Pool Care in Hot Weather

What heat and sun do to a pool

Two different things happen, and they are easy to mix up.

• Sunlight destroys unprotected chlorine. The Pool & Hot Tub Alliance (PHTA) says that without stabilizer in the water, bright sunlight can break down most of the chlorine within an hour. Stabilizer slows this. PHTA gives 30 to 50 ppm as the ideal range, and says the maximum effect comes between 50 and 100 ppm.
• Warm water raises chlorine demand. Orenda says warmer water means more chlorine demand, because both living contaminants (algae, bacteria) and non-living ones (sweat, oils, sunscreen) are more common in warm water. It also says this is not due to sunlight. Warm water lets algae multiply faster, so summer is when most algae problems start.
• Chemistry runs faster. Orenda notes that chemical reactions generally go faster at higher temperatures, so chlorine works better and also gets used up faster.

Put simply, warm water is when you need chlorine most and when you lose it fastest. We could not find a published number for how much faster FC falls per degree, so we do not give a "heat multiplier." The engine behind our calculators has no sunlight multiplier either. Test instead of guessing.

FC minimum by CYA

The minimum FC rises with stabilizer, because stabilizer holds chlorine in a less active form. We use a ratio: the minimum is about 7.5% of CYA (about 5% for a salt pool with a generator), and at least 2 ppm once any stabilizer is in the water. The reasoning and sources are on the pool chlorine levels page.

Water temperature does not change these numbers in our rule. We could not confirm a published warm-water adjustment, so in a heat wave the practical step is to test more often and keep FC comfortably above the minimum, not to chase a higher number. Pool chlorine labels say not to swim above 4 ppm. Do not raise FC above what your label allows. If your CYA is above about 50 ppm, no FC level meets both the ratio and the label, and the fix is to lower the CYA.

Do not add more stabilizer to fight the sun. PHTA advises that CYA not exceed 100 ppm, and says no extra benefit shows above that. See what cyanuric acid does and how to test it.

Heat affects the CYA test itself. A PHTA fact sheet says high temperatures cause the CYA test to read too low and lower temperatures cause it to read high. If you test a very hot sample straight from the pool, let it cool to room temperature, or confirm the reading on a cooler day before you add stabilizer.

How often to test

The CDC advises testing chlorine and pH at least twice a day in a pool or hot tub that many people use. For a home pool in a heat wave, the table is our suggestion, not a published schedule.

Test strips are fine for a daily check but they are rough. See pool test strips for what they do and do not tell you. The CDC says DPD drop kits are most accurate and strips are acceptable.

Heat wave routine

1. Test FC and pH first thing. If FC is below your minimum, add chlorine by your product's label. Use the chlorine dose calculator for your pool size.
2. Add liquid chlorine in the evening when you can. This is our suggestion, based on the PHTA finding that sunlight breaks chlorine down. Chlorine added after dusk has the night to mix in.
3. Run the pump longer. More circulation spreads chlorine and filters out debris. The pump run time calculator helps set hours.
4. Brush the walls and floor weekly, more often if the water is above 85 °F. Brushing knocks algae off surfaces where chlorine reaches it.
5. Keep the skimmer and pump baskets empty. Warm water speeds up how fast leaves and pollen rot and use up chlorine.
6. Shower before swimming and limit sunscreen on the water. Sweat and lotions add chlorine demand.
7. Shock only if you need to. If the water goes cloudy or you see green on the walls, follow how to shock a pool and use the dose on the product label.

Salt pools in the heat

A salt water chlorine generator makes chlorine only while the pump runs. In hot weather, chlorine use can outrun output. Check FC daily, raise the cell's output percentage or run the pump longer as the manual says, and see salt water pool care.

Evaporation and water level

Orenda gives 0.25 to 0.5 inch a day for an outdoor pool in summer, up to 3.5 inches a week. Dry, windy, sunny weather is at the top of that range. Evaporation takes only water. Orenda notes that calcium, metals, salt, and CYA are left behind, so the pool gets more concentrated as it is topped off. Rain and snow have no calcium or dissolved solids, so they dilute.

Keep the level about halfway up the skimmer opening. Allow a day or two of loss before it matters, but do not let the pump pull air. The pool water level page has the gallons-per-inch table for common sizes. Topping off adds minerals, so retest calcium and CYA after a lot of top-offs, and see calcium hardness.

Not sure whether water loss is evaporation or a leak? The bucket test is a common method. We did not find a primary source for it, so treat it as a practical check.

1. Fill a bucket with pool water to about 1 inch from the top and place it on a pool step so it is partly in the water.
2. Mark the water level inside the bucket and the pool level outside it with tape or a marker.
3. Leave it for 24 hours with the pump on its normal schedule.
4. Compare. If the pool dropped more than the bucket, the extra drop is not evaporation. Call a leak-detection professional.

We could not confirm a published percentage for how much a pool cover or a solar blanket reduces evaporation, so we do not give one. A cover on when the pool is not in use cuts how much water leaves, and it keeps debris out.

Water temperature

We found no U.S. standard for the maximum temperature of a home swimming pool. For hot tubs, the CDC says water should not exceed 104 °F. If you have a heater, turn it off during a heat wave. Warm water is where algae grow fastest and chlorine is used up fastest. Cloudy or green water is covered on green pool, cloudy pool, and algae types. For the weekly rhythm of tasks, see the pool maintenance schedule.

FAQ

How much chlorine does a pool need in hot weather?

The minimum is set by your stabilizer reading, not the weather: about 7.5% of CYA, so 3 ppm at CYA 40. In hot weather you will use it faster, so test daily and dose to stay above that.

Why does my pool turn green in summer heat?

Warm water lets algae multiply faster, and sun and swimmers use chlorine up. If FC drops below the minimum even for a day, algae can take hold. Brush, test, and see how to clear a green pool.

Does the sun burn off chlorine?

Yes, if the chlorine is not stabilized. PHTA says bright sunlight can destroy most chlorine in water without stabilizer within an hour. With CYA the loss is much slower.

How much water does a pool lose to evaporation?

Orenda gives 0.25 to 0.5 inch a day in summer for an outdoor pool. Dry air and wind push it higher.

Should I raise my CYA in a heat wave?

Not above the normal range. PHTA lists 30 to 50 ppm as ideal and 100 ppm as the limit. More stabilizer makes chlorine less active, so you would need more of it.

Sources

• PHTA Technical Bulletin, Cyanuric acid (sunlight and chlorine, 30–50 ppm ideal, 100 ppm limit): phta.org
• Orenda Technologies, Why swimming pool temperature matters (warm water and chlorine demand, algae): blog.orendatech.com
• Orenda Technologies, Evaporation and accumulation (0.25–0.5 in per day; what concentrates): blog.orendatech.com
• PHTA Fact sheet, Water conservation during droughts (high temperature makes the CYA test read low): phta.org
• CDC, Home pool and hot tub water treatment and testing (test twice a day, 104 °F for hot tubs, DPD kits): cdc.gov
• CDC, Controlling Legionella in hot tubs (104 °F maximum): cdc.gov