Shocking a hot tub: chlorine shock vs non-chlorine

Sanitizer kills germs, but it doesn't clear out everything bathers leave: sweat, oils, hair spray and the spent chlorine itself build up and get in the sanitizer's way. A shock is a big dose of oxidizer that burns that load off. Hot Spring describes raising chlorine to 5 ppm for 24 to 48 hours to clear waste and chloramines.

Chlorine shock (dichlor at a high dose) oxidizes and disinfects. Non-chlorine shock, usually MPS (potassium monopersulfate), only oxidizes. Master Spas notes you can get back in about 15 minutes after a non-chlorine shock, but it does not disinfect; after a chlorine shock you wait for chlorine to fall to a safe level. Hot Spring says dichlor is about six times stronger than MPS and suits heavy use.

Labels follow the same line. EPA treats the word “shock” as a claim to control germs, so an oxidizer-only product should say it is neither a sanitizer nor an algicide. Read the label to see which one you bought.

Your product label decides. As an example, Hot Spring's manual gives 1½ teaspoons of dichlor or 4 tablespoons of MPS per 250 gallons for its tubs. Strengths differ between brands, which is why we don't scale that to other tubs. You need your tub's gallons: find them by model.

• On a schedule: Hot Spring says weekly or monthly depending on your sanitizing program.
• When free chlorine reads below total chlorine (Master Spas).
• When the water smells of chlorine, or when cloudy water or a musty odor shows up.
• After a heavy session with guests, since demand rises with every bather.

After a chlorine shock, when free chlorine has dropped below 5 ppm (Hot Spring). After a non-chlorine shock, Master Spas says about 15 minutes. Either way, leave the cover open while the gas vents: Hot Spring says at least 20 minutes with the jets on, and treats cover and pillow damage from trapped gas as chemical abuse outside the warranty. Master Spas says 15 minutes.