Question:
Wire it up under the tub so the heat is as close as possible.
>> About the middle of the tub, a little closer to the drain is the
>> best place for the heating element. Some tubs have plastic
>> drain pipe, and you don't want to get that too hot. When
>> you sit in the tub the heat should be under your knees, in
>> the middle, not your feet or butt at the ends of the tub. You
>> need to put the tub up on legs, blocks, bricks, firewood, etc.
>> so you can get the heater underneath.
>What I had in mind was pumping the water threw a vessel of some sort
>which was exterior to the tub and placing the heater under it. That way,
>you could just reach over and adjust the heat provided the vessel was
>at arm's length.
Answer:
You could do that, but it's SO much simpler and
easier to just put the fire UNDER the hot tub.
I've been in a large metal stock tank [maybe 8 feet across]
which was heated by a wood fire in a hole dug under the tub.
It was wonderful, but it took many hours to heat up.
I've also been in a concrete-on-wire-mesh tub
which was heated with an internal drip-fed oil
burner: the fire box was immersed IN the water.
A simple oil fire in a metal box. It worked fine!
Of course, you could make variations.
You could have a concrete [or plywood]
tank with a bathtub in the center, making
a deep water soaking spot for heating,
and shallow lounging around the edge.
You could fill only the tub, shallow or deep,
or add enough water to flood the shallows.
