How to Drain and Refill Your Hot Tub: Complete Step-by-Step Guide

Draining and refilling your hot tub is one of the most important maintenance tasks you'll do all year — yet most owners either do it too rarely or skip it entirely until their water turns green and no amount of chemicals can fix it. This guide covers everything: how often to drain, two methods to empty the tub faster than the drain valve alone, what to clean while it's empty, how to refill correctly, and the exact chemical startup sequence to get balanced water in a single afternoon.

Why You Need to Drain and Refill Regularly

Hot tub water doesn't go "bad" the way food does, but it does accumulate dissolved solids that chemicals can't remove. Every time a bather soaks, they introduce body oils, lotions, sweat, and deodorant. Every time you add a chemical, the active ingredients do their job and leave behind dissolved byproducts. Over weeks and months, your total dissolved solids (TDS) level rises until the water becomes saturated and can no longer hold new chemicals in solution.

When TDS rises too high:

Chlorine and bromine stop working effectively even at correct dose levels
pH becomes unstable and swings dramatically between tests
Water looks dull or cloudy regardless of how much clarifier you add
Scale deposits accelerate on jets, the heater element, and the shell surface
No amount of shock treatment will restore water clarity

The only fix for high TDS is complete water replacement. There is no chemical treatment to lower dissolved solids — you have to drain and start fresh.

How Often to Drain Your Hot Tub

The standard formula for drain frequency is:

Gallons ÷ (daily bathers × 3) = days between drains

For a typical 400-gallon hot tub used by two people most days: 400 ÷ (2 × 3) = 67 days, or roughly every two months.

Tub Size	Light Use (1–2 people occasionally)	Moderate Use (2–3 people weekly)	Heavy Use (3+ people several times/week)
250 gal	Every 4 months	Every 2–3 months	Every 5–6 weeks
400 gal	Every 4–6 months	Every 3–4 months	Every 6–8 weeks
600 gal	Every 6 months	Every 4–5 months	Every 3–4 months

As a practical minimum: drain at least every 3–4 months regardless of use frequency. Water that sits stagnant for more than four months without draining will almost always have TDS and biofilm problems even with perfect chemical maintenance. See the hot tub cleaning schedule guide for a full maintenance calendar that incorporates drain intervals.

What You Need Before You Start

Gather all of these before you begin. Having everything ready means you can complete the drain, clean, and refill in a single session — ideally an afternoon when you won't need the tub for 4–6 hours.

For draining:

Garden hose (to direct water away from the foundation)
Submersible utility pump (optional but much faster — $30–$80, worth owning)
Wet/dry shop vac (to remove the last few inches the pump can't reach)

For cleaning:

Hot tub shell cleaner (specifically formulated; avoid household cleaners that leave residue)
Jet cleaner or line flush product
Filter cleaning solution (spray-on or soak-in)
Old toothbrush for jet nozzles and tight corners
Microfiber towels or soft sponge (no abrasive pads — they scratch acrylic)
Pillow cleaning spray

For refilling and chemical startup:

Garden hose with inline hose filter (prevents metals like iron and copper from entering fresh water)
Full fresh-water chemical kit — see the hot tub chemicals starter kit guide for the complete list
Fresh test strips or liquid test kit — detailed usage in the water testing guide
Calcium hardness increaser (especially important for soft water regions)
Total alkalinity increaser
pH up and pH down
Fresh sanitizer (chlorine granules, bromine tablets, or salt depending on your system — see the salt water vs chlorine comparison if you're reconsidering your sanitizer type)
Non-chlorine shock (for oxidizing startup)

Optional but useful:

Pipe deodorizer/biofilm treatment to flush the plumbing lines before draining
Surface protectant/polish for the acrylic shell after cleaning

Step 1: Run a Line Flush Before Draining (Optional but Recommended)

Biofilm — the slimy microbial layer that builds up inside plumbing lines — is flushed most effectively while water is still in the tub. Add a line flush or pipe deodorizer product (follow label instructions) and run all jets at full power for 30–60 minutes. You'll often see brown or white flakes and foam surface during this step. That's the biofilm breaking up and flushing out with the water you're about to drain.

This step is especially worthwhile if your water ever had an unusual smell or foam that was hard to control. If you do it, drain immediately after — don't let the dislodged biofilm sit.

Step 2: Turn Off the Power Completely

Before you open any drain valve or introduce a pump, turn off power to the hot tub at the circuit breaker, not just the tub's control panel. The tub's built-in controls may keep components energized even when "off." The breaker eliminates all power to the pump, heater, blower, and control board.

If your tub has a separate GFCI outlet, switch that off too.

Never run a submersible pump in the tub while the tub's own jets or heater are energized. Water and electricity do not mix, and this step is non-negotiable.

Step 3: Drain the Tub

You have two options: using the built-in drain valve (slow) or using a submersible pump (fast).

Method A: Built-In Drain Valve

Every hot tub has a threaded drain valve at or near the bottom of the cabinet — usually hidden behind a small panel on the cabinet skirt. The location varies by brand.

