I’ve wanted a hot tub for years, but never was drawn to the cost involved or time that must be spent on upkeep. I started this project two summers ago by making a thermal-siphon heating coil that attached to a 6 foot diameter stock tank. Over the years, I improved the connection of the coil to the tub. This summer, I added filtration to the little pool so I don’t have to empty/refill it regularly. Unheated stock tank pools are all the rage now. This post will show how I made it into a filtered pool for about $100

The business end of the hot tub. I’ve heated a tub of water from the low 60 degrees to hot tub temperature in 4 hours. On warmer days it takes less time.

A quick video of the tub in action

The heating coil is attached to two fittings that I can remove with a large wrench. Once all parts are removed, I can roll the drained tub anywhere I want.

The low end pump/filter I selected comes with fittings, but they connect to a proprietary plastic pool. I had to look around to find something that would allow me to connect to the tank. The company that makes the 1.25 inch intake and outtake fittings I found is called Summer Waves (and sometimes Summer Escapes for some reason). The fit to the filter’s hoses can take some jiggling to get air tight, but since both of these fittings come with plugs, I can close the ports and detach the pump without the pool leaking. This is an important feature.

All I needed was the right bit for my drill to cut the holes to mount these. I was careful to clean and paint the cut edges to prevent rust.

These fittings are much cheaper than 3d printed options currently being sold for stock tank pools.

More info on the intake and outtake fittings. I demo how the plugs work.

The Intex 330 Gph Krystal Clear Cartridge Filter Pump keeps the water clear and helps circulate hot water around the pool when heating. The copper heating element does not need a pump (since it is a thermal siphon), but only heats the top of the pool. I use a paddle to circulate water when using unplugged.

This tiny pump provides more than enough filtration for a pool this size if I run it for a few hours each day.

Intex filters are out of stock everywhere since Late Spring 2020. The Platco PIN2H is a great option for a direct replacement. Foam (right item)”Type H” filters can be found for a few dollars on sites like AliExpress, but may take several weeks to arrive from China. The foam filter worked better than I expected and is easy to clean.

These one inch chlorine tablets are great for keeping a small pool free of growth, but watch out around kids. I am careful to use inside a dispenser, and remove when kids are in the pool. My dispenser has a thermometer so I can be sure the pool doesn’t get too hot for a small child.

The oak coals heat great burgers.

Parts list (not complete) :

  1. Stock Tank (Check out your local farm supply store. These are in short supply in 2020)
  2. Tank Fittings for Coil:
  3. Hose Fitting (I use this to fill the coil):
  4. Summer Waves/Escapes Return fitting – 1.25 inch (Shop around to make sure you buy one with the plug and deflector)
  5. Summer Waves/Escapes Replacement Suction Wall Fitting for 600 GPH Filter Pumps (make sure it is for the 1.25 hose)
  6. Basic Intex Pump (Shop around, prices are inflated in 2020)

2 thoughts on “Building a Wood-Fired Hot Tub/Heated Pool with a Cheap Filter

  1. Greetings
    Thank you for sharing your tub project.
    I’d like to know more about the coil.
    * What is the length? In the video you said 50 to 60′.
    * You said 1″ diameter … did you then you reduced it to 3/4″? Why?
    * Where did you purchase the coil and for how much?
    Many thanks in advance for your response.
    Health to you and yours,

    1. The product I bought from Zoro.com was LS10060, a 60ft coil of soft copper. It has increased in price since. I paid ~$270. It comes in a 4-6 foot diameter coil (can’t remember, but it was one pipe width in depth). The coil was shipped inside a large flat box so I had to recoil it carefully by hand into the smaller diameter (but more vertical) coil you see in the photos.

      There is no good reason to reduce to 3/4. Originally I used rubber hoses to connect the tank to the coil and these were standard 3/4 diameter. These failed quickly with the heat so I replaced with 3/4 copper pipe. A wider tubing system will improve your water flow.

      There are some pre-fabricated products that might work too like this small heating element: https://theoriginalnomad.com/products/coil
      Not sure if this would work as well for a tub this big though since the tub they sell is 5 feet in diameter. They report similar heating times to what I have seen for my 6 foot tub with my home made coil.

      Good luck!

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