posted 10 years ago
1) The barrel can be dressed up or disguised in a number of ways; there is a thread here on that topic. It is even possible to substitute a masonry bell for the barrel, which will change the characteristics of operation somewhat. If you have masonry surrounding your existing insert, that would be a good backdrop and possibly part of the mass depending on details of your layout.
2) A good chimney is an asset, and there is no real limit on its height aside from throttling a very tall one if the draft is too strong. There are standard lengths of horizontal ducting in the mass, depending on the diameter of your system and the number of elbows. The book Rocket Mass Heaters by Evans and Jackson has most of the information you will need to adapt to any location.
3) With the length of heat exchange ducting, the low final exit temperatures and the initial complete combustion, you should have no exterior fire concerns, and the standard J-tube design has the wood feed such that it is very hard for fire, burning wood or sparks to fall out. A J-tube needs to be fed on a regular basis so you cannot go away and leave it for a long time. For even more safety, you can use a batch box design, which has a closed firebox, loaded once and burning steadily for an hour.