Pool closing is the most consequential maintenance event of the year. A perfect season of service is undone instantly by a single missed drain plug or an unblown skimmer line. These are the 10 closing mistakes that create the most expensive spring opening bills — documented from actual service calls, not theory.
The most expensive mistake, full stop. Every return line, skimmer line, and main drain line must be blown clear of water before the first freeze. Water left in plumbing expands when it freezes — 9% by volume — cracking pipes, fittings, and skimmer bodies. A single cracked underground return line costs $400–1,200 to locate and repair in spring. Do it right: blow each line to completion, confirm bubbles at the pool end, plug immediately.
Drain plugs get removed from the pump, filter, and heater during closing. By spring, they've been placed in a zip-lock bag somewhere, or in a drawer, or they're lost entirely. The standard fix is buying new ones every spring — which is wasteful and avoidable. Store drain plugs inside the pump basket for easy location in spring, and note their location in your closing documentation.
Closing a pool that has active algae, even in the early stages, means sealing algae spores under a cover for 6 months. By spring you have a green pool requiring triple the treatment time and cost. The correct approach: fully treat and clear any algae before closing. The water should be clear, not just "not obviously green."
Adding algaecide while chlorine is still very high (from shocking) destroys the algaecide before it does anything useful — most algaecides degrade rapidly in high-chlorine water. The correct sequence: shock, run the pump 8 hours, then add algaecide once chlorine has dropped to 3–5 ppm.
Closing when water temperature is still above 60°F leaves algae biologically active under the cover with no UV exposure to help break down the chlorine that's protecting the water. The combination of high temperature, dark environment, and depleting chlorine means a green pool by February — even with a well-dosed closing treatment.
Use a floating thermometer or the temperature reading from your automation system. Don't guess on water temperature — it lags ambient temperature by days. Wait for a genuine sub-60°F water temperature before closing.
Salt chlorinator cells should be removed, cleaned (acid washed), and stored indoors during winter. Leaving the cell installed in freezing climates risks freeze damage to the flow switch, the cell housing, and the control board connection. Most manufacturers explicitly require cell removal for winter storage.
A solid winter cover without an active pump system will accumulate rain, snowmelt, and condensation. A heavy water load can cause the cover to fail, sink into the pool, or allow debris-laden water to drain into the pool through the cover edges. A floating cover pump ($60–150) eliminates this problem automatically.
Low pH water (below 7.2) is corrosive — it etches plaster, corrodes metal fittings, and attacks equipment seals over a winter of sitting. Close with pH at 7.4–7.6, not the common mistake of "closing low" to protect against scale (a misapplication of LSI principles).
A gas heater has two or three drain plugs in the headers. These are easy to forget in the rush of closing, especially if the heater drain plugs aren't in the same bag as the pump and filter plugs. A heater with water in the heat exchanger that freezes will crack the copper tubes or the header manifold — a $300–900 repair in spring.
A pool closed without documentation creates a spring opening that starts from zero: what was the chemistry at closing? Which lines were blown? Where are the drain plugs? Did the skimmer have a Gizzmo or a plug? This information, captured in a service note in SplashLens, turns a 3-hour spring opening into a 90-minute one.
SplashLens stores every closing checklist, chemistry reading, and equipment note per pool — accessible from your phone at the next opening, no matter how much time has passed.
Open SplashLens Free →Water left in plumbing lines freezes and expands, cracking PVC pipes, splitting fittings, and potentially cracking skimmer bodies. Repair costs range from $200 to $2,000+ depending on where the damage occurs.
Failing to blow out the lines and plug returns in a freezing climate. This single oversight can crack every plumbing connection and skimmer from the equipment pad to the pool.
Closing without shocking, or closing with algae already present. Algae spores that survive under a winter cover bloom aggressively when temperatures warm.
Yes. Closing while water temperature is still above 60°F leaves algae and bacteria active under the cover. The stagnant, covered water with depleting chlorine creates ideal conditions for an algae bloom by spring.