Kevin Woods

Lol!! All I have is a conventional modern metal bathtub but I think your idea would work with it as well. In fact it would have better access to clean out T 's using your suggestion.

And no it would not require damaging the tub at all. Using its original drain to empty it and a platform to support the J pipe weight so the tub could be removed and repaired/resealed at any time.

Since all I have is a smart phone I'll do my best to describe my idea.
The J pipe would be located somewhere along the first 1/3 of the tub. This is so the exhaust pipe can be routed out next to the 55 gal drum using its heat to help draft. The pipe would lead out of the drum and loop through the tub filled with water.
It would be steam type pipe with a T for clean out. Yes, a clean out would require the water to be drained. Having a natural spring and planning to have a greenhouse will mean I'm not wasting water. All opinions are welcome. Have a blessed weekend Permies!

I recently read that really the only thing better for the mass in a RMH than cob is water. My idea is to use "8 metal piping routed through an old bathtub. It could be plumbed to a shower, laundry station, etc. Yes it would be larger than most would like but if it works I won't mind a surface large enough to sleep on!