6" rocket mass heater radiant line depth

I have been testing my new 6" rocket mass heater with bench, my radiant lines are running 180 to 210 deg on hot side 150 to 190 on low side after it heats up. I have not filled bench yet and I am wondering how deep to bury them. I saw that for a 8" heater the depth was 5" -6 " on the hot side and 4" to 5" on the low side. I have no idea if the temps for exhaust are the same as a 6" heater. I will be using pea gravel for fill above pipe maybe a  grouted paver top, bench has cinder block frame . I love my heater and the heat it produces. I  just want to warm my buns not fry them.lol  Any guidance would be appreciated thanks.

Hi Ken;
The numbers you found are minimum cob thickness for a bench.
I see that you are planning to use pea gravel as your fill. Air spaces between the gavel will allow the heat to move thru quickly.
I would suggest a minimum of 1' up to 2' over your pipes, then pavers to sit on. . If its still to hot then add extra side walls and more pea gravel.  If it is not warm enough then remove walls till comfy.  
My 8" cob bench has 16-20" of rock and cob over the pipes , with insulating cob underneath. It works awesome!  

Just curious, why pea gravel ?  It is a poor substitute for a solid rock and cob mass, generally it is used on a short term temporary mass.
You are building a bench with solid retaining walls rather than a sculpted cob bench. This makes things easy.
If your reason, is a lack of suitable clay. Be aware that a bench is composed mainly large rock /iron  with only enough clay to avoid air gaps.   In leu of clay, plain old dirt can be used to fill between the rocks.
You might be much happier with a full bench that holds heat longer rather than a pea gravel bench that cools off quicker.  
 

Thanks for the reply Thomas, I had a feeling I was about to screw up just needed a second opinion. I was wanting to hold a bench height of 18" due to physical reasons. I guess I could build a step in front of bench and take the bench up higher to keep lines  off the floor. I will cob the heater and would possibly like to cob bench internally and top, I wonder if it could work if I put expansion joints around the inside of the cinder block. I wanted to use cinder block because it is easier and I am a 1/2 man army whittling away at a much larger project than normal  one day at a time.

Hey Ken;
Around your mass the cinderblock is fine, no expansion joint needed  (do fill it with sand in the holes).   If you try to put them too near your core they will break apart from the heat.


Yes, about your floor...  the pipes MUST be insulated from the floor.  Minimum 4" of straw cob on dirt .    You can dig down and insulate with EPS solid foam board, if you wanted to  keep a low seating height.


Tell us more about your building plans.  Where you are placing this rmh? what you are hoping to heat with it? What size and style rmh did you have in mind ? Are you on a tight budget? Where are you located?

Do you have a copy of The Rocket Mass Heater Builders Guide yet ?