The Rocket Mass Heater Builder's Guide by Erica Wisner

Author:Erica Wisner
Language: eng
Format: epub, pdf
ISBN: 9781550926163
Publisher: New Society Publishers
Published: 2016-03-12T16:00:00+00:00

Insulation and Expansion Joints

The heat riser MUST be insulated for the rocket mass heater to function. In order for the thermosiphon firebox to work, the inside of the heat riser must remain very hot, and the outside (between heat riser and barrel) must remain cooler in order to flow downward. If temperatures in the barrel equalize (due to leaks, poor insulation of the heat riser, or inappropriate insulation outside the barrel), the draft stagnates and the system chokes and smokes.

Stabilized perlite or refractory wool insulation around the firebox doubles as an expansion joint, creating a “floating firebox” that can expand without cracking the casing. This high-temperature area is the most critical for expansion; the next most critical is the seal around the barrel.

Earthen masonry can be hot-fired while still flexible, to create the necessary expansion tolerance without cracking. (If cracks occur, earthen masonry is easy to patch, and can be hot-patched to spread the crack at its widest extent.)

For a project that can’t be hot-fired during construction — for any reason — consider adding an airtight expansion joint around the barrel base.

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