When to Close
Close when the water temperature consistently stays below 65°F (18°C). In most northern US states, that's late September to mid-October. Closing while the water is still too warm means algae can keep growing under the cover — and you'll open to a swamp in spring.
Why temperature matters
Algae growth drops dramatically below 60°F and essentially stops below 50°F. By closing when the water is cold, you ensure the closing chemistry (especially the final shock) lasts through winter without being consumed by biological growth.
What You'll Need
Full test kit
FC, pH, TA, CH, CYA — you need all five balanced before closing. Poor closing chemistry = green opening.
Liquid chlorine for final shock
Bring FC to shock level before covering. 2–3 gallons of 10–12.5% for a typical pool.
Shop vac or air compressor
For blowing out the plumbing lines. Compressed air pushes water out of pipes so it can't freeze and crack. A shop vac on "blow" mode works for smaller setups.
Winterizing plugs (Gizzmos)
Threaded expansion plugs for return fittings and skimmer(s). They seal the lines after blowing and absorb ice expansion in the skimmer. An air pillow floated under the cover does the same job for the pool itself — the ice pushes in on the pillow instead of on the walls.
Winter cover
Solid cover (blocks all light — best for algae prevention, needs a cover pump) or mesh safety cover (drains rainwater through, no pump needed, but lets some fine debris through).
Step-by-Step Pool Closing
Clean the pool thoroughly
Skim, brush every surface, and vacuum the entire pool. Any debris left in the water will decompose over winter, staining surfaces and consuming chlorine. This is the most important step — a clean close means a clean open.
Balance the water chemistry
Test all five parameters and adjust to these closing targets:
| Parameter | Closing target |
|---|---|
| pH | 7.2–7.6 |
| Total Alkalinity | 80–120 ppm |
| Calcium Hardness | 200–400 ppm |
| CYA | 30–50 ppm |
Balanced chemistry protects the pool surface from staining and scale over winter and makes the spring opening far easier.
Shock the pool
Hit FC to your CYA-based shock level (10× CYA for SLAM, or at least 10–15 ppm for closing). This closing shock is your last line of defense — it needs to carry the pool through months without circulation. Use liquid chlorine (not cal-hypo, which raises calcium).
Use the Shock Calculator for the exact dose.
Run the filter for 24 hours
Let the pump circulate the shock dose for at least 24 hours to distribute it evenly and filter out any remaining debris. Backwash or clean the filter at the end — you don't want dirty filter media sitting all winter.
Lower the water level
Lower the water 4–6 inches below the skimmer opening (for solid covers) or to the bottom of the tile line (for mesh covers). This protects the skimmer and tile from ice expansion. Don't drain too low — the water weight holds the pool structure in place, especially vinyl-liner and fiberglass pools.
Blow out the plumbing lines
This is the critical freeze-protection step. Use a shop vac (on blow mode) or air compressor to push water out of every plumbing line — skimmer lines, return lines, main drain line, and any auxiliary lines (spa jets, water features, slide). Blow until air bubbles vigorously from the return fitting or skimmer, then immediately plug that line with a winterizing plug.
Freeze damage is expensive
Water left in a pipe expands when it freezes and cracks the pipe underground. Repairing a cracked underground return line can cost $500–2,000+. Blowing lines takes 30 minutes. This is the single most important part of closing in freeze climates.
Drain equipment
Remove the drain plugs from the pump, filter, heater, chlorinator, and any other equipment. Store the plugs in the pump strainer basket so you can find them in spring. Leave the multiport valve between positions (sand filter) or disassemble cartridge elements and store indoors. If you have a salt cell, remove it, clean it, and store it inside.
Install skimmer plugs and return plugs
Insert a Gizzmo (expansion plug) into each skimmer throat — it absorbs ice pressure so the skimmer doesn't crack. Plug all return fittings with threaded winterizing plugs. Make sure each plug is snug but not overtightened — you don't want to crack the fitting.
Add antifreeze to lines (optional)
In very cold climates, pour pool-grade propylene glycol antifreeze (never automotive ethylene glycol — it's toxic) into any lines you couldn't fully blow out, especially the main drain line. A gallon per line is typical. This is insurance — if blowing didn't get all the water, the antifreeze prevents ice damage.
Install the winter cover
Secure the cover tightly. For solid covers: make sure the cover pump is in place and working — standing water on a solid cover can collapse it into the pool. For mesh safety covers: anchor every spring and strap. Remove ladders, handrails, and any other items from the pool before covering.
Freeze Protection Summary
The entire closing process exists to prevent freeze damage. Here's the priority:
Underground pipes — highest risk, highest cost
Blow every line and plug it. Underground pipe repairs are $500–2,000+. This is where most freeze damage money goes.
Equipment — pump, filter, heater
Remove all drain plugs. A cracked pump housing is $200–500 to replace, and a cracked heater heat exchanger is often the cost of a new heater ($1,500+).
Skimmer — ice expansion
A Gizzmo plug absorbs ice expansion. Without it, the skimmer body can crack — a $300–800 repair on gunite pools, or a liner leak on vinyl pools.
Common Closing Mistakes
Closing while the water is still warm
Algae grows in warm, stagnant water — even under a cover. If you close in September when the water is 75°F, the shock chemistry may not last. Wait until water drops below 65°F consistently.
Not blowing the lines completely
A little water left in a line is all it takes. Blow until you see strong air bubbles from the pool side, then plug immediately. Don't assume "most of the water" is good enough.
Using algaecide instead of chlorine
Algaecide is a supplement, not a sanitizer. A pool closed without adequate chlorine will grow algae regardless of algaecide. Shock properly — that's what prevents a green opening.
Draining the pool too low
The water supports the pool structure. Draining a vinyl-liner pool too far can cause the liner to shrink and pull away from the walls. Fiberglass pools can "float" out of the ground if groundwater pressure exceeds the pool weight. Lower to 4–6 inches below the skimmer, not further.