What Temperature Does Saltwater Freeze?
Seawater freezes at about 28.4°F, according to NOAA, against 32°F for fresh water. A salt water pool at 3,200 ppm freezes at about 31.6°F, a third of a degree lower, so pool salt won’t stop a freeze.
How much salt it takes
The freezing point drops with the salt dissolved, and a pool holds about a tenth of seawater’s. NOAA puts ocean salinity at about 35 parts per thousand; Hayward’s AquaRite manual calls 3,200 ppm optimal for a pool.
Salt pool
3,200 ppm
Hayward's range is 2,700 to 3,400.
Seawater
35,000 ppm
About 11 times a pool.
Road brine
233,000 ppm
The 23.3% mix road crews spray.
Pool values use the standard formula: water’s constant 1.853 and salt’s factor of 2 from the University of Illinois Physics Van, salt’s molar mass 58.443 from NIST.
Drop (°C) = 1.853 × 2 × (grams of salt per kg of water ÷ 58.443)
Example. At 3,200 ppm there are 3.21 g of salt per kg of water, or 0.0549 mol/kg. The drop is 1.853 × 2 × 0.0549 = 0.203°C (0.37°F), so it freezes at 31.6°F. The formula overstates the drop a little: it gives seawater 27.9°F, against NOAA’s 28.4°F.
Why pool salt won’t protect the pool
Trapped water in the equipment freezes, and it is exactly as salty as the pool. Hayward’s manual says the cell and flow switch “will be damaged by freezing water” and to “drain all water from the pump, filter, and supply and return lines before any freezing conditions occur.”
What breaks first: do salt water pools freeze. The shutdown: winterize a salt water pool and the salt water pool closing kit.
More salt won’t help. Doubling to 6,400 ppm only gets you to about 31.3°F, and it puts you on a drain-down to lower the salt level in your pool. Freeze protection isn’t sold in pool salt bags.
The number that matters is 50°F
A salt cell stops making chlorine at 50°F or warmer. More on what temperature a salt cell stops working.
Hayward AquaRite
50°F
The manual's cutoff.
Below that, you chlorinate by hand: salt water pool chemicals and salt water pool shock.
Frequently asked questions
Will a salt water pool freeze?
Yes, at about 31.6°F, so treat it like any pool.
What is the freezing point of salt water in Celsius?
NOAA gives about minus 1.9°C for seawater. A pool at 3,200 ppm freezes at about minus 0.2°C.
Does road salt melt ice if pool salt can’t?
Yes, because brine is about 70 times pool strength. TxDOT puts its lowest freezing point at minus 6°F, at 23.3 percent salt.
Why does my salt reading drop when it gets cold?
Cold water, not lost salt: Pentair says normal operation, “including accurate salinity report,” resumes only above 52°F. Confirm with salt levels and testing, size any real shortfall on the pool salt calculator, and keep to salt water pool maintenance.
Should I take the salt cell out for winter?
Yes, where it can freeze; Hayward and Pentair both warn freezing water damages it. More on salt cells and water care.
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