Do you like keeping warm? Do you like doing so in hot tubs? If so; this might be the Instructable for you!

I will detail how my friends and I designed and built a wood-fired hillbilly hot tub that reached 112 degrees (F) within an afternoon using items from my local Home Depot!

Step 1: Building the Frame

To make this tub easy to set up and tear down, we decided to use a combination of old railroad ties and fallen logs that I had laying around for the frame. It is important to grade the ground underneath and check for anything sharp that could puncture the tarp.

We simply used twine to lash the logs together at the corners to prevent them from moving too much.

We placed bricks and other stones around the bottom layer to prevent the tarp from bulging out into the gaps left by the log-cabin style construction.

Step 2: Building the Heating Coils

This was the most expensive part of the hottub: using 35 feet of 1/2" diameter copper tubing to make the coils. Onto each end of the coil, I soldered a 2 foot straight copper extension to get out away from the fire, and to have a cooler area to attach the vinyl tubing for the pump.

The pump is a submersible 250 gph pump that I found at Home Depot. Do note that a pump such as this is much less expensive from Amazon, so I would recommend getting one there instead.

If you are iffy on soldering the copper together, remind yourself how with this instructable:

https://www.instructables.com/id/How-to-Sweat-solder-Copper-Pipe/

Step 3: Heating the Tub

This step can take awhile. You'll want to fill the tarp with water: we have done it with both pond water and city water from a garden hose, it really doesn't matter: you won't notice the difference, I promise.

I recommend using bricks or some other weight to hold the tarp up at first: we tied it using twine and the weight of the water ended up ripping the eyelets out of the tarp. Using weights allowed the tarp to be adjusted as it is filled. Once the water is in, it shouldn't move around too much.

Once it's full, start a large fire and the pump. Depending on how hot your fire is, this can take different amounts of time: in my experience, around 3-4 hours to achieve hot tub status.

One thing we did to speed it up was attach a steel pipe to the end of a vacuum on "blow" and stick it in the fire. The oxygen being shot directly into the fire really got it cooking.

One suggestion I would make would be to place the hot tub a bit further from the fire, and account for the wind direction that day. Depending on wind, you can end up with a lot of smoke and ash blowing right towards the tub, making for a poor experience. By making the vinyl hoses longer, the tub could also be moved further away from the fire.

NOTE: If the copper coils are in the fire, you need to have the pump running and water going through it, or else your solder will melt.

Step 4: Happy Hot Tubbing!

Good luck with your hot tubbing adventure! Also, Here is a parts list compiled:
(I spent a total of about $250 on these parts, but you could probably spend a lot less if you were thrifty about it)

30-40 feet 1/2" diameter copper coil

2 foot straight copper tubes (1/2" diameter) X2

2X 1/2" copper couplings

10 feet of braided vinyl

250 GPH submersible fountain pump

2X hose clamps (3/4")

Acid core soldier

Flux

Blowtorch

Twine

Bricks/stones

Logs, railroad ties, or a big hole