Why does my hot tub water smell bad even though chlorine looks fine?

[Manikshaw]

I tested the water and chlorine levels show up normal but the spa has a weird odor every time I open the cover. It doesn’t smell like the usual chlorine smell, more like musty water. I just added chemicals a few days ago so I don’t get why it stinks already

 

[syedsam]

I had that same smell last winter and it was biofilm in the lines. Chlorine can show fine on the strip but bacteria hides in the pipes. I ran a purge product through mine, drained, refilled, and the smell was gone. Might be worth trying if you haven’t cleaned the lines in a while.

 

[Mason Blake]

Even when chlorine levels look fine, a hot tub can still smell musty because sanitizers don’t always reach every nook and cranny. Oils, lotions, and bacteria can build up in the plumbing lines, filters, and around jets. Over time, this creates a lingering odor that strips or sanitizer alone can’t fix. Doing a line flush, cleaning or replacing filters, and scrubbing surfaces like headrests and the cover usually clears it up. If the water has been in the tub for a while, a full drain and refill can also help reset everything.

 

[Ella Robinson]

Your chlorine might be fine, but that musty smell is usually biofilm in the lines. I’d flush/purge the lines, clean or replace the filter, and if it’s been a while, drain and refill. That’s what works at my place.

 

[Harper Clark]

I’ve had that same musty smell usually it’s biofilm in the lines, not your chlorine. What works for me: run a purge/line flush, clean or replace the filter, then do a full drain and refill. After refilling, pop the cover and run the jets for a bit, then keep pH/chlorine steady. Try skipping lotions before you soak. If you want, I can walk you through it.

 

[Wei Cai]

I’ve had that same musty smell too, and it ended up being biofilm in the lines, not the chlorine level. I just ran a purge, swapped the filter, then did a drain and refill. The smell disappeared right after.