How to Open a Pool for the Season

When to Open

Open your pool when daytime temperatures are consistently at or above 70°F. The NSPF Pool & Spa Operator Handbook calls 70°F the safest single trigger, and we agree. By the time water sits there day after day, algae can grow in anything above 60°F, so waiting longer just makes opening harder. Many pool owners hold off until Memorial Day, but in southern states, that can mean letting algae establish under the cover for weeks.

If your pool has a mesh safety cover, rainwater passes through and nutrients accumulate. A pool closed with a solid cover needs a pump-off before opening: running a small submersible pump to remove the standing water on top of the cover.

Equipment First, Chemistry Second

Don't add any chemicals before the pump is running. Equipment startup:

1. Remove cover carefully. Avoid dumping debris into the pool
2. Remove winterizing plugs from return lines and skimmer
3. Reinstall drain plugs on the pump and filter
4. Reconnect return fittings, eyeballs, and skimmer baskets
5. Fill the filter with D.E. (if applicable) or confirm sand level
6. Prime the pump and start equipment
7. Check for leaks at all unions and fittings. Fix before adding chemicals
8. Run for at least 30 minutes before testing water

Test Before You Add Anything

The most important step in pool opening is testing first. Adding chemicals blind is how pool owners overshoot targets and create new problems. Test:

• FC (free chlorine): expect near zero after winter
• pH: target 7.2–7.4
• TA (total alkalinity): target 80–100 ppm (per the ANSI/APSP/ICC-11 standard)
• CYA (cyanuric acid stabilizer): target 30–50 ppm for chlorine pools
• Calcium hardness: target 200–400 ppm

Use a proper liquid test kit (Taylor K-2006 is the gold standard) or take a water sample to a pool store. Test strips are adequate for quick checks but not precise enough for opening chemistry.

The Correct Order for Adding Chemicals

Per the ANSI pool-industry standard, chemical additions must be sequenced to avoid interference:

1. Total alkalinity first. Add baking soda (sodium bicarbonate) if TA is below 80 ppm (the lower bound in the ANSI/APSP/ICC-11 standard). Use the alkalinity calculator for the exact dose. Add in one dose and wait about 6 hours before retesting. TA is the buffer for pH. Fix it before adjusting pH.
2. pH second. Once TA is in range (80–100 ppm), adjust pH to 7.2–7.4 with muriatic acid (to lower) or soda ash (to raise). Get the exact dose. Never add acid and base in the same session.
3. CYA third (if needed). If CYA is below 30 ppm (the floor in industry guidance), add stabilizer through the skimmer basket in a mesh sock. Calculate the exact CYA dose. CYA dissolves slowly. It takes 7–10 days to fully register in a test.
4. Shock last. With TA and pH correct, shock at dusk. Target 10 ppm FC for a standard opening (per Trouble Free Pool community standard). Use cal-hypo 68%, not trichlor tablets, which would raise CYA.

Common Opening Mistakes

1. Adding shock before balancing pH. Shocking at pH 7.8+ wastes half your chemical. HOCl (the active killing form of chlorine) is only 33% efficient at pH 7.8 vs 66% at pH 7.2.
2. Adding algaecide as a substitute for shock. Algaecide is a preventive, not a treatment. It will not clear an established algae bloom and adds unnecessary chemicals.
3. Using the previous year's CYA as a baseline. CYA does not evaporate. It accumulates. If you used trichlor tablets all last season, your opening CYA could be 70+ ppm before you add anything.
4. Skipping calcium hardness. Low calcium (below 150 ppm) causes plaster pools to leach calcium from the surface, causing pitting and etching. If you have a plaster pool, check CH every opening.

Frequently Asked Questions

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