Glenn Littman

Paul, you are right, a column of air that is hotter than the air around it will naturally flow up. Being a commercial hot air ballon pilot I can attest to the beauty of leveraging the physics of this means of propulsion. In the case of the rocket mass heater you want that propulsion to occur in a similar manner to push the heat out your chimney but you are managing the direction of the flow in an effort to use the heat to ultimately heat a mass and thus your room.

Understand that what you are working to achieve in heating a thermal mass is the transfer of the heat in the air to heat the mass. The hot air flows up from the combustion core and as it comes in contact with the mass of the stratification chamber it begins to cool so that it changes direction and flows down, continuing to give up the heat in the air in an attempt to reach thermal equilibrium with the mass. Once it reaches the lowest point of the stratification chamber which is typically the point of exit to the chimney it reverses direction again and exits. The less friction created with those changes in direction the better the system will breath and perform. As is experienced leveraging Peter's designs, the combustion core temperatures can reach in the 1,600F range +/- and the chimney exit temperature at the base of the stratification chamber is 200F +/-. This proves the concept of heat exchange, thorough combustion of the fuel, and maximum thermal capture of the system.

I hope this helps.

Hello Paul and welcome to the wonderful world of radiant wood heating with masonry mass. Your first statement is that you are "interested in designing and building a mass heater". Can you also state your ultimate goal? Is it to tinker and find new and innovative ways to heat with masonry based on previous design concepts or are you looking to build based on proven design development? Either goal is valid with the former taking a long term commitment to experimentation while the later will leverage the experience of long term experimentation by others.

The reason for my post and question to you is that the decision to go down the road of building a heater can initially seem like a daunting task. As you do your research you will find that a batch box with masonry mass will meet your operational goal then the major work has been done and well documented. If you haven't studied Peter van den Berg's website, https://batchrocket.eu/en/, I would encourage you to take time to read it in it's entirety. Peter has perhaps done the most scientific research into refining the design and has been kind enough to share his hard work for all to benefit.

As Peter says; "Follow a proven design and get proven results". I can attest to this being fact as I fire my batch rocket in the morning and evening and it performs perfectly each and every time. The earlier post by Thomas Rubino will also provide links to help visualize the build process and end result. Best wishes for your successful batch rocket journey.

My opinion Croyden is that you have to determine what side of the scientific spectrum you wish to reside to determine the answer to your question. Having lived with and thoroughly enjoyed our batch box RMH for 9 months now I have dialed by my scientific curiosity and am fully comforted by the day to day operation of the system. I embedded 3 thermocouples into my build; 1 at the top of the riser, i embedded in the inner brick skin adjacent to the top of the riser and 1 in the exhaust port where the stove pipe meets the brick manifold near the floor. All 3 are connected to a data logger display for realtime monitoring.

I would consider my approach to be in the middle of the scientific spectrum for assessing burn performance. The two ends of the spectrum would be simple observation of the burn chamber and chimney exhaust with the other end being the method that Peter vdB uses with a Testo meter providing gas and thermal data. I believe my system burns as expected by Peter using Peter's specifications for my build. A batch will consistently burn down to coals in roughly an hour, there is never any visible smoke from the chimney and my exhaust port exit temperatures never exceed 250F while my riser temperatures run in 1,500F range during the active burn. As for cold outdoor temps and determining the cleanliness of the burn. We are consistently in the -10 to +10F range at sunrise this time of year. The exhaust from my chimney is white but it totally dissipates within 10 feet of the chimney exit indicating that this is steam. If it were smoke you would definitely see the smoke trailing off, especially on mornings where the winds are calm which is our normal condition.

Incidentally, the operation of my system has been so consistent I no longer leave the temperature display turned on. I'll flip it on a few times during a burn session just to check the temperature of the inside brick skin to determine if I want to burn a 2nd batch, or in the case of days like we had last week when it was -17F outside, a 3rd batch. My inner skin maintains a temperature of 350F when I first wake up and I target a temp of at least 500F this time of year for a 2nd or 3rd batch. All the while the system burns clean and never seen an over-fueled condition.

Bottom line for me... I could run with no temp probes if I wanted and I would be confident that the system is burning clean. Of course, this is dependent on the build design and following a proven method. Deviation in build design could affect performance and require more scientific methods for analysis. I hope this helps.

For the folks that like data to support your decisions. The best reason to build a batch rocket heater... because it works... really really well.

It's presently a chilly -12F (-24C) outside at 5:00 AM. The last fire in the batch rocket was at 7:00 PM last night, 10 hours ago and the bell skin temperature was 176F (80C). My 2,000 sq ft shop with only 2" of fiberglass blanket insulation is 57F (14C) for a 69F (38C) differential and the bell skin temp is now 143F (62C).

Form may follow function for many folks but not so in this case... Thomas the Dragonmeister proves that form and function can go hand in hand.

Shortie sure is a cutie! Perhaps a little red lipstick on the air intake to go with the party dress...

