How to Flush Hot Tub Lines and Refill
Add a line flush (spa purge) cleaner to the old water before you drain, run the jets for the time on its label, then drain, rinse, and refill. In the example, a label dose of 4 fl oz per 400 gallons treats a 400-gallon tub, and refilling it with one garden hose at 5 gallons per minute takes about 1.3 hours.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (9 sections)
A line flush cleans the inside of the plumbing, where a scrub brush can't reach. The calculator scales the cleaner's label dose to your tub and estimates how long the refill will take with your hose. The example label numbers are placeholders, not a recommended dose; use the numbers on your product. Always follow the product label.
Why flush the lines
Hot tub plumbing stays warm and wet, and a slimy layer called biofilm can grow on the inside of the pipes, jets, and air channels. Biofilm shields germs from sanitizer and can come loose as flakes. The CDC names sediment and biofilm as key factors in Legionella growth in hot tubs, and a CDC outbreak report notes that biofilm can protect Pseudomonas even when disinfectant levels are adequate. Jacuzzi's cleaning guide likewise describes biofilm as resisting normal sanitizers. Draining and refilling replaces the water but leaves the pipes as they were, which is why a flush cleaner goes in before the drain.
Signs your lines need a flush
- Brown, gray, or white flakes come out of the jets when they start.
- Sanitizer disappears fast even in fresh water.
- Foam or a musty smell comes back soon after a refill.
- The tub sat unused, uncovered, or unsanitized for a while.
Even without signs, many owners flush at every drain and refill. How often to drain is on the hot tub water change calculator; this page is the procedure.
What you need
- A line flush or spa purge product labeled for hot tubs.
- A garden hose, and a submersible pump if you want a faster drain.
- A spa filter cleaner, or a spare filter cartridge.
- A soft cloth and a non-foaming spa surface cleaner.
- Your test kit and start-up chemicals for the fresh water.
Step by step: flush, drain, clean, refill
- Measure the cleaner. Scale the label dose to your tub with the calculator. Add it to the old water as the label directs. Some labels (Spa Purge, for example) say to take the filter out first so the gunk doesn't clog it.
- Run the jets. Turn the jets and air controls on for the time on the label, cycling each pump if your tub has more than one. Keep the cover open while they run.
- Skim the foam and flakes that rise, so they don't settle back into the tub.
- Turn off the power at the breaker before you drain, so the heater and pumps can't run dry.
- Drain the tub through its drain valve with a garden hose, or with a submersible pump. Check your local rules on where spa water can go.
- Rinse and wipe the shell. Spray loosened residue out of the footwell and wipe the surface with a non-foaming spa cleaner. Rinse well so no cleaner ends up in the new water.
- Clean or replace the filter. Biofilm from the lines ends up in the cartridge. Soak it in filter cleaner, or swap in a clean spare. See hot tub filter cleaning.
- Refill with the filter in place, through the filter housing or the fill pipe your manual names, not straight into the footwell (see below).
- Restore power, check that every pump runs and water flows from the jets, and let the tub heat.
- Balance and sanitize the new water before the first soak.
How long it takes
| Stage | How long |
|---|---|
| Flush cleaner with jets running | The time on your product's label: from about 5 minutes plus an overnight soak to about 30 minutes, depending on the product (see below) |
| Drain | Depends on your drain valve or pump; a submersible pump is much faster than a hose |
| Rinse, wipe, and filter | Usually under an hour |
| Refill | Gallons ÷ hose flow: 1.3 hours for 400 gallons at 5 gal/min in the example |
| Heat-up | Several hours; see hot tub not heating for the math |
To find your hose flow, time how long it takes to fill a 5-gallon bucket and divide 5 by the minutes. Enter that number in the calculator.
| Product or guide | What the directions say |
|---|---|
| Leisure Time Jet Clean | Run jets 15 minutes, let it sit 1 hour, run jets and blower 15 minutes, then drain |
| Natural Chemistry Spa Purge | Filters out, water at least 95 °F, jets at least 5 minutes, then circulate overnight with jets off |
| Ahh-Some | Run jets and blower up to 30 minutes, then drain and rinse |
| Jacuzzi cleaning guide | Run jets on high 15–20 minutes with a pipe flush before draining |
We read these directions on retailer and maker pages; your bottle's label is the one to follow.
Refill without an air lock
An air lock is a bubble trapped in the pump or heater plumbing after a refill. The pump hums but moves little water, the jets sputter, and the heater won't run. Hot Spring's troubleshooting guide lists an air lock after filling as a cause of no heat. To avoid one:
- Fill through the filter housing. Jacuzzi says to put the garden hose directly in the filter housing to prevent air locks.
- On tubs with a fill pipe, use it. Hot Spring says to fill from the correct fill pipe instead of putting the hose into the shell; on some models it is the standpipe under the filter.
- Fill to the level in your manual before you turn the power back on.
- If the jets still sputter, follow your manual's steps to purge air, or call your dealer.
First-day chemistry
Fresh water has no sanitizer, and its pH and alkalinity come from your tap. Test, then bring alkalinity, pH, and sanitizer into range before anyone soaks. The order and the reasons are on the beginner's guide to hot tub chemicals, and chlorine doses by tub size are on hot tub chlorine. If foam shows up in the new water, leftover cleaner is the usual cause: see foamy hot tub.
Safety
Turn off power at the breaker before draining, and keep it off until the tub is full again. Bullfrog warns that an empty tub must not be left in direct sun, so refill soon after cleaning. Never mix a line cleaner with chlorine, bromine, or other chemicals in a bucket. Wear gloves and eye protection. Don't soak in the tub while the cleaner is in the water. Poison Control: 1-800-222-1222. Always follow the product label.
FAQ
How often should you flush hot tub lines?
A common habit is to flush at every drain and refill, and whenever you see flakes or foam from the jets. Leisure Time's Jet Clean directions say every 3 months or at each drain and refill. Hot Spring and Bullfrog suggest draining every 3–4 months.
How long do you run a hot tub line flush?
For the time on the product's label, with all jets and pumps running. Times differ by product.
What is the gunk that comes out after a line flush?
Mostly loosened biofilm, body oils, and residue from the pipes. That is what the flush is meant to remove. Skim it off and drain the tub.
Can you use bleach or vinegar to flush hot tub lines?
We don't recommend it. Household products aren't labeled for this use, and some can foam or harm seals. Use a product labeled as a spa line flush, or ask your tub maker.
Sources
- CDC, Controlling Legionella in hot tubs (sediment and biofilm): cdc.gov
- CDC, MMWR 2024, Pseudomonas outbreak (biofilm protects bacteria): cdc.gov
- Jacuzzi, How to clean a hot tub (pipe flush; fill through the filter housing): jacuzzi.com
- Hot Spring, How to drain a hot tub and refill it (fill pipe; every 3–4 months): hotspring.com
- Hot Spring, Complete troubleshooting guide (air lock): hotspring.com
- Bullfrog Spas, Draining, cleaning and refilling: bullfrogspas.com
- Leisure Time Jet Clean directions (retailer copy, secondary): inyopools.com
- Natural Chemistry Spa Purge directions (retailer copy, secondary): inyopools.com
- Ahh-Some directions (retailer copy, secondary): hottuboutpost.com
- Dose and refill time: label scaling and gallons ÷ hose flow; see methodology.