Heat Risers...

Hi All,

Just finished reading Ianto Evans' & Leslie Jackson's book on rocket mass heaters and one question came up about the heat risers.
There's instructions on how to build a brick RMH including a brick riser and says that it must be insulated but doesn't say why...
So, why does a brick riser need to be insulated (I would have thought that the bricks themselves would be sufficient)?

The bricks are not insulative enough. You need HIGH temps in the riser to get the clean burn/re-burn. You also need a large temperature difference between the riser and barrel to "pump" the exhaust through the heat exchanger/bench.