Cold water lowers output
Salt cells generate chlorine through an electrical reaction in the water, and that reaction slows down as temperature falls. The same cell at the same setting simply makes less chlorine in January than it did in July.
Many systems also have a low-temperature cutoff — often somewhere in the low fifties — where the cell stops producing entirely to avoid damaging the plates. That's a protective feature, not a failure.
Why it usually isn't a crisis
Cold water also demands far less chlorine. Sunlight is weaker, algae growth nearly stalls, and bather load is close to zero. A pool that needed constant production in August can coast on very little in December.
The trouble comes when output drops to nothing and demand doesn't quite reach zero — a warm stretch, heavy leaf drop, or a pool that isn't circulating enough. Then chlorine bottoms out quietly and the pool starts to turn.
What to do instead of panicking
Keep testing through the winter. The whole point of digital readings is catching a slow slide before it becomes a green pool.
When the cell isn't producing, chlorine gets supplemented by hand — a small, deliberate amount based on the reading, not a guessed dose. Turning the cell's percentage to 100 in cold water mostly wears the cell out faster without producing much.
Salt level still matters. Cold water reads lower on many salt sensors, so a reading that looks low may be temperature, not actual salt. Adding salt based on a cold reading is how pools end up over-salted in the spring.
Winter is the right time to clean the cell
Scale on the plates lowers output year-round, and it's easy to blame cold water for a problem that's actually buildup. A quarterly clean — removal, acid bath, inspection, reinstall, and flow check — keeps the two causes from hiding each other.
If a cell is genuinely at the end of its life, cold weather is when it becomes obvious. Replacing the cell involves the electrical side of the system, so that work gets handed to a licensed pro.
