10' x 30" Intex Easy Set Pool for about $50. I set it up on our concrete patio. It was great fun for the kids, but 2 years ago we put a roof over the patio. That means the pool gets no sun until after 3pm, resulting in water temperature never higher than 72 degrees during a heat wave. So this year, armed only with determination, a roll of black irrigation pipe, some 1x2x8 lumber, screws and various plumbing parts laying around the garage, and a Water Pump from Amazon or Harbor Freight, I created a basic solar pool heater for about $50. Of course, I had a lot of stuff "laying around", mainly a coil of 1/2 inch black irrigation hose, but you can buy 500 feet of that for about $35-$50 at your local home store, keeping your total still well under $100. The collector sits on the patio roof and the pump is located next to the pool filter. It's not pretty, it's functional and to the point and built in one afternoon. The project will require a little creativity in making it all fit together. I'm not going to supply step by step instructions, sizes, or a parts list. My goal here is to show you that it can be done, how I did it, and encourage you to use your brain to create something useful, instead of killing it with beer.
Below is picture of the completed solar collector. It is about four feet in diameter, built from 3 lengths of 1x2x8 lumber, some deck screws, and about 200 feet of irrigation tube.
Next, you'll need a way to get water out of the pool and to the pump. The best place to get water would be the bottom of the pool, where the coldest water is located. Fortunately, the Intex pool has a convenient threaded drain plug at the bottom. Unfortunately, it does not easily mate to any pipe size known to man. You would think it would mate to a garden hose, but no. My solution was to force a right angle fitting commonly used in irrigation (for sprinkler "funny pipe") into the drain plug, snug it with a hose clamp, and connect to irrigation tube as seen below.
The hose runs around the bottom of the pool to the pump:
http://www.harborfreight.com/34-horsepower-clear-water-pump-69297.html because I wanted to complete the project and didn't want to wait for delivery of a less expensive 1/2 horsepower pump on Amazon. Water from the pool enters the pump through the black hose then is pushed about 15 feet to the roof through the green garden hose. I chose a garden hose to lift to the roof because it was convenient, and allows me to easily add a "T" on the roof and a second collector. The collector drains down into the pool via a through-wall garden pond connector I found at Lowe's, meant for use on a plastic garden pond tub as a water inlet. This will require you to cut a hole in your pool about 6 inches below the water line. If you don't want to cut your pool liner, then just run the outlet hose into the pool, but watch it - careless kids can knock it out of the pool, draining it of all your nice warm water onto the ground in no time - even with the pump off. (One word: SIPHON.) Now, if I could find a good and inexpensive 12 volt pump, I could run it on solar power... But the price of solar panels would kind of ruin the whole "cheap solar pool heater" thing.
Results have been pretty good. Today was a very hot day, reaching 100 degrees. Water in the pool right now at 7 pm, June 29 is 84 degrees, and the water coming out of the collector is 86. Remember, this is a pool that's in the shade and never got much warmer than 70 before. One word of advice is to keep the pool covered when not in use to minimize heat loss. You're probably wondering why I didn't just use the pool filter as my pump. Well, first; the hose size is difficult to adapt to 1/2 inch irrigation hose, and second; it wasn't designed to pump water 15 feet to the roof. Now, if the solar collector was at ground level, that would work and make it worth my effort to find a way to adapt the pump hose to the irrigation pipe.
What's next? Definitely a timer so I don't have to remember to turn the pump on and off every day.