How to Drain and Refill a Hot Tub (Step-by-Step)
Drain your hot tub every 3-4 months. Before draining, add a system flush product and run the jets for 30 minutes to clear biofilm from the lines. Then drain completely, clean the shell and filter, refill with pre-filtered water, add a sequestering agent, circulate 30 minutes, and balance chemistry in order: total alkalinity first, pH second, sanitizer last.
As one long-time spa owner put it on DIYourself forums: “Changing the water is relatively simple compared to messing around with chemistry.” We agree. When your spa water has become a constant battle, a fresh start beats fighting chemistry indefinitely. We also recommend staging all your post-fill chemicals before you start draining, so there is no wait time once the tub is full.
For the full scope of what a drain-and-refill fits into, see our hot tub maintenance guide.
When should you drain and refill?
The standard interval is every 3-4 months for residential spas with regular use, per SwimUniversity’s hot tub water change schedule{:target=“_blank”}. Master Spas recommends every 6 months for well-maintained spas, though their interval assumes consistent filter care and regular shocking. Drain sooner if any of these apply:
- TDS above 2,500 ppm, total dissolved solids accumulate from chemical additions, sweat, and skin oils. Once TDS exceeds 2,500 ppm, chemistry becomes erratic and unreliable.
- Persistent foam despite treatment, foam that keeps returning after defoamer means your water is saturated with organic compounds.
- Musty or chemical odor, “chemical smell” is often chloramines, not excess chlorine. If shocking doesn’t clear it, the water is spent.
- Water that won’t clear after 24 hours of correct chemistry, at this point, further chemical treatment wastes money and time.
When chemistry feels like a constant battle instead of routine maintenance, it’s time. Drain intervals: every 3-4 months is the most common recommendation for residential hot tubs. Master Spas recommends every 6 months for well-maintained spas; drain sooner if TDS exceeds 2,500 ppm or foam won’t clear.
Video guide
Video: “How to DRAIN and CLEAN a HOT TUB” by Swim University
What you need
Gather everything before you start. You don’t want to discover you’re missing the pre-filter while the tub is empty and water is running:
- System flush/purge product (Ahh-Some, Natural Chemistry Spa Purge, or equivalent)
- Submersible pump or access to your spa’s drain spigot and a garden hose
- Garden hose for refilling
- Pre-filter for garden hose (removes chloramines, iron, and minerals; around $15-25)
- Spa surface cleaner and soft cloths
- Filter cleaning supplies (rinse solution and overnight soak solution if a deep clean is due)
- Sequestering agent / metal remover
- Chemistry test kit (full-panel, not strips)
- Post-fill chemicals staged and ready, have your total alkalinity increaser, pH adjusters, and sanitizer prepared before you start draining
Step-by-step: draining your hot tub
Step 1: add system flush (30 minutes before draining)
Turn the jets to all available settings. Add your system flush or purge product per the label. Run for 30 minutes.
Before draining, add a system flush product and run the jets for 30 minutes. The grey or brown foam that appears is biofilm from inside the plumbing lines, the same biofilm that would otherwise contaminate your fresh fill.
Both SwimUniversity and Master Spas drain and refill protocol{:target=“_blank”} recommend this step. Skip it and you’re seeding your fresh water with whatever built up in the lines over the past 3-6 months.
Step 2: turn off power at the breaker
Switch the spa off at the dedicated circuit breaker, not just at the control panel. The control panel power-off may not cut all circuits. Verify power is fully off before draining, running a pump dry even briefly can damage it.
Step 3: connect your drain method and start draining
Locate the drain spigot, which is typically at the base of the equipment compartment. Attach a garden hose to the fitting and direct the other end away from your foundation and toward an appropriate drainage point.
Alternatively, a submersible pump placed in the footwell drains faster, useful for larger 400-500 gallon spas. A 1/3 HP submersible pump can empty most spas in 30-60 minutes versus 1-3 hours for gravity drain.
Step 4: open the drain and monitor
Full drain time runs 1-3 hours for a 300-500 gallon spa. While the spa drains, use the time productively: clean the cover, inspect the equipment compartment, check the seal around the equipment access door, and inspect the jets for visible scale buildup.
Step 5: clean the shell
Remove remaining water with a sponge or shop vac. Apply spa surface cleaner to the shell interior, wipe with a soft cloth, and pay attention to the waterline scum ring where oils and minerals concentrate.
