I built a mock-up of my rocket mass heater core outside (no mortar, no barrel). Both the firebox and the heat riser are made out of dense firebrick. I noticed that it took quite a while before the outside of the heat riser got so hot that I couldn't touch it. At least a half hour. My question is: if the outside of the heat riser takes so long to heat up (due to the thermal mass of the bricks), what role does insulation play in keeping the interior of the heat riser hot? It takes so long for the outside of the brick to get hot enough to the point where insulation is needed that I might already by approaching the end of my burn (I live in a mild climate). Is the insulation needed to keep the hot gases from heating the outside of the brick? Why? I have the feeling that I'm missing something here, and I would greatly appreciate advice from some of the old-timers.
Posting because my question is very relevant...
I just finished insulating my heat riser with ceramic fiber board. The dimensions were luckily quite close to my dense firebrick riser but I stabilized the boards together by pushing steel wire through them. There are some air gaps in the corners some possibly as big as a half inch, and as my riser isn't perfect there's a small air gap between the boards and brick. I know this is not optimal but will it still make a huge difference? Also I offset my barrel forward (so the chimney is more towards the back of the barrel) and removed the insulation on the back side in thought that the heat in the back but cooler air in the front (where my transition is) will help push the air towards to cooler part. Would this help me or hurt me and why or why not? I already set the barrel and forgot to take pick <.> Thanks!
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
You won't have cooler air in front (where the gap is wider); as the space is larger the gases will flow more easily there and it will be hotter. Taking the insulation off one side of the riser will not affect this in any meaningful way, but it may make the riser lose more heat and become cooler and less efficient. Keep the whole riser insulated!
As Al said, it is good for there to be ample free space where the gases collect to enter the horizontal ducting. A sharp bend or constriction there can cause a system to be choked.