Biofilm in hot tub pipes, and flushing the lines
Short answer
Biofilm is a slime of microorganisms stuck to the pipes and shell, where sanitizer struggles to reach it. Master Spas' manual says to run a biofilm product before you drain, and CDC's guidance says to scrub every surface each time the tub is emptied.
What is biofilm in a hot tub?
A colony of microorganisms that stick to each other and to a surface, such as the inside of the plumbing or the shell. It builds up over time with use. Because it clings to surfaces instead of floating in the water, draining alone leaves it behind; CDC's answer is to scrub every surface each time the tub is drained. CDC lists sediment and biofilm among the main conditions that let Legionella bacteria grow in hot tubs.
How do you flush hot tub lines?
- Before draining, add a biofilm or line flush product as its label directs.
- Run the jets for the time the label gives, with the cover open.
- Drain the tub, then scrub every surface.
- Clean or replace the filters, since they hold what the flush knocked loose.
- Refill and start up as normal.
Master Spas says to do this periodically before draining, and lists a system flush as the next step when water odor persists after sanitizer is corrected. See how to drain and refill a hot tub.
How much to add
The product label sets the dose, by your tub's gallons.
Strengths differ between brands of the same chemical, so we don't print amounts. Test, read the label, add a little at a time with the jets running, wait, and test again. Your tub's gallons are in its manual or in our gallons and filter size table.
What are the signs of biofilm?
Odor that comes back after you have fixed the sanitizer. That is the point where Master Spas' table moves on from adjusting sanitizer to a system flush. See also hot tub smells.
Common questions
What is hot tub biofilm?
A slime of microorganisms that sticks to the plumbing and shell. Master Spas' manual says it builds up over time with use.
How often should you flush hot tub lines?
Periodically before a drain, per Master Spas, and whenever odor returns after the sanitizer is corrected.
Is hot tub biofilm dangerous?
CDC lists biofilm among the conditions that let Legionella bacteria grow in hot tubs, which is why it says to scrub surfaces at every drain.
Sources
2
- Master Spas Twilight Series owner's manual, 2026
- CDC: Controlling Legionella in hot tubsControl limits, the water replacement formula, scrubbing and filter care (written for public hot tubs)
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