Draining and Refilling a Hot Tub

A water change is an afternoon, most of which is waiting. Three details make the difference between doing it properly and doing it twice: purge the plumbing before you drain, wipe the shell while the tub is empty, and fill through the filter housing to avoid an airlock.

Before you start

• Pick your weather. Not a freeze, and ideally not the hottest day either — you will be working outside.
• Plan where the water goes. Several hundred gallons has to end up somewhere that is not your foundations or a neighbor's garden. Check what is acceptable locally for treated water.
• Have the refill chemicals ready. Nothing is worse than a full tub of unbalanced water and a shop that has closed.
• Have a clean filter cartridge. After a purge you want a fresh one, not the one that just caught everything.

Step 1: purge the plumbing

With the tub still full. Remove the filter cartridge, add a plumbing purge product at the label dose, and run every jet on every setting for the stated time, cycling the air controls so flow reaches the branches that normally sit still.

On a tub that has never been purged, the result is genuinely unpleasant. That is the product working. Which product to use depends on whether this is a rescue job or routine maintenance — see the line flush roundup and the full flush procedure.

Step 2: turn the power off

At the breaker, not just the control panel. A heater that runs dry is a dead heater, and it is one of the more expensive mistakes available during a water change.

Step 3: drain

Either through the built-in drain valve, which is slow, or with a submersible pump, which is not. A pump turns most of an afternoon into about twenty minutes, and removing that friction is the single biggest reason people manage to change water on schedule rather than stretching it.

Watch it as the level drops. Utility pumps do not like running dry, and the last inch is a wet-vac or a sponge job regardless.

Step 4: clean the empty shell

This is the step that gets skipped and it is the only time you have access. Everything the purge released is now sitting on the surfaces.

• Wipe the whole shell with a non-abrasive cloth and a spa-safe cleaner.
• Pay particular attention to the waterline, where the scum ring is.
• Wipe the jet faces and the housings behind them.
• Clean the filter housing itself, not just the cartridge.
• Wipe the underside of the cover while you are at it.

Do not use household abrasives or bleach on acrylic. A spa-safe cleaner or a mild dish soap thoroughly rinsed is enough, and rinsing thoroughly matters because residue foams on the refill.

Step 5: refill through the filter housing

Put the hose down into the filter housing rather than pointing it at the water surface. Filling from the bottom up pushes air out of the plumbing ahead of the water, which is what prevents an airlock — the situation where the pump runs but nothing comes out of the jets.

If your fill water is hard or comes from a well, this is the moment for a hose pre-filter. Keeping metals and sediment out is far easier than treating them once they are in, and it is covered on calcium hardness.

Fill to the manufacturer’s mark, which is usually mid-way up the skimmer opening. Under-filling starves the pump; over-filling means the skimmer cannot skim.

Step 6: power on and check for an airlock

Restore power and start the pump. If jets are weak or silent, you have an airlock. The usual fix is to loosen the union on the pump slightly until water appears and air stops hissing, then retighten — but check your manual first, because procedures differ between tubs and some have a bleed valve for exactly this.

Step 7: rebalance from scratch

Let the tub come up to temperature before dosing, because chemicals behave differently in cold water, then work through the sequence in order: alkalinity, pH, calcium hardness if needed, sanitizer, shock. Targets are pH 7.2 to 7.8 and the published hot tub sanitizer level.^[source]

The full version, with the reasoning for the order, is on first chemicals after a refill. Do not follow the order printed on the individual bottles; several will tell you to adjust pH before alkalinity, which is backwards.

Step 8: do the cover while you are there

The cover is off and you are already outside. Clean and condition it now rather than promising yourself you will do it later, because that is the habit that delays a several-hundred-dollar replacement. Products are on the cover care roundup.

Things that go wrong

• Running the heater dry. Power off at the breaker before draining, every time.
• Draining in a freeze. An empty tub with residual water in the pipes is a burst-pipe risk.
• Skipping the purge. You have just installed fresh water into dirty pipework.
• Filling from the top. Airlock, and an unnecessary hour of troubleshooting.
• Dosing cold water. Balancers behave differently and your readings will chase you for days.
• Refitting the old cartridge after a purge. It is holding everything the purge released.

None of that is difficult, but all of it is easier to get right the first time than to correct afterwards. And the reason for the whole exercise is the same as always: fresh water holds a sanitizer residual far more easily than old water does.^[source]