Attach a garden hose to the drain valve spigot (standard hose thread)
Run the hose to a safe drainage area: a floor drain, lawn area (chlorinated water diluted by a full tub is generally safe for established grass), or driveway where it drains to the street
Open the drain valve by turning the cap counterclockwise
Expect 1–2 hours for a 400-gallon tub to fully drain by gravity alone, depending on your hose diameter and elevation

Method B: Submersible Utility Pump (Recommended)

A 1/6 or 1/3 horsepower submersible pump can empty a 400-gallon tub in 8–15 minutes. This is the professional-grade approach and makes the whole process much faster.

Lower the pump into the footwell of the tub (the deepest point)
Run the discharge hose to your drainage area
Plug the pump into an outdoor GFCI outlet (not the tub's breaker — the tub's power should still be OFF)
The pump will run until it starts to cavitate (making sputtering sounds) when it can no longer draw water
At that point, remove the pump and use a wet/dry shop vac to remove the remaining inch or two of water

Leave the built-in drain valve open while pumping — this allows the last bit of water to drain by gravity after the pump finishes.

Step 4: Clean the Shell While Empty

You have a limited window to clean the shell effectively — it needs to be wet enough to work with but not have standing water. Right after draining is ideal.

Shell Surface

Spray hot tub shell cleaner directly on the acrylic surface and let it dwell for the time specified on the product label (usually 3–5 minutes). Wipe in circular motions with a soft sponge or microfiber cloth. Pay extra attention to:

The waterline ring (oils and lotion buildup look like a bathtub ring)
Jet housings (gunk accumulates in the gap between the jet face and the shell)
Seat surfaces and footwell (heaviest bather contact)
Any yellow or brown staining (often iron/manganese deposits or sunscreen residue)

For stubborn staining, a dilute white vinegar solution (1:4 vinegar to water) works well on mineral scale without damaging acrylic. Do not use abrasive scrubbers, bleach, or household bathroom cleaners — they can scratch the surface or leave chemical residue that affects your fresh water chemistry.

Jets and Jet Nozzles

Many jet nozzles unscrew counterclockwise and can be removed for cleaning. Soak removed nozzles in a bucket of hot water with jet cleaner for 20–30 minutes, then scrub with an old toothbrush. For fixed jets, spray directly into the jet face and scrub with a brush.

Headrest Pillows

Hot tub pillows are a common mold and mildew source that most owners completely ignore. Wipe them down with pillow cleaning spray or a 10:1 water-to-white-vinegar solution. Check the back of each pillow — if you see black spots (mold), replace the pillow rather than trying to disinfect it.

Filter Canister Housing

Pull the filter(s) out (they should have been cleaned recently — if it's been more than 30 days since the last filter rinse, follow the filter replacement guide). Rinse the interior of the filter housing with clean water and wipe down the inside walls.

Final Rinse

After scrubbing, do a light rinse of the entire shell with clean water from a hose. You're not trying to create a full rinse cycle — just remove any remaining cleaner residue before you start filling. Then give the shell a final wipe-down with a dry cloth.

Step 5: Refill Correctly

How you fill your hot tub matters more than most owners realize. Two common mistakes during refill can cause weeks of chemistry problems:

Mistake 1: Filling from the top Always fill your hot tub by putting the hose into the filter compartment rather than spraying it into the tub. Filling through the filter housing primes the air out of the plumbing lines and prevents air locks — pockets of trapped air that prevent jets from flowing properly or cause the pump to make grinding noises.

Mistake 2: Skipping the inline hose filter Tap water often contains dissolved metals — especially iron, copper, and manganese — that can stain your shell or cause chemistry imbalances before you even add your first chemical. An inline hose filter (fits between your garden hose and the tub) removes these metals for less than $20 and is reusable for several fills. It's especially worth using if your tap water is hard or if you've ever had staining problems after a refill.

Fill until the water level is at the manufacturer's recommended mark — typically 2–4 inches above the highest jet. Do not underfill (the pump will cavitate) or overfill (water will spill when people enter the tub).

Step 6: Chemical Startup Sequence

Once the tub is full, restore power at the circuit breaker and turn the tub on. Let the circulation pump run and the water reach the target temperature (100–102°F is standard for most users — see the temperature guide for therapeutic use cases) while you begin chemistry.

Always add chemicals in this sequence. The order matters because each adjustment affects the next reading.

1. Test the Source Water First

Before adding anything, test the raw tap water. Record:

pH (target: 7.4–7.6)
Total alkalinity / TA (target: 80–120 ppm)
Calcium hardness / CH (target: 150–250 ppm)

Your tap water's baseline tells you exactly how much of each chemical you need. Skipping this step leads to over-dosing. The water testing guide covers all three testing methods — strips, liquid kits, and digital testers.

2. Adjust Total Alkalinity (TA)

TA acts as the pH buffer. If TA is too low, pH will swing wildly with every chemical addition. If TA is too high, pH locks up and resists adjustment.

