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Solar Heating your Swimming Pool
11 years ago, I bought a house with an in-ground swimming pool, something of a rarity in chilly Berkeley but it was 40 years old. I don't like cold pools, and decided to put in solar heating panels. It took some time, but I worked out how to operate the heating/filter system at a very low energy cost. I wrote a brain-dump yesterday for someone who just got a house with a pool, and then decided to put it online, rather than lose the information.
Safety
Unfortunately, this starts with a difficult topic. Before you start work on your pool, please do a google search for "pool evisceration" and read the references. A number of children have been horribly injured by innocent-looking plumbing.
In current pool construction, evisceration is avoided by having any pipe with suction on it be connected to more than one drain, so that suction does not build up on one drain.
Making sure a current pool is as safe as it can be from evisceration hazards
1. Make sure the skimmer bucket, where the basket that holds things the skimmer catches goes, has a working cap on top of it. You might want to screw it down so that curious kids can't pick it up, the last one I bought came with screws. One of the bad injuries was from a baby sitting down in the skimmer bucket. Modern skimmer buckets have a second drain built below them in the pool wall so that there can not be suction buildup. Make sure that's not blocked.
2. Make sure the bottom drain has a grate screwed down on to it.
3. If there is an outlet for a pool vaccumm, fit a spring-loaded cap on to it.
Stores
Leslie's pool supply is the only good local store. Use web/mail order to get everything else.
Water table
Draining a pool isn't good for it. If the water table is high as in the winter, it can float the pool right out of the ground. Sometimes you can get the thing back where it was, but you'll lose the plumbing, including that hard-to-replace bottom drain pipe. It can also cause the plaster to start popping off the base of the pool, which will make it leak. I live in the hills and the water is only 7 feet down in the middle of summer. I have not drained my pool since a pebble-tek replastering 7 years ago. It's easy to keep clean without ever draining it, and having a diver come from a leak-detection company and do any underwater repairs you need is less expensive than I would have expected.
Type of panel
There are a number of different kinds of panel that you can put on your roof. Most these days are plastic of some kind. The panels I'm using are made of aluminum and copper, and I can't get any more. The cheapest kind come in a roll of about 20 feet by 4 feet and are just rolled out onto the roof. Between thermal expansion and wind, they are going to chafe the life out of your roof shingles. More practical units come in solid rectangular panels that connect end-to-end. Cover as much of the sun-facing roof as you can. Since heat is the free part of this energy equation, you will save the most electricity with more panels, and a shorter run-time. You will also get a longer season in which you can heat the pool with more panels. About 3/4 of your pool surface area is a practical minimum size to be making a significant difference in the pool temperature. A web search will find various brands, plan on doing this by mail-order, as it's hard to find the parts otherwise. My panels can get the pool up to body temperature for two to three months in the summer, here in Berkeley's fog-belt, on the shady side of the hills. Most people will have a longer season. Today the system is running and water coming out of the panels is about 79 degrees, probably 10 degrees above air temperature.
Roof mounting
I built a frame for my panels using redwood blocks bolted into the roof studs and ground-contact-rated 2x4s running between the blocks. This holds them several inches off of the roof. Some plastic panels come with stainless steel mounting hardware that holds them off the roof. Prosolar makes beautiful hardware for clay tile roofs, foam roofs, etc., but it's pricey. They actually made my panels, and have either exited that business or just refuse to ship them anywhere.
Air-relief valve
Panels come with an air-relief and vaccumm breaker valve. This removes air from the system and keeps it from developing a syphon. The most important thing it does is drain the panel reliably for those Northerners who have freezing weather. But it is a tremendous energy waster for us, because it breaks the syphon that actually saves us energy in pumping water up to the roof and back. Block off the air relief valve and you will be able to use the low setting of a two-speed motor to pump through the panel. Leave it open and you'll need the high-speed setting. The air in the system just comes out the pool jets, it doesn't get stuck anywhere. Use a manual valve to make sure the panel is drained when you close the system for the winter. If I lived in a place where frost is less predictable, I'd use an electric valve for air relief, and have it shut when the motor is on low speed and open otherwise. You will not close the pump for the winter, in a really cold winter you will circulate the water to keep the pool from icing over. Ice is bad for it. Yes, it really does ice over in the Berkeley hills some years.
Cover
Use a solar pool cover to conserve that heat and keep the mosquito larvae out. They are made of bubble-wrap with some ultraviolet-resistance added. UV and chlorine rot them in 2 years or so. Leslies pool supply has nice cut-to-shape ones with reels.
Controller
The controller turns on the system when it can heat the pool and turns it off before it starts cooling the pool. It uses two thermistors, and turns on the system when one thermistor is hotter than the other, and allows you to set a maximum temperature. If you live in Arizona, it also allows you to set the panel to go on at night and cool off the pool. Mine is from Goldline controls. It comes with instructions that expect you to run the filter on a timer all day, and then have the controller just turn the valve to the heating panel on and off. This might have been a good idea 15 years ago when electricity was cheaper. To be able to turn the motor on and off with the controller, you need a controller that puts a thermistor in the panel and one in the pool.
Motor
A two-speed motor is essential to save energy, since it's going to run all day on sunny days. These days you can also get variable speed motors. Mine is from Pentair and does about 3/4 HP at full speed and 1/16 HP at low speed. The faster you pump water, the more back-pressure there is that wastes energy. Thus, when you are not cleaning the pool or doing some heavy cleanup, it will be much better to run the motor slowly, and for longer.
Drain
It is important to take the water to be heated from the BOTTOM drain, since that is where the coldest water will be, and your just-warmed water will float to the top. My bottom drain was abandoned when the pool was rebuilt, so we use the pool cleaner, a "Kreepy-Krauly", to pull the water from the bottom. This is the only acceptable unit for pebble-textured pools, if you have smooth plaster you can use something else. It slowly cleans the pool with the motor on low power. Once I do the season opening and pull out the big branches, the pool just keeps itself clean for the rest of the summer. We replace the rubber skirt and ears on that Kreepy-Krauly every year, that ends up costing about $100.
Sanitization
I got a big canister that holds 30 chlorine tablets and runs the pump water through them slowly whenever the pump is on. I can set that up and leave it alone for a month.
PH
PH and so on are mostly done at season opening, once you have the chemical balance right you just test it every week or so. The rain will tend to make the water acid, you have to buffer it so that it doesn't dissolve your plaster.
Algae
I used to use a copper additive for algae. Now that there are vapors of suspicion that unbound copper is implicated in Alzheimers disease, I am holding off on that. In general running the filter a lot gets rid of the nutrients that algae needs, on occassion a chlorine shock or some chemical additive are necessary to handle a bad case.
Murkiness
Leslie's has a "clarifier" solution that handles murkiness very well. I only need it when I open the pool for the season.
Plumbing
PVC pipe is like lego for grownups. Very cheap and easy to build with, and it holds 200 PSI when you are done. Pay attention to cleaning parts to be attached, use primer, make sure glue covers all surfaces that will be welded together, keep the joint still while it's curing. Do not leave unused PVC in the sun, UV makes it brittle. Paint installed PVC that would be sun-exposed black so that UV won't make it brittle. Use 2 inch pipe to carry water, bigger pipe has less resistance.