How To Shock a Pool

My husband and I dread closing up our pool for the winter. Not because it’s particularly complicated, but because it signals the end of those relaxing, carefree summer days.

And while cold temperatures might be a shock to our systems, it turns out a shock is just what a pool needs before winter. Shocking a pool is a routine part of pool care and essential to pool winterization if you’re closing it down for the season.

Read on to learn why you need to shock your pool, and when and how to do it.

Why Should You Shock Your Pool?

In simplest terms, shocking a pool means adding a large dose or “shock” of concentrated chlorine, either powder or liquid, to the water. It rapidly rebalances and adjusts chlorine levels in the water.

Shocking raises the levels of “free” chlorine molecules that find and eliminate bacteria, algae, and other elements that can discolor or cloud the water or affect pool equipment performance. Pools need to be shocked periodically during the months they’re in use — more on that below — and before winterization. Here’s how to drain a pool for passive winterization.

When Is the Best Time To Shock Your Pool?

When it’s time to winterize, introduce a chlorine shock at least two days before you cover your pool. This lets the chlorine disperse and off-gas. And whether you cover your pool or not, always shock first, then add an algaecide a couple of days later.

Never shock and treat for algae at the same time or on the same day, because the intense dose of chlorine will negate the algae-eating properties of the algaecide. For winterizing, consider an algaecide specifically formulated for low temps.

For the rest of the year, pool pros recommend you shock your pool about once a week. Consider an extra shock if you’ve had a big crowd in the pool, if it’s been especially hot and sunny, or if it’s rained a lot. Rain can increase the pH of your pool, which reduces the effectiveness of chlorine.

If you use your pool regularly, shock it on a weekday evening. Everyone must wait 24 hours before they can use the pool again.

Note that even saltwater pools, which create their own chlorine, need to be shocked periodically and before winterization.

How To Shock a Pool

Shocking a pool isn’t rocket science, but you should observe a few rules as you proceed.

• Test your pool chemistry first with test strips or a testing kit.
• Adjust chemicals as needed so that pH, alkalinity and calcium hardness are at desired levels before shocking.
• Start with granulated or liquid pool shock.
  • Liquid shock can be added directly to pool water.
  • Granular shock needs to be dissolved in a bucket of water before going into the pool. Note: Never do it the other way around. The granular shock needs to go into the water; you can’t add water to the shock.
• Use one pound of shock (liquid or granular) per 10,000 gallons of water.
• Turn on the pool pump.
• Add the liquid or dissolved granular shock to the main part of the pool, not the skimmer.
• Run the pool pump for at least eight hours after adding shock to disperse the chlorine.
• Measure the chlorine levels, which should be three to five parts per million (ppm). If the water hasn’t reached that after 24 hours, repeat the process. Note: If chlorine levels are only a little low, try a half-dose the second time.
• If you over-shock the pool and chlorine levels are too high, turn off any automatic chlorination, wait a few days and test again. If levels haven’t receded, keep the pool uncovered and exposed to sunlight so some of the chlorine burns off. Note that chlorine levels of about five ppm are not safe for swimming.
• If you’re shocking a pool while it’s in use, wait 24 hours before letting anyone swim. Be sure to test your pool chemicals to make sure the water is balanced before you let anyone dive in.

Granular, Liquid, or No Chlorine Shock at All?

When deciding what type of shock to use in your pool, you’ll find proponents of liquid or granular chlorine shock, or even chlorine alternatives. Here’s a little information on the differences:

• Granular chlorine shock is more expensive, easy to transport, dissolves easily and can be stored for long periods of time. But it must be separately dissolved in water before added to the pool.
• Liquid shock is heavy and harder to transport. But it costs less than granular shock and doesn’t need premixing. Most pool supply stores sell liquid chlorine shock in refillable containers, eliminating waste.
• Chlorine-free shock will clear up water and eliminate some organic debris. It’s also easier on your pool liner and equipment. But it doesn’t kill algae or bacteria, and is generally recommended only when the pool needs cleaning and chlorine levels are already too high.