I stumbled upon this thread while searching another topic. I love hot tubs!
At any rate, I don't know the status of this project as I don't visit the forums much any more but thought I'd share my experiences in case the project is still being considered....or for anyone interested. I have a 5'x3' cedar tub (~300 gallons) with the larger Snorkel underwater stove. Starting with a clean firebox (since ashes insulate), and filling it with very dry wrist to forearm sized pine....and keeping it fed, I can raise the water temp by 45° every hour. It takes a wheel barrow of wood to take the water from 55° to ~104°.....which, I'm guessing is ~40 pounds of wood.
The stove smokes until the fire is going good then, when it is kept going good, there is very little to no detectable smoke. The stove smokes terribly with less than totally dry wood...or with bigger pieces of wood...or hardwood that burns more slowly. Hence my reason for preferring pine or similarly soft wood. It burns fast
My tub has a cedar cover but it is not insulated nor is the tub itself. Even in freezing temps, the water loses less then 10° overnight. I can soak Fri night then, with only a small fire with several pieces of wood, can get it back up to temp Sat morning.
Re sanitation: I use 30% hydrogen peroxide. Took me a while to figure it out....but, I just add ~1/2 cup after each soak and stir. That gives me 5 months before the water/wood gets too slimy. We (family of 3) shower and thoroughly wash before each soak. When it comes time to clean the tub, I just drain the water on our landscaping. Plants thrive on it.
This tub has no jets, pumps, or filters. I'd really like to add a PV powered direct pump/filter system to remove the varying things that find their way in. I've just been using a head bug net. Not as slick as a solar filter would be but it works ok.
Cheers to Hot Water!!
Best with the project.