Hot tub line flush: how and when to clean the plumbing

A hot tub line flush cleans the inside of the plumbing. Do it right before every drain: remove the filter, add a line flush product per the label, run all the jets for the time stated (usually 20 to 60 minutes), then drain, wipe the shell, and refill. It removes the biofilm and oils that make new water go cloudy or foamy within days.

What you are cleaning out

A hot tub has 50 to 100 feet or more of pipe and hose behind the shell. The inside of that plumbing is warm, dark, and coated with a thin layer of body oils and lotion. Bacteria settle on it and build a slimy protective film called biofilm.

Normal chlorine or bromine levels kill free-floating bacteria but do not penetrate established biofilm well. So the biofilm sits there, consuming sanitizer and shedding into the water. Draining alone does not remove it. A line flush product loosens it so it leaves with the old water.

Signs you need one

• Sanitizer drops to zero overnight with nobody using the tub.
• Water goes cloudy within a week of a fresh fill.
• Flakes or specks (white, brown, green, or black) shoot out when the jets start.
• A musty smell that shock does not fix.
• Foam with no obvious cause.
• You just bought a used tub, or the tub sat with stagnant water.
• It has been more than one water change since the last flush.

How to do a line flush

1. Do it on drain day. Flush the old water, not the new.
2. Remove the filter. Turn off power at the breaker, pull the cartridge, then restore power. You do not want the released gunk caught in the filter. Clean the filter separately while the flush runs.
3. Check the water level. It must be above the highest jets.
4. Open everything. Turn every jet fully open. Open the air controls. Set diverter valves to the middle so water reaches all zones.
5. Add the line flush product per the label for your tub's volume. Pour it into the water near the filter intake.
6. Run all pumps on high for the time on the label. Turn on waterfalls and any secondary pumps. If the jets time out, restart them. Halfway through, move each diverter to each end position for a few minutes.
7. Watch. Foam and floating debris are normal. A dirty system can produce a surprising amount of brown or gray scum.
8. Turn off power at the breaker and drain. See how to drain a hot tub.
9. Wipe the shell. Residue will cling at the old waterline. Wipe it off with a soft cloth and rinse. A wet/dry vacuum on each jet pulls out more loosened debris.
10. Refill through the filter compartment with a clean or new filter installed, then follow startup after draining.

Follow the product label on dose and time. Do not combine a line flush with other chemicals, and do not add shock at the same time unless the label says to.

If the flush was really dirty

Heavy scum on the first flush means there was a lot of buildup. In that case:

• Refill only to just above the jets, run a second flush, and drain again.
• Replace the filter instead of cleaning it. An old cartridge can carry biofilm back in.
• After the final refill, shock with chlorine and hold sanitizer in range for a few days before soaking.

What a line flush does not do

Problem	Does a line flush fix it?	What does
Biofilm, oils, lotion residue in pipes	Yes	Line flush before draining
Calcium scale in the heater and lines	Mostly no	Balanced pH and hardness, scale control. See the calcium hardness guide.
Dirty filter	No	Filter cleaner soak or a new cartridge
Weak jets from an air lock or closed valve	No	Bleeding air, opening valves
Old water with high dissolved solids	No	Draining and refilling

New and used tubs

New tubs are water-tested at the factory and can sit damp for months before delivery. Used tubs often sat stagnant. Either way, it is worth filling once, flushing, and draining before the first real fill. If a brand-new tub has cloudy water or vanishing sanitizer in its first weeks, this is usually why.

Keeping the lines clean between flushes

• Never let sanitizer sit at zero. Biofilm gets established during those gaps.
• Shock weekly. See hot tub shock.
• Run every pump and open every jet and diverter at least once a week so no line sits stagnant.
• Shower before soaking to cut the oil load.
• Keep the filter clean.
• Change water every 3 to 4 months and flush every time.

Is the foam during a flush a problem?

No. Foam is the product lifting oils out of the plumbing. If it threatens to overflow, turn the air controls off or drop the pumps to low for a few minutes. It all leaves with the drain water.