Why Does My Hot Tub Smell? Fixes by Smell Type

Smell-to-cause table

This table is our own summary of the sources below. No source ranks the causes by how common they are, so work through the test order in the next section rather than guessing.

Test in this order

Test before you add anything. A smell is a symptom, and the numbers tell you which cause you have.

1. Free chlorine (FC). The CDC sets a minimum of 3 ppm in a hot tub. If FC is under 3 ppm and the water smells strongly of chlorine, that fits chloramines. Bromine tubs: the CDC minimum is 4 ppm bromine.
2. Total chlorine (TC). Combined chlorine (CC) is TC minus FC. The CDC says most health departments limit combined chlorine in public pools to 0.4 ppm or less. Our hot tub shock calculator uses 0.5 ppm or more as the trigger to shock. The page on free vs total chlorine explains the gap.
3. pH. The CDC range for hot tubs is 7.2 to 7.8. Out-of-range pH weakens chlorine, so fix it first. See pH too high and pH too low.
4. Water age and filter. If the numbers look fine but the smell stays, the cause is probably in the plumbing, the filter, or the water itself.

The CDC advises testing at least twice a day when a hot tub is in use. A home tub used a few times a week should be tested before every soak.

Chlorine or chemical smell

The CDC says that if you smell chlorine at a pool, you are probably smelling chloramines. They form when chlorine combines with urine, sweat, skin cells, and personal-care products. They irritate the eyes, skin, and nose, and they are weak sanitizers, so the tub can be under-protected while it smells strongly of "chlorine." Jacuzzi's guide says the same: a strong chlorine odor often indicates too little sanitizer, not too much.

The fix is to oxidize the chloramines with a chlorine shock, using the product and dose on your label. For a worked amount for your tub size, use the hot tub shock page. Leave the cover off and run the jets while the shock works, so the gas escapes. The CDC says to ventilate the swimming area during superchlorination, and a closed cover traps the smell.

Prevention is cheaper than shock. The CDC suggests rinsing off before getting in, since even a quick rinse removes much of the body waste that forms chloramines. A tub with a lot of soakers also needs water changes more often: see the hot tub water change calculator.

Bromine tubs can smell too. We could not confirm from a primary source what the smell is called or how it differs from chloramines, so check the hot tub bromine page and your product label.

Rotten egg or sulfur smell

Rotten egg is hydrogen sulfide gas. Two sources are possible, and the first check separates them: fill a clean glass with fresh water from your hose and smell it.

• The fill water smells. Well water is the usual cause. Vermont's health department lists sulfur bacteria and natural decay of vegetation or sulfur minerals as sources of hydrogen sulfide in wells. The tub is not at fault. Treat the source water, or fill from a clean one. See filling with well water for what else to test.
• The fill water is fine, the tub smells. Bacteria are growing in the tub, usually where chlorine or bromine has been low or the water is old. One commercial guide we found describes sulfur-reducing bacteria in the plumbing, but we could not confirm that cause from a primary source for hot tubs specifically.

Do not soak in a tub that smells like sulfur until you have treated it. Our advice for the tub itself follows the CDC's hot tub guidance on biofilm: drain, scrub vigorously, clean, and refill. The CDC says to scrub all surfaces every time the tub is drained, and to replace or clean the filter. See how to clean a hot tub filter, and use a new filter if the old one is past its life.

The CDC's higher-dose disinfection procedure (hyperchlorinating to 20 ppm for 10 hours) is for tubs linked to a Legionella case. If someone who used your tub is ill with fever, cough, or a rash, see a doctor and contact your local health department.

Musty or earthy smell

A musty smell that is strongest when the jets start usually means biofilm: a slime layer of bacteria that sticks to pipes and jets and resists normal sanitizer levels. The CDC names failing to scrub hot tub surfaces to remove biofilm as a factor that lets Legionella grow. Biofilm can also contain Pseudomonas aeruginosa, which the CDC links to hot tub rash.

Typical steps, from the sources we read:

1. Add a hot tub line flush product to the existing water, following its label. Jacuzzi's guide uses a pipe flush cleaner and runs the jets on high for 15 to 20 minutes. Other brands give different times, so go by your label.
2. Run every jet, valve, and diverter so the cleaner reaches all the plumbing. Foam is common and is a sign it is working.
3. Drain completely. Do not refill yet.
4. Wipe the shell and the waterline. Rinse well, because leftover cleaner makes foam. See foamy hot tub.
5. Clean or replace the filter, refill, and balance the water.

We have no published source for how often to flush the lines. Jacuzzi advises changing the water every 1 to 3 months depending on use, and the Pool & Hot Tub Alliance suggests every 3 to 4 months. Many owners flush the lines at each water change. That last point is our suggestion, not a standard.

If the smell is there with the cover on but the water tests fine and the jets smell fine, check the cover and the cabinet vents too. We could not confirm this from a source. It is a practical check, not a rule.

When draining is the answer

Shocking cannot fix every smell. Drain and clean when:

• Shock and a clean filter do not clear the smell within a day or two.
• The water is older than your usual change interval. Use the water change calculator. The CDC formula is days between changes = (tub gallons ÷ 3) ÷ average soakers per day.
• A sulfur or musty smell comes back after a line flush.
• The water is cloudy or green as well. See cloudy or green hot tub water.

After refilling, start fresh with the first-fill steps and keep your readings in range using the hot tub chemistry chart. Keep the water at 104 °F or lower: the CDC says hot tubs should not exceed 104 °F.

FAQ

Why does my hot tub smell like chlorine when I have not added any?

The smell is chloramines, not free chlorine. They form when chlorine reacts with sweat, oils, and lotions. Test FC and TC. If FC is low or CC is high, shock according to the product label.

Why does my hot tub water look clear but smell?

Clear water can still carry chloramines or biofilm. Test FC, TC, and pH. If those are in range, flush the lines and check the filter and the age of the water.

Why does my hot tub smell after I shock it?

The waste is being oxidized, and the by-products gas off. Keep the cover off with the jets running, wait until FC is back in the label range, and retest. If the smell persists after a day, suspect old water or a dirty filter.

Is a rotten egg smell dangerous?

Hydrogen sulfide gas can be hazardous at high levels in enclosed spaces, according to Vermont's health department. Do not soak until you have found the cause. Smell the fill water first.

How do I get a musty smell out of a hot tub?

Use a line flush product as its label says, run the jets, drain, scrub the shell, and clean or replace the filter. The CDC says to scrub every time you drain.

Sources

• CDC, Home pool and hot tub water treatment and testing (3 ppm chlorine, testing twice a day): cdc.gov
• CDC, Controlling Legionella in hot tubs (pH 7.2–7.8, 104 °F, biofilm, scrub and drain, water-change formula): cdc.gov
• CDC, Chloramines and pool operation (how chloramines form, 0.4 ppm combined chlorine, ventilation): cdc.gov
• Vermont Department of Health, Hydrogen sulfide in drinking water: healthvermont.gov
• Jacuzzi, How to clean a hot tub (strong chlorine odor, line flush, water changes): jacuzzi.com
• Pool & Hot Tub Alliance, Maintaining your hot tub (drain every 3–4 months): phta.org