This is the Hot Tub / Pool I made for my family to use. This project was inspired by the great plastic pool shortage this summer. In late June I called every Walmart and they said they had no pools and no air conditioners. eBay had price-gougers with 5x markup. Think back to the heat bubble in July. So I had this idea to make my own dual-use hot tub and pool. I know MMM mentioned the $20/swim pool with the high initial cost, low use rate all counted against the future value of money. So keeping that in mind I designed my pool to be low cost, usable year round, and when I’m done with it, the parts can be re-purposed. Also, since I keep about 42” of water in it its deep enough to actually swim in, since its 12’ corner to corner, like a swim spa.
The pool itself was done in late July, so we did enjoy it for a time with the 90+ degree weather too cool off in, but now into fall it has transitioned into its second use, A hot tub. There is an 80K BTU natural gas fuel heater ($100) , a 7GPM stainless pump ($63), and a stainless plate/plate heat exchanger ($10 used) that takes antifreeze from the heater to an insulated box that houses the 1000GPH pump and sand filter ($55). The wood is almost all pressure treated so will resist rot from moisture. The screws are all stainless steel for the same reason. I learned how to use the KREG pocket hole jig to attach the structure together from the outside-in. There is not one fastener head that touches water. The basic design is based on a sheet of 4x8 plywood. The sides are one sheet each and the bottom is two sheets butted up. I sealed the edges with marine below-the-waterline caulking. The knots of the plywood were sealed up with 2-part epoxy and the water touching surfaces are painted in oil-based boat paint. If I could do it again, I would have saved time and money and painted it in white Sherwin Williams 2-part epoxy paint, it would have been better for many reasons.
The structure was engineered to be both economical and have a large safety factor. Dusting off my statics books, I used hydrostatic equations to determine loads, I found that 2x4 studs spaced 16” OC will resist the many thousands of pounds force 4’ of water will exert on the vertical surfaces. I installed 2” foam insulation between the joists/studs on the bottom and sides adding up to a R10 value. It will loose only $0.20 of heat on the coldest days of winter. The rim of the pool is 2x8 joists turned sideways to resist the bowing force of the water, with corner braces that double as seats. The outsides are skinned in the cheapest plywood I could buy, and painted with water-resisting oil paint. I also made an elevated swim deck that encloses the mechanicals and a dorm-sized beverage fridge. I hung outdoor CAFÉ lights from the Unistrut metal racking that supports the solar panel roof of this screened-in space. I made a separate post about all that a year ago.
To make it 21st century ready it had to be IOT, so I got a clever device from SONOFF, it’s a 16 amp power relay that is married up to a RTD thermometer, $18 at Walmart.com. this deal here measures the water temperature and turns the pump and heater on and off based on rules that I make in my smart phone. I can turn it on manually, and also activate the automatic control with has ON/OFF temperatures.
Overall, the $1600 I spent was about 1/3-1/2 what a smaller commercial hot tub would have cost. Its run rate cost will be much less than an electric hot tub since my marginal gas usage is contracted at $.30/CCF/100KBTU and I only need ~60Kbtu/day to keep it 100F on 0F days. Also, the electric to run the pumps is coming from a solar powered circuit so it runs without grid electrical power.
