Solar covers and heat retention for Ohio pools

Because our nights are cool, even in summer. National Weather Service normals for Akron-Canton, the 1991 to 2020 averages, put the typical daily low at about 50 degrees in May, 59 in June, 63 in July, 62 in August and 55 in September. Daytime highs in May and September sit in the low to mid 70s. A pool warmed by the afternoon sun spends every night sitting under air that is 10 to 20 degrees colder than the water.

Most of that heat leaves through the surface. Water evaporating from a pool carries a great deal of energy with it, and wind across the surface speeds it up. That is why a pool can feel noticeably colder on a breezy morning after a clear night, even when the day before was hot. Stop the evaporation and you stop most of the heat loss, which is what a cover does.

If you live in the lake-effect snowbelt counties to the northeast, such as Geauga or Lake, where spring can be slow to arrive, a cover matters even more if you hope to open the pool by Memorial Day.

A lot, according to the research. The EPA's WaterSense guide to pool water efficiency says pool covers are the most effective way to reduce evaporation, that solid covers can reduce evaporation by more than 90 percent when in use, and that on heated pools they can save 50 to 70 percent of heating costs. The same guide rates solar covers, including bubble covers, solar rings and thermal blankets, at 50 to 95 percent effective at cutting evaporation, depending on their shape and how much of the surface they cover.

A 2016 study by Cal Poly for the National Plasterers Council measured it directly on test pools. Bubble covers reduced evaporation by about 95 percent. Floating solar disks, which leave gaps between them, cut it by about 50 percent. Coverage is what counts: a cover that fits the pool edge to edge works far better than one that leaves open water at the steps and corners.

Evaporation also costs water. EPA estimates that an uncovered 500 square foot pool can lose 12,000 to 31,000 gallons a year to evaporation depending on climate, and a cover saves most of that along with the chemicals in it.

Four types are common on Northeast Ohio pools, and they are not interchangeable.

• Solar blanket or bubble cover. A sheet of plastic with air bubbles, cut to fit the pool and floated on the water. Inexpensive and effective at stopping evaporation. Usually rolled on and off with a reel at the end of the pool.
• Solar rings or disks. Floating circles that partly cover the surface. Easy to handle, but the gaps between them mean roughly half the evaporation reduction of a full cover.
• Liquid solar cover. A product added to the water that forms a very thin film on the surface. Aqua Magazine reports typical evaporation reductions of 15 to 40 percent. It helps a little and needs no handling, but it is not a substitute for a physical cover.
• Automatic cover. A cover that runs in tracks along the pool and opens or closes with a key switch. It holds heat like a solid cover, and when it is listed as a safety cover, it adds a barrier as well.

No. This is the most important thing to know about solar blankets. A floating bubble cover is not built or listed to hold the weight of a child, and a child or pet who walks onto one can end up underneath it. The EPA's own table of pool cover types marks solar covers as not suitable for safety use.

Safety covers are listed to ASTM F1346 and are either automatic covers or anchored covers strapped to the deck. If you use a solar blanket, keep the pool barrier fence and gates closed as usual, and never treat the blanket as a reason to relax around a covered pool.

Every night the pool is open, and on cool or windy days when nobody is swimming. The shoulder seasons are where covers pay off most. In late May and early June, and again in September, the sun is still strong in the afternoon but nights drop into the 50s. A cover lets the pool keep the daytime gain, which is often what makes those weeks swimmable at all.

If you have a heater, the math gets simple. Every degree the cover saves overnight is a degree the heater does not have to make up in the morning, which is why EPA puts the savings on heated pools at 50 to 70 percent of heating costs. Our guide to heat pumps and gas heaters goes into running costs, but a cover is the first step whichever heater you choose.

In peak summer, some owners leave the cover off on hot, still nights because the pool is already warm enough. Watch the forecast: a clear night after a hot day loses more heat than a cloudy one.

Mostly they hold heat. A clear or translucent bubble cover lets sunlight through to the water and traps what it gains, so a covered pool on a sunny day can warm up more than an uncovered one, largely because it is not losing heat to evaporation at the same time. Opaque covers block more sun but still stop evaporation.

The bigger effect by far is overnight. In our climate, the most useful way to think about a solar blanket is as a lid that keeps yesterday's warmth in the pool, not as a heater.

Edge to edge, because the gaps are where the heat escapes. Blankets are sold in rectangles and trimmed to the pool's shape with scissors, bubble side down. On a freeform fiberglass pool, trim it to follow the curves and the steps, leaving it slightly oversized so it lies flat against the walls rather than pulling back from them. A blanket that has shrunk after a few seasons, leaving a strip of open water along one side, has lost much of its value.

Tanning ledges and benches are the awkward spots. A separate piece cut for the ledge often fits better than one sheet trying to cover both depths.

A cover keeps leaves, seeds and pollen out, which matters in Northeast Ohio's wooded neighborhoods. It cuts evaporation, so you top up the pool less and lose fewer chemicals with the water. The tradeoff is handling. A solar blanket on a large pool is awkward without a reel, and a cover left in a heap in the sun on the deck breaks down quickly.

A few habits help a blanket last:

• Use a reel and roll it up rather than dragging it across the deck.
• Shade the rolled cover or use a reel cover when it is off the pool.
• Take it off when adding a large dose of chlorine, and let the water circulate before putting it back.
• Hose off leaves before rolling it so debris does not end up in the pool.

An automatic cover removes most of that effort, which is why people who find themselves not bothering with a blanket often switch to one.

A solar blanket is a swim season cover, not a winter one. When the pool is closed for the winter, usually after the September nights turn cold, the blanket comes off, is cleaned, dried and stored out of the sun. The winter goes to a winter or safety cover built to carry snow and keep debris out, anchored to the deck or running in the automatic cover's tracks. A solar blanket left on over an Ohio winter collects leaves and water, tears under snow load and can sink into the pool.

If you use our pool service for closing, the crew can handle the swap as part of the job.

Covers do the most, but the pool itself matters too. Wind speeds evaporation, so a fence, hedge or wall on the windward side helps. A pool placed where it gets afternoon sun warms more than one in the shade of the house or tall trees. Water features that spray or spill increase evaporation while they run, so it helps to shut them off when you are not enjoying them.

Construction plays a part. Insulated concrete form walls, which we use for ICF concrete pools, hold heat better and help stretch the swim season. Every package includes a pool heater, and an automatic cover is an upgrade we install on any of our pools. Our fiberglass packages start at $68,400 installed with Pentair equipment, including a variable-speed pump that can run slowly and quietly for long hours, plus a pool heater, and upgrades such as a cover are priced on top.