How to shock a hot tub, and when you should

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“Shocking” a hot tub means adding a large dose of oxidizer to burn off the sweat, oils and spent chlorine (chloramines) that build up with use. It is a reset on top of your normal sanitizer, not a replacement for it.

This is general water-care guidance. Your spa’s owner’s manual and your product’s label win wherever they differ.

What “shock” means

Your everyday sanitizer (chlorine or bromine) has to be there at a steady level whenever the tub is in use. Shock is different: an occasional, bigger dose that clears the backlog.

Hot Spring’s Hot Spot owner’s manual describes a weekly or monthly super chlorination or shock as something your sanitation program may require. It says raising chlorine to 5 ppm for 24 to 48 hours “removes excess waste and chloramines from the water.”

Chlorine shock vs non-chlorine (MPS) shock

Chlorine shock (super chlorination) raises chlorine well above its usual level for a while. Hot Spring’s manual uses sodium dichlor for super chlorination and calls it 6 times stronger than MPS. CDC, though, lists dichlor among chlorine products that contain cyanuric acid, and recommends not using cyanuric acid or products with it in home hot tubs. If you use chlorine shock, pick a spa product without it (check the label).

Non-chlorine shock is usually potassium monopersulfate (MPS). Hot Spring describes MPS as an oxidizer used to prevent contaminant buildup, maximize sanitizer efficiency, minimize combined chlorine and improve water clarity, and says it is “an oxidizer, not a sanitizer.” It is used alongside an EPA-registered sanitizer, never instead of one.

The weekly oxidizer planned for Cycle Box is MPS too: a non-chlorine oxidizer, not a sanitizer. More on the weekly routine in hot tub foam fixes.

When to shock

On your program’s schedule. Hot Spring frames it as weekly or monthly, depending on the sanitation program you use.
After heavy use. Hot Spring notes that sanitizer demand goes up sharply when usage climbs, for example with invited guests.
When the tub smells strongly of chlorine. See below.
When the water turns cloudy and sanitizer is low. Cloudy hot tub water walks through the order to check things.

A strong chlorine smell means chloramines, not too much chlorine

Hot Spring’s manual says free chlorine “does not have an odor whereas combined spent chlorine (chloramines) has a strong chlorine odor that can be eliminated by shocking the water.” Its advice when the spa smells of chlorine: super chlorinate or shock.

So a sharp chlorine smell is a sign to test and shock, not to skip your sanitizer.

Step by step

Test first. Check free chlorine (or bromine) and pH. CDC’s guidance for hot tubs is free chlorine at least 3 ppm and pH 7.0–7.8.
Read the label. Use a product made for spas, and measure the dose the label gives for your tub’s volume.
Jets on, cover off. Hot Spring says to add spa chemicals into the filter compartment with the jet pump on high, and to run it at least ten minutes with the cover off.
Let it vent. After a super chlorination or non-chlorine shock, Hot Spring says to run all jets and leave the cover open for at least 20 minutes so the oxidizer gas can vent. Trapped gas can discolor and damage the cover and pillows.
Wait before soaking. Hot Spring says to let free chlorine fall below 5 ppm before using the spa.
Retest. Confirm sanitizer and pH are back in range before anyone gets in.

Never leave an open spa unattended, especially with children around.

Stay at the floors

Shock does not change the everyday rule. Keep free chlorine at 3 ppm or higher (or bromine 4–8 ppm) whenever the tub is in use, with your own registered product. CDC recommends testing chlorine and pH at least twice a day, and more often when a number of people are using the tub. Nothing planned for Cycle Box replaces sanitizer.

FAQ

Is shock the same as sanitizer?

No. Sanitizer is the steady level you keep all the time. Shock is an occasional big oxidizer dose. Non-chlorine shock (MPS) is “an oxidizer, not a sanitizer,” in Hot Spring’s words.

How much shock do I add?

Follow the label on your product for your tub’s volume. Spa makers and products differ.

Can I get in right after shocking?

Not until levels are back in range. Hot Spring says to let free chlorine fall below 5 ppm first.

Why does my tub smell like chlorine when the strip reads low?

That smell comes from chloramines (spent chlorine), per Hot Spring. Shock, then retest.

What to do next

Try Water Match to see whether your fill water runs Soft, Balanced or Hard.
Join the free founding list for Cycle Box, which isn’t on sale yet.

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