I am building the RMH heater manifold out of brick. My first question is, does it matter what kind of brick? I have posted a few pictures of the miscellaneous bricks I have on hand.
Second question...The gap between the heat riser within in the manifold and the heat exchange (clean out pipe), can it be more than 3 inches?
Third question...My gap above the heat riser and the drum is more than 2 inches...is this ok? I think its closer to three inches.
Fourth question...can I use the clay slip or is it better to use a regular brick mortar when building the manifold?
A few notes on my pictures...
I just did a mock up of how many bricks I need, the general size, layout etc...I know that I have to make a few adjustments so the drum can sit directly on the manifold and fill in the bigger gaps with half size bricks.
The bricks at the top of the rise are temporary. I plan to add another course of fire brick to make it ab 51 inches tall instead of the 47 inches, which is too short. The other small bricks are just the spacers to measure the 2 inch necessary gap I need between the top of the drum and the riser.
Type of brick doesn't matter for the manifold, use what you have.
The gap between the manifold and heat exchange should be more than three inches. This is where quite a few people run into issues as the gas tries to squeeze itself into the heat exchange piping. I would put it further away and use a transition from at least 10" pipe to the 8" pipe if you are doing an 8" system. You want to funnel the gas so that it is a smooth transition and doesn't get hung up on edges
The gap between riser and barrel can be anything you want as long as you don't exceed the maximum cubic feet allowed in the system. I tell people to move it closer to three inches so they don't have any problems blocking the gases.
You can use slip for the manifold. I still wouldn't use any mortar that is cement based here. Just use cob or slip for stacking the bricks.
Also, it looks like you are using splits for your firebrick. You will want additional insulation around the riser, use fireblanket or perlite.