Every spring, many pool owners open their pools to find something puzzling: the cyanuric acid (CYA) they added last summer has dropped dramatically, sometimes even disappearing entirely. Because CYA is typically stable and not consumed by chlorine, where does it go during the winter months?
Obviously, some decrease in CYA will be attributable to dilution. Even in covered pools, winter rain and snowmelt can enter, while water is displaced through overflow lines or around the cover edges. This dilution effect lowers CYA along with everything else. Pools with mesh covers, in particular, are more prone to significant dilution over winter.
And some CYA “loss” may actually be inaccuracies in the CYA test at lower concentrations. At lower concentrations (below ~30 ppm), the standard turbidity test becomes less precise. That means some of the “mystery loss” may actually be a mix of true depletion and testing limitations, especially when comparing spring readings to summer baselines.
Of course, any mysterious chemical drop may signal a leak. In cases where water was partially drained during closing, or if a leak went unnoticed during the off-season, the lost water would take dissolved CYA with it. While less common, it’s another way levels can mysteriously drop.
But when CYA drops dramatically, the most widely accepted explanation is bacterial activity. When pools are closed for the winter, water circulation stops, chlorine dissipates, and conditions become favorable for certain bacteria. These microbes can actually use cyanuric acid as a food source, breaking it down into other compounds.
Research suggests that under lowoxygen, chlorine-free conditions, bacteria degrade CYA into ammonia and nitrogen-based byproducts. That’s one reason spring openings sometimes come with both low CYA and a heavy chlorine demand — the leftover ammonia reacts aggressively with chlorine, forming chloramines until the demand is satisfied.
Cyanuric acid doesn’t vanish into thin air, but over the winter it often gets consumed by bacteria, diluted by fresh water, or reduced to undetectable levels. For service professionals, the key is to expect this seasonal shift, plan for higher chlorine needs at opening, and educate customers so they’re not surprised when stabilizer levels need topping off each spring.
What This Means for Pool Pros
• Expect to add CYA at opening. For many pools, a spring boost is simply part of the program.
• Anticipate high chlorine demand. If bacteria converted CYA to ammonia, shock treatment may require much higher chlorine doses than normal to reach breakpoint.
• Check covers and water levels. Pools with mesh covers or overflow issues may lose more stabilizer than those with tightly fitted solid covers.
• Pro Tip: For pools that consistently show CYA depletion over winter, keeping a record of pre- and post-winter levels helps set expectations and makes spring openings smoother. It also positions you as a knowledgeable service professional who anticipates and prevents problems before they escalate.
• Educate the Homeowner. Explain that winter loss of CYA is normal and that high chlorine demand in spring often accompanies it. Advise them on proper maintenance to prevent surprises next season: o Use solid covers where feasible to reduce dilution. o Maintain a small residual of chlorine even in winter (if water stays in the pool). o Check water periodically during off-season if possible.
