You don't say what style of RMH you're building, but as a J-tube only uses a simple plate for the P-channel, I presume you are doing a batch box. You can not practically run the P-channel through the top of the firebox, as it would get too hot. It is generally run on top of the firebox roof, to pick up some preheating without damage to the tube, then a right angle to drop straight down in front of the port between firebox and heat riser.
batchrocket.eu - core designs - p-channel.
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