Garden Hot Tub
The South-facing black aluminum roof is angled to give maximum exposure to the sun. It has a roll of black plastic water pipe which is supplied by my well pump. The flow must be regulated to generate water that is about 110 degrees on sunny summer days.

I made the regulator by punching the very tip of an ice pick/awl into an aluminum disc cut to fit inside the female end of a garden hose. I found it better to have a slow trickle of very hot water and add cold if necessary, than to have a faster flow and wish it was hotter. The actual output is about 1 Quart a minute/1 Gallon in 4 minutes/15 Gallons per hour. My most productive hours are from 11:00 to 2:00.

This view shows the three Triple-Track aluminum Storm Windows on the back side.
The 3-place seat is made like a garden bench with storage underneath, and then the seat and floor are covered with a piece of carpeting that is only stapled to the back of the seat behind the head rest. This allows the carpet on the floor to be folded back to keep it dry, or rolled onto the seat for ventilation.
Here the capet is rolled up and held back to show the bench seat and the flooring  which is 1/2" spaced to allow splash drainage and ventilation.

  The pressure-treated 2" X 8" floor joists are covered with window screen before the pressure-treated 1" X 6" flooring was put down with Galvanized screws. This keeps out the crawling critters.

"Come on in, the water's fine." This is where I invariably fall asleep on a HOT summer afternoon. The yellow pool ring has just enough air to support a head when the ring is folded in half. The material on the walls is Masonite. I think they call it Marlite. It has a smooth face that is impervious to water.

  When you fill the roof generator, make sure there is a  full stream of water coming from the opposite end with no air bubbles. If air is allowed to accumulate, it prevents water from touching the hot plastic and will decrease the heat transfer. I drain mine in the winter months. The plastic pipe may not burst when frozen, but the fittings might.

I may convert the system to a heat exchanger. Then the pipes would be filled with AntiFreeze, avoiding the potential for frozen pipes and allowing me to pressurize it to avoid air accumulation.

When I get the outdoor furnace hooked up,
guess where you will find me on a chilly winter afternoon  : )