To properly size a solar pool heating system, the following questions must be answered.

1.)	What is the desired temperature of the pool?
2.)	How early / late in the swimming season do you want to use the pool?
3.)	Is the pool shaded? If so, how many hours between 9 and 5?
4.)	Do you use a solar cover on the pool (in the Spring and Fall)?
5.)	Is the pool exposed in a windy location?
6.)	Is there ground water movement around the pool?
7.)	Will there be shade on the solar collectors between 9 and 5?
8.)	What direction must the solar collectors face?
9.)	At what angle will the collectors be mounted?
10.)	What brand of solar collector is being used?
11.)	How large is the surface area of the swimming pool?

1.) The desired pool temperature is usually a function of how the pool is going to be used. A pool used for vigorous exercise cannot be more than 78 Degrees (F) (25 Degrees C) or the person using the pool will overheat. Pools used for family recreation, especially if there are small children, are ideally maintained at 85 Degrees (F) (29 Degrees C). If the pool is being used for therapeutic purposes, 90-95 Degrees (F) (32 - 35 Degrees C)is not uncommon. Of course, the warmer the pool the larger the solar heating system needed to maintain the desired temperature.

2.) The "standard" swimming pool season in (Southern) Ontario is from the May 24th weekend to the Labour Day weekend - typically 14 -15 weeks. Unheated pools will often not be warm enough to swim in as late as 3 weeks after being opened. They often are too cool during the last 2 -3 weeks before labour day. If you don't want to swim earlier or later than this abridged season (of 9-10 weeks), you don't need any kind of heater - a solar blanket should keep the natural solar energy in your pool. If you would like to swim from the May 24th weekend to labour day, then you will need a heater. It is not unusual to swim in a pool as much as 4 weeks earlier (from late April) and 4 weeks longer (until late September) by using a solar pool heating system. The larger the solar system, the further into the shoulders of the season you can use your pool.

3.) If the swimming pool is shaded between the hours of 9 am and 5 pm, the solar system must be increased in size to compensate for the lack of natural solar heating. The only reason unheated pools get to a reasonable temperature during the summer is because the sun shining on them heats them up. If the pool is shaded it won't get as warm. A fully shaded pool will require a solar system twice as large as one for an unshaded pool.

4.) Solar covers are very important to the successful operation of a solar pool heating system in the Spring and Fall. Pools lose most of their heat through evapouration. Solar covers dramatically reduce the heat loss from pools in the Spring and the Fall, when pools can lose much more heat than a solar system can replace - if a cover is not used. If a cover is not used, the size of the solar system should be doubled.

5.) If the pool is located in a windy location, and not protected by a fence or hedges, it will suffer larger heat losses due to increased evapourative and convection losses. It is best to protect the pool from the wind (without shading it!!) to cut down on the heat loss. If this isn't practical, the solar system should be increased in size.

6.) Ground water movement around a pool is difficult to access. If the pool is difficult to heat, or doesn't get warm even after several sunny warm days, there may be ground water stealing the heat from the pool. Another sign is the problem of water behind the pool liner, or water flushing dirt in through the pressure balancing valve at the bottom of the pool. This situation may make it impossible to properly heat the pool with a solar system (or even a gas heater). If ground water is suspected, a solar system should not be installed until the issue is resolved.

7.) Shade on the collectors between the hours of 9 am and 5 pm can significantly reduce the amount of heat delivered to the pool. If the collectors are shaded for one hour (out of the 8 hour period) then the collector area must be increased by 1/8 to compensate. The collectors should never be shaded between 11 am and 3 pm!!

8.) The direction the collectors face is actually fairly flexible, providing adjustments are made to the area. The starting size of the solar system remains unchanged if the direction the collectors face varies plus-or-minus 45 degrees from SOLAR South (not magnetic South) (there is a difference which varies depending upon where you live).

The sizing compass above illustrates how the starting size of a solar pool heating system varies depending upon what direction the solar collectors face - IF all other sizing factors are perfect!!

9.) The solar collectors used for outdoor swimming pool heating systems can be mounted anywhere from flat to 60-degrees-from-flat and still deliver 90%+ of the energy they would deliver if angled at the "optimum" angle. The condition is that they must face due solar South if at 60 degrees. The shallower the angle, the less sensitive the performance is to the direction they face. Flat obviously has no direction:) Vertical fences are a VERY BAD CHOISE for summer use solar systems.

