RMH and copper exhaust.. is it possible

New to this but looking at building one for a winter cabin 24x8 and wondering if its possible to use copper tubing as the exhaust pipe to disperse the heat along the floor and under the elevated beds.

NO! Danger! Danger!

Sorry to yell, but copper oxidizes at a very low temperature, creating a known carcinogenic compound.

Since your cabin is only 8' wide [in the southern states, those are called a "shotgun shack" ], how about building a narrow mass bench along one side?

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.