You have a gas heater or heat pump, and whether the bubble blanket goes on decides how hard that heater works. Here is the heat a solar cover saves a heated Brevard County pool, and why it is never a safety cover.
Evaporation takes more heat from a pool than anything else
Water carries heat away as it turns to vapor at the surface. The U.S. Department of Energy calls evaporation “by far the largest source of energy loss” from a pool.
The numbers explain why. Raising a pound of water one degree takes 1 Btu. But each pound of 80°F water that evaporates takes 1,048 Btu out of the pool, per the same DOE page. The evaporation rate climbs with a warmer pool, more wind across the surface, and drier air.
Heater manufacturers say the same thing. The Rheem/Raypak heat pump manual for models 2450–8450 states that “evaporation is the main cause of heat loss from a pool”. It adds that keeping the pool covered when not in use “will greatly reduce the cost of keeping the pool heated.”
Do solar covers work?
They do, because a cover is a vapor barrier laid over the place the heat escapes. The DOE calls covering a pool when it isn’t in use “the single most effective means of reducing pool heating costs.” DOE says savings of 50% to 70% are possible.
How well a given blanket works depends on fit. The EPA’s WaterSense pool guide, citing a 2016 Cal Poly study, rates solar covers (bubble covers, solar rings and thermal covers). It puts them at 50 to 95 percent less evaporation while in use, varying with shape and coverage. Coverage is the part you control. Trim the blanket to the walls and steps rather than leaving a strip of open water along one end.
The savings go past heat. With a cover, DOE puts the reduction in make-up water at 30% to 50% and chemical use at 35% to 60%.
Does a solar cover heat a pool?
Its main job is holding heat in. An outdoor pool already absorbs 75% to 85% of the solar energy striking its surface with no cover, according to DOE.
A cover takes a little of that away. DOE says a transparent bubble cover may reduce the pool’s solar absorption by 5% to 15%. A completely opaque cover may reduce it by 20% to 40%. That gap is the reason to choose a clear blanket over an opaque one if it will stay on in daylight.
When the cover comes off depends on when you swim. DOE’s advice for a pool used during the day is short. Take the cover off just before swimming and put it back as soon as you’re done. Say you use the pool only at night. In dry or windy conditions, DOE advises, a transparent or bubble cover is generally worth keeping on in daylight. In warm, humid conditions, DOE adds, evaporation slows and it may pay to leave the cover off during the day. Night is simpler: even in warm weather, the Rheem/Raypak heat pump manual recommends keeping the pool covered at night.
What a night without the blanket costs your heater
The overnight number is the one a heater owner feels. The Rheem/Raypak heat pump manual is plain about it. Unblanketed pools can lose up to 10°F per night, compared with 4°F or less under a blanket. The same troubleshooting entry adds that without a blanket, “the total heat gained during the day can be lost overnight.”
Whatever the pool loses overnight, the heater has to put back the next day before it can hold the setpoint. On a gas heater that means more burner time and more fuel. A heat pump already runs long hours by design. The same manual notes a lower Btu-per-hour capacity than a gas heater for the same pool. It also says a heat pump may operate up to 24 hours a day at certain times. An uncovered night stacks more hours on top of that.
Which heater type comes out ahead with or without a cover is a separate question. It has its own page: gas vs heat pump pool heaters, side by side. For what buying and running a heater costs, see the pool heater cost guide. The short version for both: a covered pool asks less of either one.
Liquid covers save far less
A liquid cover is a chemical you add to the water instead of a sheet you pull over it. The EPA’s WaterSense guide describes it as a non-toxic evaporation suppressant applied to the pool surface on a regular basis. Citing the same Cal Poly study, the guide rates it at 15 percent less evaporation. That compares with 50 to 95 percent for a solar cover.
That makes it a fallback for a pool where a blanket is impractical, not a substitute for one. The same EPA table marks liquid products as unable to serve as a safety cover. That leads to the part that matters most.
A solar blanket is not a safety cover
A bubble blanket is a buoyant plastic sheet that lies flat on the water. That is how the CPSC describes a solar pool cover. The EPA’s WaterSense table answers “No” to whether a solar cover can serve as a safety cover.
Florida law draws the same line. Under the Residential Swimming Pool Safety Act (F.S. ch. 515), a residential pool has to meet at least one safety option. That is required to pass final inspection, and one option is an “approved safety pool cover” (F.S. 515.27). The Act defines that as a manually or power-operated cover meeting ASTM standard F1346-91 (F.S. 515.25). The CPSC’s barrier guidelines likewise say power safety covers should conform to ASTM F 1346-91 (Pub. 362, p. 13).
The hazard is specific to how a solar cover behaves in the water. In 1980 the CPSC issued a national alert. It came after a woman and her two young sons drowned when they apparently became entrapped under a solar pool cover. Its warning explains that a swimmer under part of the cover can surface into it and be unable to breathe. It adds that the swimmer can swim further under the cover while trying to find air. The CPSC’s advice is direct. Never leave a solar cover partially in place, and don’t swim in a pool where one partly covers the water.
So the blanket comes all the way off before anyone swims, and it goes on only with nobody in the water. The barrier, exit alarms or approved safety cover that Florida law counts still do the safety job. The blanket only does the heating job.
Public pools play by a stricter rule
Running a community or hotel pool? Florida’s public-pool rule, FAC 64E-9.008(5), allows a cover or solar blanket only while the pool is closed. Heating for those pads is on the commercial pool heating page.
When the cover is on and the water still won’t hold
A blanket that fits and goes on every night is doing what the heater manual asks of it. If the pool still comes up short, the cover has done its part and the question moves to the heater. Start with pool heater maintenance and what a tune-up checks.
Sources
- U.S. Department of Energy, Energy Saver, Swimming Pool Covers (archived January 2, 2021).
- Raypak, Installation and Operation Manual, Heat Pump Pool & Spa Heater, Models 2450–8450 (P/N 241909 Rev. 2, hosted by Rheem), PDF pp. 5 and 36.
- U.S. EPA WaterSense, Jump Into Pool Water Efficiency, “Cover Up,” p. 2, and Table 1, p. 3 (cover effectiveness figures cited from Muleta, “Cal Poly Study: Effectiveness of Pool Covers to Reduce Evaporation from Swimming Pools,” 2016).
- Florida Statutes, s. 515.21, Short title, s. 515.25, Definitions, and s. 515.27, Residential swimming pool safety feature options.
- U.S. Consumer Product Safety Commission, Safety Barrier Guidelines for Residential Pools (Pub. 362), p. 13.
- U.S. Consumer Product Safety Commission, CPSC Warns Consumers of Potential Drowning Hazard Posed by Solar Pool Covers Used on Swimming Pools (1980).
- Florida Administrative Code, r. 64E-9.008, Supervision and Safety.
