Why would you want to use copper? Are you thinking of smaller copper pipes to divide the exhaust (see picture below)? Apart from the toxicity factor, taking smaller pipes for the exhaust is NOT a good idea as it would restrict the flow.
Especially if it is your first RMH, I would suggest you stick to a working plan. Ernie and Erica Wisner have different proven plans (6 inch RMH and 8 inch RMH). There is also a thread on underfloor heating systems with a video. If you really really want to do it anyway, you should do a prototype outside before building everything and realizing that it is not optimal.
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