TA below 80 ppm: Add alkalinity increaser (sodium bicarbonate). Dose per label instructions.
TA above 120 ppm: Add pH decreaser (sodium bisulfate) slowly, in small increments. TA will drop as you acidify.
TA 80–120 ppm: Move to the next step.

Run jets for 15–20 minutes after adding, then retest before moving on.

3. Adjust pH

With TA in range, pH becomes stable and adjustable.

pH below 7.2: Add pH up (sodium carbonate)
pH above 7.8: Add pH down (sodium bisulfate)
Target: 7.4–7.6

The water chemistry guide explains why these ranges matter and what happens when they're out of spec.

4. Adjust Calcium Hardness (CH)

Calcium hardness matters especially for owners in soft-water regions. Water with low CH (below 150 ppm) is corrosive — it will leach calcium from your shell, jets, and heater element to satisfy its mineral deficit.

CH below 150 ppm: Add calcium hardness increaser in increments; do not add more than 10 ppm per hour to avoid clouding
CH above 250 ppm: If tap water CH is very high, a partial drain and refill with softer water is the only fix
Wait 2–4 hours and retest before proceeding

5. Shock the Fresh Water

Even fresh tap water should be shocked before adding sanitizer. Non-chlorine shock (potassium monopersulfate) works well for this purpose — it oxidizes organics and prepares the water without adding chlorine at this stage.

Add the manufacturer's recommended startup dose, run all jets at high speed for 20 minutes with the cover off, and let the tub circulate for 1 hour.

6. Add Sanitizer

Once pH, TA, and CH are all in range and the initial shock has been given time to off-gas:

Chlorine: Add chlorine granules to reach 3–5 ppm free chlorine
Bromine: Add bromine tablets to the floater; tablets will dissolve slowly to maintain 3–5 ppm
Salt system: Add the manufacturer's recommended salt amount for your tub's volume, then run the salt cell

7. Final Test and Confirm

After the sanitizer has circulated for at least 30 minutes, run a complete test of all parameters:

Parameter	Target Range
Free chlorine / bromine	3–5 ppm
pH	7.4–7.6
Total alkalinity	80–120 ppm
Calcium hardness	150–250 ppm

If everything is in range, the tub is ready to use. If any parameter is still off, make one adjustment at a time and retest before making another. Do not add multiple chemicals simultaneously — they can interact and push other readings out of range.

How Long Until the Tub is Ready to Use?

With an efficient process, you can go from empty to soakable in 4–6 hours:

Drain + clean: 1–2 hours
Refill (400-gallon tub): 1–2 hours
Heat to temperature: 1–4 hours (depends on starting tap water temp)
Chemical startup and testing: 1–2 hours

Plan your drain day accordingly. If you start by 10 AM, you can realistically be soaking by 4–6 PM. If the tub heats overnight, run the startup chemicals before bed and it will be ready by morning.

What It Costs to Drain and Refill

The full cost per drain cycle is lower than most owners expect:

Cost item	Typical range
Water (400 gallons at average US rates)	$3–$8
Chemical startup kit (startup doses only)	$30–$60
Filter rinse/treatment	$5–$15
Hose filter replacement (every 3–4 fills)	$5–$7
Total per refill	$43–$90

Over a full year with four drains, that's $175–$360 in drain-and-refill costs. See the annual hot tub maintenance costs guide for how this fits into the full cost picture.

Common Draining Mistakes to Avoid

Draining in freezing weather: If outdoor temperatures are below 35°F, the residual moisture in plumbing lines can freeze and crack fittings. Either schedule drains for warmer days or winterize properly — the winterize guide covers full cold-weather protocol.

Draining directly against the foundation: Gallons of water pumped fast and close to the tub can erode soil under the deck or pad. Always direct water at least 6 feet away from the foundation and downhill.

Skipping the line flush: Biofilm in the plumbing lines will re-contaminate your fresh water within days if you refill without flushing it out. Thirty minutes of line flush product before draining pays off in weeks of clearer water.

Adding chemicals before the water heats: Many chemicals — especially calcium hardness increaser — dissolve poorly in cold water and can cloud the tub or cause white precipitation that settles on the shell. Wait until the water reaches at least 80°F before beginning the chemical startup sequence.

Testing once and calling it done: Fresh water chemistry shifts as the tub heats and circulates. Test again 24 hours after the initial startup and make any fine adjustments before settling into your regular weekly testing routine.

Draining and refilling every 3–4 months is the single maintenance task that most directly determines how easy your water chemistry is the rest of the time. Clean water from day one means fewer chemical corrections, clearer water, and a longer lifespan for your jets, heater, and shell. Keep a drain log (date, starting TDS if you have a meter, and chemical amounts used) to identify patterns and dial in your startup process over time.

For the full ongoing maintenance picture — weekly, monthly, and seasonal tasks beyond the drain — the cleaning schedule guide is the companion read.