[Cob] cob/earth oven also heating hot tub intake?

[KC Chandler]

"your hottub is a (very large) safety valve" is true unless the 
overheat mechanism isolates the "fire loop" to stop the hottub heating process.
With all due respect... we're over-thinking it here. You plumb it like any water heater, pulling the cold water off the bottom, through the coil in the stove. Heat rises. Of course it will create its own syphon. It can't do anything else. 

 iron pipe is a good solution for a hot tub but plumbing can be tricky 
since the fitting connections (t-tape, pipe dope ect) usually leak when 
exposed to extreme heat and especially open flame.

 The fittings don't go into the stove; only the coils do. Your fittings are outside of your heat source. 
Your problem won't be in overheating the hottub (let the fire go out) ...it's not likely that you can heat it faster than the water in the tub can cool, even if a small tub, even if it's covered. 
But, I'd be interested in seeing someone try.