Rinse the shell thoroughly. Any cleaner residue left behind will foam in your fresh fill. If you see mineral scale on jets, a wipe with white vinegar breaks it down without leaving harmful residue.
Step 6: clean or replace the filter
This is the time to do a proper clean filter during drain. At minimum, rinse the cartridge. If a monthly deep soak is overdue, remove the filter and soak it overnight in filter cleaning solution, reinstall a clean cartridge before refilling. Never run the pump without the filter installed.
Step-by-step: refilling your hot tub
Step 7: attach a pre-filter to your garden hose
A pre-filter on your garden hose removes chloramines, iron, and dissolved minerals from the source water. This reduces the chemistry correction work after filling and prevents the initial staining that high-iron well water can cause. Master Spas specifically recommends using a pre-filter on the garden hose when filling. Pre-filter elements run $15-25 and are worth every cent if your tap water is hard.
Step 8: fill to the proper level
Fill to the manufacturer’s recommended water level, typically just above the highest jets. If your home uses a water softener: use no more than 50% softened water. Over-softened water has very low calcium hardness, which causes foaming and can strip minerals from the spa surface. Master Spas states this limit explicitly: “never more than 50% softened water.”
Step 9: add sequestering agent immediately
Add the sequestering agent while the jets are running as the spa fills. A sequestering agent bonds to metals and minerals the moment they enter the water, before they can cause staining or scale on the shell and equipment. Add it with the jets running for even distribution.
Step 10: turn on power and circulate
Restore the breaker. Let the system circulate for 30 minutes before running your first chemistry test. This ensures water is fully mixed and the pump has purged any air from the lines. If jets aren’t flowing correctly after power-on, you may have a temporary airlock in the lines.
After refilling, wait 30 minutes for the system to circulate and purge air before testing chemistry. Then adjust in order: total alkalinity first, pH second, and sanitizer last.
Step 11: balance chemistry in order
Test the full panel: total alkalinity, pH, calcium hardness, and sanitizer.
Adjust in this exact order:
- Total Alkalinity, target 80-120 ppm. TA stabilizes pH and must be correct before pH adjustments will hold.
- pH, target 7.4-7.6. Wait 1 hour after TA adjustment before adjusting pH.
- Calcium Hardness, target 150-250 ppm. Low calcium causes foaming.
- Sanitizer last, chlorine shock after first fill: raise free chlorine above 5-8 ppm to disinfect the fresh water. Wait until FC drops below 5 ppm before entering. Then shock after refilling on your normal schedule going forward.
For pools, see our guides on pool shock after refilling and water clarity problems if you’re managing chemistry across both a pool and spa.
FAQ
How long does it take to drain and refill a hot tub?
Plan for a half-day. Drain time runs 1-3 hours for a 300-500 gallon spa (faster with a submersible pump). Shell cleaning takes 30-60 minutes. Refill takes 1-2 hours. Chemistry balance, including wait time between adjustments and the initial shock, adds another 2-4 hours. Total: 4-8 hours if you include chemistry settling time. You can start the drain in the morning and have the spa ready by evening.
Where do I drain my hot tub water?
To the sanitary sewer via a floor drain or utility sink, or to lawn or garden if your chlorine or bromine level is very low (below 1 ppm). Check local regulations before draining outdoors, many municipalities prohibit draining to storm drains even at low sanitizer levels. Do not drain large volumes onto the same area repeatedly, as sodium from water treatment can accumulate in soil.
Do I need to shock a hot tub after refilling?
Yes. Shock with chlorine immediately after filling to disinfect the fresh water and any residual contamination from the shell or plumbing. Raise free chlorine above 5-8 ppm with a chlorine shock product. Wait until FC drops below 5 ppm before entering, then transition to your normal sanitizer routine (chlorine or bromine).
Can I use a wet/dry vac to drain my hot tub?
For the last few inches of water after gravity drain, yes, a shop vac handles residual water efficiently. For the full drain, it is not practical. A submersible pump rated for 1/3 HP or higher is the right tool for full drainage. Gravity drain via the spigot works fine for most homeowners; it just takes longer.
Why does my hot tub foam after refilling?
Four common causes: residual cleaner left in the shell (rinse more thoroughly next time), high mineral content in the fill water (use a pre-filter), insufficient calcium hardness (test and add calcium hardness increaser to reach 150-250 ppm), or softened water exceeding the 50% limit. Add a sequestering agent during fill and test calcium hardness as one of your first post-fill checks.