A batch box style rocket mass heater, unlike a J-tube style, requires a door. But most people don't have the skills or equipment to make one. Paul and Jen discuss the experimental low-tech casserole-lid door which uses cob to make a perfect fit, and how it fared in real-world use. So far at the lab, around ten casserole-lid doors have failed, in various interesting ways. They discuss the reasons for failure, and why Paul is no longer recommending their use despite their being low-tech and cheap and easy to replace, especially if purchased from goodwill.
Paul considers that this is but one reason that batch boxes are inferior to J-tubes, and refers to the podcasts he made with Kirk “Donkey” Mobert . He also mentions that Matt Walker uses a pop-riveted metal door on his designs which more people would be able to construct. Matt also told him that all types of glass will eventually fail with that kind of use.
Jen points out that many of the failures could be down to running the rocket stove in Allerton Abbey 24/7 for around ten days, which is far longer than a RMH is usually run for, as part of the thermal inertia testing for the WOFATI design. It may be that with more normal usage an easily replaceable, and cheap, casserole lid may still be the right solution. Currently they are testing how well a cast iron skillet functions as a door and are brainstorming further solutions.
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