How to drain and refill a hot tub, step by step

Draining and refilling isn’t hard, but it’s easy to do out of order — refill before the filter’s clean, balance chemicals before the water’s even up to temperature, or spend two hours babysitting a gravity drain valve because nobody mentioned a $60 pump does the same job in twenty minutes. Here’s the whole process in the order that actually works, plus how to know it’s time if you haven’t already decided.

The short version: shut off power, flush the lines, drain (a submersible pump beats the gravity valve badly), clean the shell and waterline while it’s exposed, service the filter, refill through the filter compartment, then balance in order — alkalinity, pH, calcium, sanitizer, shock.

What you’ll need

• A way to move the water out. The built-in drain valve works but is slow — a submersible utility pump is the difference between a 20-minute job and a 2-hour one.
• A garden hose, for both draining (routing water away from the tub) and refilling.
• A foam-free, spa-safe shell cleaner for wiping down the drained shell and waterline.
• Optionally, a floating scum absorber run in the days beforehand — less oily film means less scrubbing once the tub is empty.
• A clean or new filter, since refilling through a dirty one is a head start on cloudy water.
• Your usual chemicals — alkalinity increaser, pH adjuster, calcium hardness increaser, sanitizer, and shock — to rebalance the fresh fill.

Step 1: Shut off power and flush the lines

Cut power at the breaker or GFCI before you touch anything. Then, every drain or two, flush the plumbing: add a line-flush product to the old water and run the jets as directed. Biofilm builds up inside the pipes where sanitizer never fully reaches, and it re-contaminates the fresh fill within days if you skip this. Expect the water to look worse before it drains — that’s the gunk leaving.

Step 2: Drain the tub

You have two options. The built-in drain valve works by gravity alone, which for a 300–400 gallon spa easily runs 2 or more hours — longer if the valve is small or partially clogged. A submersible utility pump rated around 1,800 GPH clears the same tub in roughly 20 minutes, and it’s not spa-specific equipment — it’s the same pump people keep around for a flooded basement or garden irrigation, with a hose adapter that routes water wherever you need it to go. Drop it in, run the hose to a drain or a spot in the yard that can handle the volume, and let it run dry before pulling it back out. Never let the tub’s own pump or heater run without water in the shell.

Step 3: Clean the shell and waterline

This is easiest in two passes, not one. In the days leading up to draining, drop a floating scum absorber in the water — it soaks up the body oils and lotion residue that would otherwise concentrate at the waterline, so there’s meaningfully less to scrub off once the tub is empty. Once it’s drained, wipe the exposed shell and the waterline ring with a spa-safe cleaner — a household cleaner left on the shell will foam the instant the jets come back on. If what you’re looking at is actual mold or mildew rather than routine grime, that needs a different product and approach; see hot tub mold and mildew: prevention and removal.

Step 4: Service the filter

An empty tub is the easiest time to deal with the filter, since it’s already out of your way. Rinse it if a rinse is enough, deep-soak it if it’s been a while, or swap in a new cartridge if it’s past due — our filter cleaning guide covers how to tell which one you need. Refilling through a filter that’s still clogged with the old water’s buildup just gives cloudy water a head start.

Step 5: Refill through the filter compartment

Run the garden hose into the filter compartment rather than directly into the shell. Filling this way pushes trapped air out through the plumbing as the level rises, which avoids the airlocks that leave pumps sputtering and priming badly the first time you fire the jets up.

Step 6: Balance the fresh fill, in order

Fresh water has no sanitizer or balance yet, so bring the heater up to temperature and then work through the numbers in sequence: total alkalinity first (80–100 ppm), then pH (7.4–7.6), then calcium hardness (150–250 ppm), then sanitizer — and finish with a shock dose to set a clean baseline. Alkalinity leads because it controls how pH behaves; dose out of order and you’ll end up redoing the work. The full picture of how these five numbers interact lives in the complete hot tub water care guide.

Frequently asked questions

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