Dale's Oil Tank Hot Tub

This topic concerns using a standard oval oil tank laid on it's side as a hot tub and thermal flywheel within a greenhouse/spa. I have cut many of these up with a grinder in the past. The going rate for disposal of tanks is about $125.00 , so this project will start out in the black.

The tank needs to be raised a comfortable distance off of the floor. A cob platform or one made with used concrete blocks will bear the rather heavy weight. (see photos). Most of the space under the tank will be filled with subsoil, leaving 8 inches or so for the exhaust from a fire brick J-tube. The exhaust enters the big flue under the tank and proceeds toward the back and then returns to the exit at the same end where it came in. From there, the exhaust enters a standard stove pipe and exits the greenhouse at about 7 ft. above the floor.

The small portion of the support walls that are exposed to flame will be made of firebrick. The fill will be capped with a layer of clay or firebrick. Flue gasses will travel about 10 feet while beneath the tank. The tank will be heated directly when the fire is going. Later, heat stored in the brick will continue to heat the tank. Trial and error will determine firing times.

In order to accommodate a 4 ft. heat riser, the first 2 ft. of the J-tube is recessed into the dirt floor. This area will be covered with flag stone, for a fire pit look.

After the tank is cut, the oil will be cleaned up and a tiger torch used to get the last of it. A suitable paint will be applied to the interior of the tank. The Sharp metal edges will be dulled and cedar gunnels applied so that the edge is covered. The tank will be set on just enough slope so that it can be drained through a pipe attached at the low end.

After everything is working, the support walls will have soil heaped around them and landscaping will cover the slopes. The burn pit is to function as a seating area. The wood feed will be set up to burn fairly long wood. This will allow for less frequent fueling. I'm not silly enough to keep combustable materials near the feed tube. Soil, rock and glass don't burn. If the tank ever burns out, water will kill the fire. Complete draining onto the dirt floor, would do no harm.

Legionella and other unwanted bacteria will be controlled through temperature and regular draining.

Let's see how much wood might be needed. I plan to load 200 gallons of water into the tub. Assume a starting temperature of 50 F and we want to raise it to 150 F, a 100 degree rise. A gallon of fresh water weighs 8.34 pounds. 200x8.34= 1668 The water weighs 1668 pounds.

A pound of decent quality air dried firewood contains roughly 7000 BTU of energy.

One Btu is the heat required to raise the temperature of one pound of water one degree Fahrenheit.

7000 divided by 1668=4.12 Let's call it 4 -------- There is enough energy in a pound of wood to raise the temperature of the water by 4 F at 100% efficiency. At 50% efficiency, which is more likely, we would get a 2 degree rise in temperature. Therefore we need to burn 50 pounds of wood to raise the temperature to 150F. This would be enough heat to hold a small greenhouse in my climate at a comfortable temperature for several days.

Suppose the tub is required for bathing and 100F is desired. We need to raise the temperature by 50F. --- 25 pounds of fuel are required.
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I've seen some pretty wild numbers tossed around. There are those who figure that 5 pounds might do it, and others who figure it would take a couple hundred pounds. There will be some heating of the firebrick in the burn tube, and some will be absorbed by the cob. Some heat will radiate from the water during heating (insulative blanket). The actual usage could be a little more or less.

Some of the heat lost to the brick will be recoverable. After the burn is done and the feed tube is sealed, the coldest portion of the environment will be the bottom of the tank. The firebrick will be quite hot after a burn like that and they will heat the air that is prevented from flowing up the chimney. Because of this, the water in the tank will continue to be heated after the fire is out. It will take time to determine when to stop firing. A bucket of cold water should always be on hand, within easy reach of bathers, so that the tub can be cooled if it gets too hot.

I've been rethinking some aspects of this one. If it can be obtained cheaply, I would prefer to use a 500 gallon propane tank. It might hold 300 gallons once it is sliced. They are cleaner and a simple front to back flue under the tank would work well for something long compared to width.

Heat with wood gas --- I want to make some charcoal and it would be nice to be able to load up all of the fuel for a burn at once. This could be accomplished by having a charcoal retort feed wood gas into the J-tube. The start up process would involve --- 1. Starting a rocket feed for the retort.
2. Wait for the water to boil off and wood gas to begin flowing.
3. Light the gas and check on things a few times. Adjust airflow, feed the retort rocket etc.

With practice, it should be possible to make batches of charcoal that produce the right amount of gas to heat the tub to the desired temperature.