10.) Despite what every manufacturer says - their product is not the best, and the longest lasting, and the easiest to install, and delivers the most heat, and is the most freeze-proof, etc. etc.... Not all products deliver the same amount of heat per square foot of area. Some are significantly worse than the average. This means you need significantly more of them to deliver the same amount of heat to the pool. The best means of comparing the different solar pool heating products is to check the 3rd party testing agencies, such as the National Solar Test Facility (located in Mississauga Ontario) or the Florida Solar Energy Test Center (located in Florida - of course). Not all products are listed, as not all are tested. If they are not tested - they are not to be trusted!!

11.) The larger the pool, the larger the solar pool heating system. The sizing compass above illustrates that the minimum size of a solar pool heating system is 50% of the pool's surface area under ideal conditions. The previous 10 factors all tend to increase this area. In florida, the starting size of their solar pool heating systems ranges from 75% in the Southern part of the state to 100% in the Northern areas of the state. Mind you this sizing is for pools that are used all year! (They actually use the solar systems to cool their pools during the summer by running them at night!)

 

A few words about solar blankets

Solar Blankets are great things for keeping the heat in a swimming pool - They're just so dambed awkward to handle! As pool blankets are very important in keeping the costs of operating a swimming pool down, here are a few pointers that will help you get the best use of them.

1.) There are two types of blankets available. The bubble type solar blanket which lets about 50% of the sunlight into the pool, and the foam type, what we call "lunar" blanket which lets 0% of the sun into a pool. Both types of blanket work very well at preventing water from evapourating from the pool when they are used (properly).

2.) Every solar blanket should have a roller to wind the blanket off the pool when not in use. It makes the blanket easier to handle (so you will be more inclined to use it) and will prolong the life of the blanket.

3.) Blankets can be awkward to use on other than rectangular pools. Sometimes it works to place the roller across the middle of the pool rather then at on end of the pool. Sometimes it works to cut sections off the blanket so they don't catch on the sides of shaped pools.

4.) Some pool blanket coverage is better than none. If you can't cover all of the surface of the pool with a blanket, cover as much as you can. As a last resort you could try products that claim to disperse a film over the surface of the water to slow down evapouration - We are skeptical of the effectiveness of these products - a blanket WILL do a much better job. (See item 10)

5.) Bubble solar blankets have very little insulating value - they are best at preventing evapourative cooling. The foam "lunar" blankets reduce evapourative losses and actually provide a small amount of insulating value, plus they will also reduce the radiative losses better than bubble blankets.

6.) The drawback to "lunar" blankets is that they block 100% of the sunshine from entering the pool. This is great if you are leaving your pool for a week (or so) and you want to keep it from overheating and growing a crop of algae, however if you want to heat your pool, you can't use them during the day. (Hence our term "lunar" blankets). Lunar blankets usually last at least twice as long as bubble blankets. (5-7 years)

7.) Either type of blanket's ability to keep heat in the pool will be significantly compromised if the top of the blanket is covered with water or has puddles of water on it. Water will evapourate from the puddles and pull heat right through the blanket and cool the pool. After it rains, you should roll it up the blanket and drain the water off of it.

8.) One myth about solar blankets is that they cause algae to grow. Algae grows in pools that are warm and don't have enough chemical to kill the algae. What often happens, is that pool owners will cover the pool and neglect it for several days at a time. The (bubble) solar blanket covered pool heats up and the chlorine in the water disappears, providing a perfect breeding ground for algae. Once algae gets this type of foothold, it requires a lot of chemical to get rid of it. Plus, the solar blanket has now has algae growing on it. The blanket must be cleaned or it will re-introduce algae back to the pool when reused!

9.) A major advantage solar pool heating systems provide over fueled pool heaters, is that you don't need to use your pool blanket in the middle of the season, and your pool will still be warm. The reason for this is that although the pool will lose a couple of degrees overnight, the solar system will recover that heat quickly the next day - for free! If you have to pay to recover that heat, you may be more inclined to keep the pool cover on the pool....

10.) There are a number of products on the market that claim to keep heat in the pool by reducing the evapouration of water from the pool by spreading a thin layer or film of liquid (material unknown) over the surface of the water. While the science of this effect is well known, and used on ponds and canals in arid regions of the US and Canada, it can't work effectively on swimming pools. The reason is swimming pool pumps & filters actively clean the surface of the water with skimmers - effectively removing the film and filtering it out. The only way to see any benefit from these fishy items is to only use the bottom drain or an automatic pool cleaner (Kreepy-Krawly, Barracuda, etc.) all the time to take water from under the surface of the water. Of course, then the leaves and bugs and dust that fall into the pool, accumulate on the surface of the water of the pool.