podcast 413 – The problem with batch box rocket mass heaters – Part 1

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Paul Wheaton, Kirk "Donkey" Mobert, Erica and Ernie Wisner with Fred Tyler and ant Jeremy in attendance discuss Allerton abbey and the batch box heater that is installed onsite. According to Paul, Allerton abbey is now about 96% complete, and the door has finally been re-installed. The new door is approximately 280 lbs. and opens smoothly and has had magnets installed to help keep it closed without a latch. The discussion rapidly turns to how dangerous doors can be and the risk doors can pose to children, especially unattended children.

The participants were onsite at wheaton labs for Natural Building Week priorities - first was to hang the giant door. The second priority was to peel back the WOFTATI roof there was an estimate of about 8-10 tons of dirt on the root, the actual amount of dirt on the roof was probably four times that amount. It would have been almost physically impossible with the people available to remove the dirt from the roof in the amount of time available and still repair the roof so it was determined that power equipment would be needed to remove the dirt from the roof. Using the on-site fourteen ton excavator would not allow the operator to see what they were removing.

After a search it was found that there were no 32-ton excavators were available for rent in the local area. There was a 27-ton excavator available but there was no low-boy trailer available to move it to the site. Because of the difficulty in getting a bigger excavator onsite It was decided to build up a platform to allow the 14 ton excavator to remove the dirt from the roof and allow the operator to see what they were doing. It turned out that a lack of running vehicles large enough to haul the excavator to the site was still an issue so the Natural Building crew decided to attempt to 'fix' the airflow issues by repairing the roof from the front. Donkey had a FLER camera so that was used to attempt to document the air flow and temperature and then the camera battery died.

While the camera battery was recharging the intrepid crew decided to start a smoky fire in a metal can to trace airflow by determining where the smoke was flowing out of the WOFATI roof. The general consensus was that Donkey did almost burn the structure down during the attempt so it was determined while it was a lot of fun it was not productive so Fred went to town and rented a 2-ton excavator (which the crew found extremely cute).

Turns out Paul had more excavator driver time than all the rest of the participants combined, so Paul became the excavator operator. Paul was able to remove probably 8-10 tons of earth the first day. WOFATI's are designed to have about 3 feet of dirt covering, the current layer on Allerton abbey was approximately 4-5 feet.

After discussing the current state of Allerton abbey the conversation then moves on to a discussion of batch box rocket mass heaters with Paul's take on the issues (which is mostly negative) and Ernie and Donkey defending. Paul's number one problem is batch box being built without doors and then calling it a Rocket Mass Heater and once it's lit without a door the unit smokes and then the people trying to light it say RMH's don't work. Donkey was willing to admit that batch box rocket heaters are finicky so if you have never built a RMH or any other type of Rocket Stove then a batch box is NOT the project to start with. They have to be built correctly or they will probably not work at all or if they do they will smoke.

Paul feels that there is a steep learning curve on how to light a batch box heater and Donkey points out that on the recent Rocket Stove tour a participant was able to start the batch box heater in the Red Cabin with about two minutes of instruction and the only time it smoked was when Donkey was "messing with it". Erica's concern with batch boxes is that people like what they are used to so they are attracted to the stove because it looks and feels more like a stove they are familiar with and they light it incorrectly because they light them like they are used to lighting a wood stove. So because it looks and feels familiar people don't follow the instructions and use them correctly. Donkey points out that Trucks that run diesel look just like trucks that run gas but people can learn to operate them properly.

It was then brought up that Peter van de Berg created the 8" batch box that is currently in the shop and Paul's contention is when Peter is running it's fantastic but if anyone else attempts to run it they don't have as much success, especially when starting the batch box. Ernie takes exception to that and states he has not experienced any issues getting the batch box in the shop to light.

Erica then spends some time describing the "slow" start method that is ideal to start any stove. First step is confirming the chimney is primed, using either a small amount of newspaper or a candle. Once the chimney is drafting correctly, then start the stove and check the output from the chimney. If the chimney output is clear or white then you're good, if it's any other color you may have to adjust what is happening in your stove. Paul points out you can use a small propane torch to heat the chimney prior to lighting the fire in order to heat the riser.

Paul is mostly incredulous that everyone in the room uses matchstick kindling to start their fires. Paul also points out that with batch box stoves when the primary wood is loaded into the batch box Rocket Mass Heater you have to leave at least a 1 inch gap in front of the "port" inside the run chamber. Erica remembers Peter mentioning that there should be at least a 2" gap all around the wood being burned. Discussion turns to how the Wisner's typically operate their Rocket Mass Heater at their house. Usually for Erica and Ernie they tend to only add more wood every 90 minutes to 2 hours. At Paul's house he tends to add wood every 30 minutes or so for a few hours and then he's done for a couple of days. At Donkey's house he "Donkey-fied" a Tim Barker non-Boom Squish water heater that takes about 60 minutes to get the water to shower temperature. Donkey's original great-grandfather prototype of the batch box is not as efficient so he runs about two burns of two hours each to get that stove to run through the night.

Discussion turns into what are actual Rocket Mass Heater vs. the Freak Show of Flaming Death stoves that are created by the "instant experts" on YouTube. Beware of posting without follow-up videos of how the units operate.

Credit: Eric Tolbert

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Relevant Threads

Wheaton Labs
WOFATI (Woodland Oehler Freaky-Cheap Annual Thermalized Inertia)
Natural Building Project
Batch Box Rocket Mass Heaters
Boom Squish Rocket Water Heaters
Donkey Forum


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