Question about heat riser insulation

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.

Thanks,

Daniel

Daniel Granovsky : Welcome to permies.com and our sister site richsoil.com This is a good question, and one we have had before, it comes up periodically !

Basically ether Soft Lightweight firebrick, or Hard Dense firebrick can only affect the temperature of the hot exhaust gases from its surface!

The hotter the surface Temps the quicker the Burning Wood Gases reaches the freaky high temps that produce Your rockets great efficiencies.

This surface temp and NOT the Insulation around the firebrick produces these temperatures !

The function of the insulation is to establish a different temperature profile between the Hot rising Gases in the Heat Riser, and the Cooler rapidly falling

hot exhaust gases within the outer shell of the barrel .

This is what creates the push-me, pull-you effect that propels the Hot Exhaust Gases* through Your Thermal Mass Bench!

Hope this was useful and timely ! For the good of the Craft ! Big AL



*30 feet horizontally with your 6'' Rocket and 50 feet horizontally with your 8'' rocket !

Hey Allen,

I appreciate your response. I'm still somewhat confused. The gases in the heat riser are hotter, and they cool down when they come in contact with the barrel. What do you mean by temperature profile?

Thanks,

Daniel

Daniel Granovsky : You Have heard of 'grading on the curve' the curve has a profile. Measuring and plotting the temperature radiated off of the barrel

is one way of measuring the 'Work performed'; in this case the acceleration of the Exhaust gases is the product of the work done , and the Heat or

Temperature profile is the Work done -

Too much Cob on the outside of the barrel or not enough insulation on the Heat Riser could cause the temperatures to equalize and stall the 'pumping effect'

This is only close and as the radiation of that heat off of the barrel is a positive side-effect to us, we don't need to care ! Big AL

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!

Shad : As long as the Heat Riser is a solid monolithic structure without holes in it your build is probably all right !

An important consideration for shifting the Barrel Away from dead center over the Heat Riser is to create an expanded or wider gap at the Transitional Area

Where the rapidly falling Hot Exhaust gases turn 90º to flow horizontally into Your Thermal Mass !

When the Barrel is off-set, pinching the barrel closer to the outside of a Heat Riser You usually create a cooler side to the barrel as the expected lesser flow

of hot exhaust gases reduce radiation off of the barrel at that point Usually - - - Y.M.M.V. !


The cooler side of the barrel should help protect Flammable materials on that side.

Again shifting the barrel in any direction may require a total rebuild of the Rocket Burner base to assure the wider gap in the off-set Barrel Favors the Transition

Area !


I hope this answers the questions you were trying to ask- For the Good of the Craft ! Big Al

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.