Why Wheaton Labs Doesn't Build 4-Inch Rocket Mass Heaters Anymore

This excerpt from the Free Heat movie discusses the various reasons why Paul and crew aren't planning to implement 4-inch Rocket Mass Heaters anymore.  

When compared to an optimized 6-inch system, a comparable 4-inch system:
- Has more smoke-back
- Emits more smoke in exhaust
- Is more finicky to operate

So although it is possible, in most cases it's simply not worth the trade-off.

And you can't reach a big hand down into it.  And cleaning ash out is trickier.