Bob Lowry Explains: Cold Weather Pool Care Made Simple

Winter changes the rules for pool care, and the smartest pros shift with it. As water temperatures fall below 60 degrees, biological activity slows and the familiar algae pressure that dominates summer nearly disappears around 55 degrees. That lull tempts many owners to ignore the water, but chemistry never sleeps. Lower temperature pushes the Langelier Saturation Index downward, nudging water from balanced toward corrosive. When LSI slips, surfaces become the source, and plaster, grout, and heaters can pay the price. The goal in winter is not more chemicals; it is smarter balance with fewer inputs. Less demand does not mean zero demand. It means adjusting set points so the water stays gentle on the vessel while remaining sanitary and clear.

The first lever is understanding how temperature affects LSI. A rough field rule is that every 15-degree drop pulls the index by about 0.1. Fall from 90 to 60 and you lose roughly 0.2, enough to turn a barely balanced pool into a surface eater. You can counter with modest increases to pH and total alkalinity, often via sodium bicarbonate, to restore equilibrium without overcorrecting. Calcium hardness remains important, but big winter swings are rarely needed if you finished summer near target. The point is to plan ahead: as your thermometer slips into the 50s, test more intentionally and adjust the buffer. Balanced water in cold weather preserves plaster, resists metal corrosion, and prevents that chalky, stubborn ring that shows up when stagnant water slowly evaporates against tile.

Salt chlorine generators introduce another winter variable. Most cells throttle down or stop producing around 60 degrees, sometimes closer to 55, so electrolysis cannot be your primary disinfectant in cold water. The flip side is that you need much less chlorine because algae is barely reproducing. That opens room for simple liquid dosing as needed. A smart hardware habit is to remove or swap the good cell for a dead “winter plug” so minor stray currents and trapped water do not age a working unit. If you cannot remove it, at least power down properly. Protecting the cell for a season can add meaningful life, and with replacement prices high, that is real savings for service pros and owners alike.

Circulation strategy matters as much as chemistry. Many people try to trim run time to a token two hours, which is the worst of both worlds: too little to turnover, just enough to stir debris and create uneven chemistry. Either commit to a proper daily turnover or, in true winterized regions, shut the system down after protecting lines and equipment. Freeze protection automation provides a safety net by switching on when the sensor detects near-freezing conditions, but it is only as reliable as your power grid. The Texas deep freeze showed that days of subfreezing weather plus power loss can overwhelm unprepared systems. In hard-winter areas, pros plug lines, add pool-grade antifreeze, lower water below returns, remove portable gear, and accept a dormant but protected vessel until spring.

Cyanuric acid deserves a winter plan too. After seasons of trichlor tabs, many pools carry CYA far above the efficient 30 to 50 ppm range. Winter is a strategic window to partially drain and dilute without heat waves or peak bather loads. CYA also slowly degrades on its own, sometimes around 5 ppm per month, and in rare cases specific bacteria can consume it to nearly zero when chlorine is low. That variability underscores why testing still matters in cold months. If you run open year-round, keep CYA modest so free chlorine remains effective at lower demand. If you close, document levels before winter so you know what to expect upon opening and can avoid surprises that delay swim season.

For borderline climates—parts of the Southeast and Mid-Atlantic—deciding to close or stay open can change year to year. The risk calculus is simple: if you can reliably power freeze protection and you are willing to maintain balance, staying open keeps water moving and surfaces cleaner. If deep freezes and outages are plausible, closing correctly will cost less than a spring equipment rebuild. Either way, avoid letting water sit half-treated for months. Stagnant water etches rings into tile, concentrates minerals at the waterline, and sets you up for stains that cleaners struggle to lift. A few targeted winter moves—buffering alkalinity, tuning pH, validating calcium, and planning CYA—make the difference between a smooth spring start-up and a costly rehab. Winter is quiet, not idle; use it to protect the vessel, extend equipment life, and lighten your workload when the sun returns.