Julian Adam wrote:I would like to add to Glenn; if you are using the buildup with insulation under the slab, if you have the option, it's best to use XPS or EPS. Especially XPS does not suffer from moisture, PUR (nowadays the standard insulation, at least here in the EU) does. I will be using XPS for my underfloor heating (backup/alternative to my RMH).

Thanks for the additional detail Julian. You are correct, I couldn't recall the exact material I used as it was 9 years ago, I checked and it was XPS board.

Trace Oswald wrote:I just want to say thanks to Glenn for posting this in such detail.  I've read the entire thing at least 4 times, and every time, I've picked up on something new.

I can't wait to start my build.  It will be in the greenhouse I'm building.  I've already started clearing the site for the greenhouse.  I'll be pouring a slab for the RMH to sit on.

I'm super glad to hear that my build thread has been helpful. It serves as a double blessing. First, the participation by the community was invaluable to me as a first time builder to get it right the first time and have a superb functioning heater. Second, it helps folks like yourself to get a jump on build planning.

For your greenhouse build I'll throw out a thought that I failed to do with my greenhouse which I keep growing year-round, but did learn when I built my home which is slab on-grade with hydronic floor heating in the slab. Floor mass, whether concrete or dirt can serve as a thermal mass but up front planning will help with efficiency. If you insulate the perimeter you will minimize heat loss out the sides. For my slab I laid down crushed stone with a vapor barrier on top then 2" thick construction foam board. The perimeter was dug a little deeper and foam board was cut and laid vertically to insulate from heat loss out the sides. You can definitely count on the slab that the RMH will be built on as a part of your mass. The slab that my heater is built on maintains about 80+ degrees 12" out from the footprint of the mass. at all times. If budget allows you may also want to consider trenching around the perimeter of the greenhouse and placing foam insulation vertically to help retain the warmth of the dirt mass of the greenhouse footprint. I'm not sure how deep to go but I would think at least 18" or more.

Pablo Kulbaba wrote:This build is superb. Thanks for sharing your path.
Glenn:  Do you have any news to share on the performance and endurance of the blast door?

Hello Pablo, so sorry that I missed seeing your post. I hope you see my reply.

The door is functioning perfectly. I do typically leave it cracked open for the first half of the burn on start-up in the morning or evening. Typically about 1" (~25mm) for the first 15 minutes and then about 1/2" (~12mm) for the next 15 minutes. Keep in mind that I live at 8,000' above sea level (2,450 meters) so my air density is much less than most people and I simply made an educated guess on making my primary air intake larger. Once I have a good amount of coals it is burning fine and I close the door fully.

Note that this is Thomas Rubino's door design for which he offers a build book at Dragon Technology: https://dragontechrmh.com/how-to-build-a-batchbox-door/.

Peter van den Berg wrote:...But there's a steel piece inside the firebox which could give up the ghost somewhere in time.

Thank you for your excellent response Peter and assessment of the different designs. As for the steel piece in the basic (original) design, I assume you are referring to the secondary air intake tube on the firebox floor. I built mine in a manner that will allow me to simply lift it out and replace it. I may have to remove the door but I believe I can just lift it up from the end by the port and gain enough angle to pull it toward the port and out of the hole in the door frame.

Peter van den Berg wrote:...But, given no restrictions, the Shorty core would be my number one favourite...

This is great to hear. One of my good friends is doing a major remodel/expansion on an Earthship house and is going to build an 8" system in the main room. He was planning a basic (original) design but with your comments above I'm sure he will be rethinking that and going shorty. In this case my question would be whether there is no reasonable limit in the height inside the bell from the exit port of the Shorty core to the underside of the roof?

Hello Julian... a few comments to toss out.

My system was built to 6" specs due to having an existing chimney in my shop. The system is a bit undersized for my shop area (42' x 48' x 15') so with my mid winter outdoor temps going below zero F, I will typically run 2 or 3 batches back to back. I like to keep my outer brick skin in the 135 to 160F range in the hottest zone.

I'm running a traditional combustion core with tall riser. The riser is built from insulated fire brick rated at 2,600F and wrapped with superwool. My roof is 12" above the exit of the riser and triple layered; refractory brick, then superwool then clay brick. With the brick layed in dry but tight and the SW layer snugly sitting wall to wall. I have no gas leaks proven by the fact that I keep a CO monitor on the rooftop. This provides me with a clean and relatively easy means to remove the roof in the future for inspection. I used the high temp rated IFB for my riser with the expectation that it will last for many years unaffected by the heat generated.

When I light off the first batch in the morning or evening I will typically keep my door cracked just a touch to aid in getting the load lit but then close the door. There is no further fiddling with air intake, it just runs on its own with a nice low dragon roar. Even with a 3 batch back to back burn I have never seen my exit temperatures exceed 245F (118C). It will usually run in the 225F (107C) range.

If you have no height or weight restriction don't discount the traditional core design. I'm actually curious to know Peter's thoughts on this. Specifically, given no restrictions on size, weight and materials cost/access what would be his order of preference for the combustion core: traditional tall riser, DSR2, DSR3, Shorty core?