MIT has recently published the paper "Up in Smoke" by Rema Hanna, Ester Duflo and Michael Greenston which studies the a randomized installation of the Chullah in India. The study participants received skilled help in installing the Chullah stove and minimal help in maintaining them.

The Study is published Here:
http://www.charcoalproject.org/2012/04/news-clean-cookstoves-draw-suppor...
this link is shorter http://bit.ly/IfI0ZL

It is unsurprising that the study found that stove failures and poor maintenance of the chimneys lead to a failures of the stoves over time. This stove also features a 90 degree turn in the chimney, and a combustion chamber design that is prone to maintenance problems, and those problems have been discussed before. It's also worth noting that most households in this study are starting with a mud stove and not a traditional three stone fire. Often we talk about the emissions from a 3 stone fire versus an improved stove, but in some areas we may now be looking at improving an existing stove.M2k Tekno