How to drain and refill a cold plunge

The first thing to get straight is that nobody can tell you “change it every X weeks” and be honest about it. How long your water lasts depends on three things working together: your filtration, your sanitation regime, and how heavily the tub is used. A plunge that is filtered, dosed with a proper sanitizer and kept covered can go a genuinely long time on one fill. The same tub with no treatment, no cover, and several sweaty users after workouts can turn cloudy in a fraction of that. The variables are too different from setup to setup for a single number to mean anything.

So let the water tell you. Three signals decide it: clarity (is it still clear, or going hazy and cloudy?), smell (fresh and neutral, or starting to turn?), and your treatment plan (is your usual dose still holding the water clean, or does it stop working sooner than it used to?). When clarity drops, the smell shifts, or you find yourself dosing more and more to keep up, the fill is done and it is time for a full change. That is a more reliable trigger than any date on a calendar.

The practical upshot is that most of the effort belongs upstream, in how you treat and cover the water, not in how often you drain. A solid routine stretches the interval a long way, which is why our water-treatment guide and the broader maintenance routine do more to save you work than any draining trick. Get those right and draining becomes an occasional reset rather than a weekly chore.

Once you have decided the water is done, getting it out is the easy part — you just have three options and a few cautions. The simplestis the built-in drain valve most plunges have: screw a garden hose onto it, run the far end to wherever the water should go, open the valve, and let gravity do the work. It is slow — often the better part of an hour for a full tub — but it needs almost no supervision and makes no mess.

The fast option is a submersible or utility pump. Drop it into the tub, run its discharge hose to your drain point, switch it on, and the same tub empties in minutes. The catch is the tail end: as the water level drops, you have to be there to shut the pump off before it starts sucking air, because running it dry can burn out the motor. The no-equipmentfallback is a plain siphon — a hose full of water with one end lower than the tub — which is slow and fiddly but costs nothing and works in a pinch.

Wherever the water comes out, it has to go somewhere sensible. A floor drain, a sewer cleanout, or a wide, thirsty lawn area that can soak up the volume are all fine. The one real warning is chemistry: if you have just dosed the water or run it heavily chlorinated, do notdump it straight onto garden beds or sensitive plants, because concentrated sanitizer can scorch roots and leaves. Lightly treated water spread across a large grassy area is usually harmless; when you are unsure, send it to a drain instead of gambling with your landscaping. And remember a full tub is heavy and the surrounds get slick — move deliberately and keep your footing.

This is the part that separates a routine chore from an expensive one. Your chiller and pump are the most vulnerable things during a drain, and the single rule that protects them is: never let either one run without water moving through it. The water a chiller circulates is also what carries heat away from it and keeps its internals happy; a pump relies on the water it moves to stay cool. Let the level drop below the intake while the unit is still running and it can overheat or seize in a hurry. So the first move, before you open any valve, is to cut the power to both.

Then disconnect or bypass the chillerbefore the water goes, and clear its lines afterward so it is not left holding a slug of water in a half-drained loop. This is good practice any time you drain, but it becomes critical before a freeze. Water left sitting in a chiller, pump housing or a length of line will expand as it freezes and can crack whatever is containing it — the kind of damage that turns a seasonal shutdown into a repair bill. If you are winterizing rather than just changing the water, treat the lines as the priority and follow the fuller steps in our cold plunge in winter guide.

One more equipment note that catches people out: if the plunge has been struggling to reach temperature, draining and refilling will not fix an underlying cooling fault. A tub that is not getting cold usually points to airflow, sizing or a maintenance issue rather than the water itself — our guide to a plunge that is not getting cold walks through the real culprits before you blame the water.

An empty tub is the only time you can actually reach every surface, so do not waste it. With the water out, wipe down the interior walls and floor and pay particular attention to the waterline, where a greasy film of body oils, skin and residue tends to build into a visible ring. That film is exactly what your fresh water will inherit if you skip it, and it is far easier to clear now than to fight through treatment later.

You do not need harsh chemicals for a routine reset. Wipe the surfaces down, sanitize them with a suitable cleaner, then rinse thoroughly so nothing lingers to react with your next dose. The goal is simply to clear biofilm and residue so the new fill starts genuinely clean rather than picking up where the old one left off. If your tub has a filter, this is also the natural moment to rinse or replace it — the maintenance guide covers filter care alongside the rest of the between-fill routine. A five-minute wipe now is what makes the next fill last, so treat the cleaning as part of the drain, not an optional extra.

Refilling is the reverse of draining, with the same care about equipment. Fill the tub with fresh water to its normal level — only then reconnect the chiller and pump, so they never spin up against an empty or half-full tub. With the water back in and the units reconnected, re-dose your treatment to your usual routine. Fresh water is a blank slate; it needs its sanitizer set up again from scratch, and getting that dose right at the start is what keeps the new fill clear for as long as possible.

Then be patient. A full tub of fresh, room-temperature water takes time for the chiller to pull back down to your target — this is not instant, and it is normal to wait a good while before the plunge is cold enough to use. Cover it while it chills so the chiller is not fighting the sun and warm air, which is one of many reasons a snug cold plunge cover earns its keep. The cover also keeps leaves, dust and pollen out between sessions, which is half the battle in stretching the time to your next drain.

That is the whole loop: change the water when clarity, smell or your dosing tells you to, protect the chiller and pump so nothing runs dry, clean while it is empty, refill, re-dose and let it chill. Do the upstream work — a real treatment plan and a good cover — and this becomes an occasional reset rather than a recurring hassle.