area to prevent bottle necking. My heat riser is offset so I am able to do this no problem. If your heat riser is in the center of the barrel make sure the area between heat riser insulation and barrel is equivalent to your ducting.
ie (Area barrel - area heat riser and insulation) = cross sectional area inside barrel.

Give yourself a little extra area at the ash pit for fly ash collection.

Video update: RMH ducting clean out and inspection with borescope and still images.



Other notes: The external chimney is not very structurally strong and will sometime blow over with really strong winds. The galvanized chimney is not really suppose to be load bearing and needs something else to support it. At the moment it is secured to the original house chimney and a 14 ft 2x4 cemented in the ground. The cement footing holding the 2x4 has already cracked during one of the stronger wind storms (suspected microburst rainstorm/virga as branches were everywhere).

Update with rocket mass heater:

Problem:
I had a lot of problems getting my rocket mass heater to draw today. It was a cold system and was only getting a fraction of the draw I would normally get. I would occasionally get a puff of back smoke into the house while trying to get it started.

Solution:
I found that the 3rd clean out port, at the external chimney, had fallen out. I was essentially trying to run my RMH without the external chimney. I put the cap back in place and it is roaring again with its normal might.

Update: The amount of ash produced from burning twigs and wood


This video I describe and show how much ash is produced from burning twigs (apple wood)


(9) 5 gallon buckets, about 1 to 2 pounds of twigs produced about a 1 gallon worth of ash

This video I describe and show how much as is produced form burning particle board and wood


10 pounds of particle board and 5 or 6 pieces of pine from pallets produced about a handful of ash