Home made Hot tub from barrel stove

[Cord]

Thermosiphon is what you're thinking of. Just remember, with a thermosiphon you will need to oversize the lines. At least double them. At least.

They're expensive, but you might be interested in a Chofu. Island Hot Tub Company/Chofu Heaters - Your Source for Chofu Hot Tub Heaters and Accessories

 

[boutym]

These are actually pretty cool tubs now that I think about it. Certainly more inexpensive than commercial units and around here you could probably get away with not getting a permit since a stock tub is just a stock tub. Nobody says you can't sit in a stock tub now and then if you like.