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You'll need dark colored hose pipe - black rubber hose is ideal and is relatively inexpensive. Alternately, use black garden irrigation pipe. It is thin-walled to allow quick heat transfer, inexpensive, but will kink easily. It is readily found in most home supply stores.
The more hose pipe you use, the more the water will be heated as it flows through to the pool. How much you will need will depend on how much you want to heat the pool, the size and regular temperature of the pool etc.
Consider setting many pipes in parallel to increase solar gain. If you use one long hose, the water temperature near the end may be to high to allow for further collection, and may in less efficient in gaining heat. The greater the difference in temperature between the water and the potential gain, the more efficient the system.
Most in-ground pools will have a hose bib that may be used to drain water from the pool. If you have something like this you can use it instead to send water through the hose and back to the pool. If not you will need a small pump, which will pump the water at slow speeds.
You can start with 50 or 100 feet of hose and add more if necessary.
Attach the hose so it circulates the water from your swimming pool through the pipe.
Let the water circulate for a few minutes so that the water coming out is not what was left in the pipe (which may already be quite hot).
To determine how much pipe you need or what kind of temperature rise you will get you need to start taking some measurements. You will need to know how fast the pipe fills a 1 gallon milk carton or bucket, how many gallons your pool contains, and a measurement of the difference between the pool temperature and the temperature of the water coming out of the hose (subtract the pool temperature from the temperature after it leaves the hose).
You can get an approximation of the impact of this method by dividing the temperature difference by the number of gallons in the pool. This will tell you roughly how much the temperature of the pool will rise when one gallon is circulated through the hose. For example, if the temperature difference is 10 degrees, and your pool is 5000 gallons, then the temperature of the pool will rise approximately 10/5000 = 1/500th or 0.002 of a degree for each gallon going through the pipe.
To find out how long it will take to warm the pool by one degree you divide the number of seconds it takes to pump one gallon by the degree rise from the last calculation. For example, if it takes 20 seconds to pump one gallon, it will take 20/0.002 = 10,000 seconds to heat the water by one degree.
Divide the number of seconds by 60 to get the number of minutes, and then by 60 again to get the number of hours - for example 10000 / 60 = 167 minutes. 167 / 60 = about 2.8 hours.
Slowing down the water will make the water warmer in the pipe but it will take longer to heat the pool. To increase the speed of heating the pool you can add more pipe.
If installing a parallel pipe system above the pool such as on a roof, be sure to install a vacuum break at the highest point of the pipes, otherwise the system can easily get vacuum trapped, and will not cycle water through all the pipes.
Solar heating for pools can be added by zoning in a bypass in the mechanical room. By using a three zone bypass configuration you can control the amount of water that is going to the pool and the amount of water being directed through the solar system.
It is not an advantage to cycle water through the solar heater during periods of time without sun. To avoid a manually intensive system you can install a 24 hour electrical timer and valve actuators to turn the solar on during the day and off in the evening when the sun goes down.
Lifting solar to second and third floor elevations will usually require upgrading the pump size to accomodate the additional lift forces. Also the solar pipes themselves represent a large resistance to flow that the pump must overcome. As a general guideline upgrading the pump size by 1/4HP is usually enough.
In most cases this is not even worth considering unless you have some kind of thermal cover for the pool. The rate of warming is typically too slow to compensate for overnight cooling that will occur if the pool is not covered.
An alternative to hose are vinyl solar panels that add some heating efficiency to this system and cost less than $150 per 2'x20' panel. Solar panels equal to the area of the pool might be ideal, but vinyl solar panels are easy to add so the DIY installer can test the effects of a few panels on pool temperature and pump power before investing in more panels.
Do not expect major increases in temperature - this is unlikely to help make the pool usable in the winter months but may extend the swimming season - most people should be able to get 5-10 degrees Fahrenheit increases using this method with enough hose.
Remember that if you cover the pipe with plastic bottles or a glass sheet, the covering will reflect sunlight, reducing the amount of solar energy gained.
If your system is working correctly, your pipes should be cool to the touch, as this means the solar heat is being harvested and transferred to the water, instead of being released to your hand.
80-81 degrees Fahrenheit is what most people consider a comfortable pool temperature for activity in the pool although some can swim in 70 degrees. 84-85 degrees Fahrenheit is good for most people for relaxing pool usage.
Above-ground pools will lose more heat overnight than in-ground pools (when overnight temperatures drop below the pool water temperature).
The calculations do not cover all factors and are very approximate. They also assume the availability of constant warming sunlight. You may need to adjust for the hours of daylight that fall on the hose.
You may want to mount the hose on a board that can be angled to point more directly towards the sun or leave space between your hoses so they do not cast shadows onto each other, thus allowing a greater angles for the moving sun to strike the hose.
Do not use spaflex, tigerflex or any form of flexible PVC to run the solar plumbing as these pipes deteriorate quickly when exposed to direct sunlight and UV
Any pipes or hoses that are to be used for making a pool solar heater need to be PVC and not nylon based. The "C" in PVC stands for chlorine where nylon is reactive to chlorine and will fail
Be careful not to let water drain from the hosepipe so that the pump is running "dry" as this will damage the pump. Once water is flowing keep the end of the hose in the pool to prevent this. You might need to fill the pipe with water by connecting it to a tap and then closing off the ends initially before connecting to the pump.
In some cases you may want to mount the hose on a board and attach it to a sun-facing roof. This is not recommended unless you have a suitable low roof (e.g. shed) and/or are skilled at working at roof height.
If you want to use this method for warming water for other purposes note that most garden hoses are not suitable for use with drinking water. You will need to set up a 'heat exchange' system or use food-quality hoses.