Hot Tub Water Change Guide: Drain, Clean, Refill and Balance

A full hot tub water change takes about two to three and a half hours of hands-on work with a submersible pump, then an overnight wait while the heater brings the water up to temperature. You flush the plumbing, drain, clean the shell and filters, refill through the filter well, and then heat and balance. Start after lunch and you can be soaking the next morning.

Everything below assumes a typical 400-gallon tub and a garden hose. For a tub this size on city water with a spigot nearby, the hose is the right tool. A water truck only earns its place on a well, at a house with no usable tap, or for a swim spa.

How often and when to change the water

Many manufacturers suggest a full change every 3 to 4 months for a regularly used tub, and sooner with heavy use or once the water stops responding to chemicals. A common rule of thumb for the interval in days is one third of the tub's gallons divided by the number of bathers per day. For 400 gallons and two people a day that's about 67 days, so roughly every two months. Our guide to how often you should change hot tub water covers the signs that it's time sooner.

Pick a mild day if you can. Tap water in spring or fall can come out in the 50s, which means a long heat. A hard freeze makes the job risky, because an empty tub's plumbing can freeze while you clean, and the changing hot tub water in winter guide covers how to do it quickly and safely if you can't wait.

The afternoon plan with timings

Hands-on time adds up to about 2 to 3.5 hours with a submersible pump, and more if you drain by gravity. The elapsed time is the afternoon plus the heating overnight.

Step 1: flush the plumbing lines

Biofilm builds up inside the pipes and jets where cleaning never reaches, and it's a common reason a freshly refilled tub foams or smells within a week. A line flush product loosens it before you drain, so it leaves with the old water.

Take the filters out, add the flush product per the label, and run the jets on high with the air controls open for as long as the label says, usually 20 to 60 minutes. Expect brown foam and flakes, which means it's working. Scoop off the worst of the foam before draining so it doesn't dry on the shell.

If the tub is new, or was flushed at the last change and the water has behaved since, you can skip this step once. Skip it two changes in a row and it usually shows up as foam.

Step 2: drain (pump, hose or drain valve)

Turn the power off at the breaker before the water level drops. Running a pump or heater without water can damage it quickly, and that's an expensive repair.

There are three ways to get 400 gallons out.

• Drain valve and gravity. Attach a garden hose to the drain spigot at the base, run it somewhere lower than the tub and open the valve. It's slow, usually 1 to 3 hours at the 2 to 6 gallons a minute a gravity hose manages, and it stalls if the hose has to climb.
• Submersible pump. A small utility or sump pump dropped in the footwell, with a hose to the yard, empties a 400-gallon tub in 15 to 30 minutes. It's worth owning if you change the water three or four times a year.
• Wet/dry vac for the last inches. Neither method gets the footwell dry. A shop vac finishes the job, and held to each jet it can blow out water sitting in the lines, which matters in cold weather.

Where the water goes matters. Let the sanitizer drop close to zero for a day or two before draining, or use a neutralizer, and let the water cool. A lawn or gravel area can usually take 400 gallons if you spread it out. Keep it away from the foundation, the neighbor's lot and any storm drain, and never drain it into a pool. Local rules on spa discharge vary, and our guide to where to drain hot tub water covers the options and the rules to check.

Step 3: clean shell and filters

With the shell empty, wipe it down with a cleaner made for spa surfaces or a diluted white vinegar solution, using a soft cloth or a non-scratch sponge. Skip household cleaners and anything that suds, because a film of dish soap left in a footwell can turn the next fill into a foam machine. Rinse with a bucket of clean water and vac or sponge the rinse water out.

Clean off the waterline scum with the spa surface cleaner, check that the jets turn, wipe the underside of the cover and the pillows, and look the shell over for cracks while it's dry.

For the filters, rinse them with a hose nozzle between the pleats, then soak them overnight in a filter cleaning solution. With a second set, the soaked set can dry while the clean set goes back in. Pleated spa filters wear out, and most need replacing every one to two years, sooner in hard water.

Step 4: refill through the filter well to the right level

Put the hose end down the filter well or standpipe instead of over the side of the shell. Filling through the filter compartment pushes water through the plumbing from the bottom up and helps prevent the airlock that leaves a pump humming with no flow. Check your manual, though, because a few models say otherwise.

If the house is on a well, or the tap water is very hard, screw a pre-filter onto the hose end. It catches sediment and some metals before they reach a small, hot body of water where they concentrate fast.

At 5 to 10 gallons a minute, 400 gallons takes 40 to 80 minutes. Fill to the level in the manual, which is usually a couple of inches above the highest jets. Too low and the pump draws air, and too high and the first two people in push water over the side.

Sometimes the tap isn't the answer. On a well with iron or sulfur, with no usable outdoor tap, or for a swim spa of 1,500 to 2,500 gallons or more (roughly 2.5 to 8.5 hours by hose), a delivered fill makes sense. Deliveries are priced per trip, though, so a 300 to 500-gallon tub usually costs the same as a minimum load, and on city water the hose almost always wins. Our hot tub fill service page says so plainly, and the guide to filling a swim spa covers the larger case.

Step 5: heat, test and balance

Turn the power back on. Prime the pump and watch for water moving at every jet before the heater runs. If the pump hums with no flow, there's an airlock. Turn the power off, loosen the union nut on the pump just enough for air to hiss out and water to weep, then retighten it and try again.

Set the thermostat. A typical 240-volt spa heater raises the water about 3 to 6 °F an hour, so a 55 °F spring fill reaches 102 °F in roughly 8 to 16 hours, and a 120-volt plug-in tub takes a good deal longer. Keep the cover on, since an open tub loses heat fast through the surface.

Balance once the water is warm and has circulated for an hour or two, because readings shift as it heats. The order is the same as a pool, in smaller doses: total alkalinity first, then pH to 7.2 to 7.8, then calcium hardness to the level your manufacturer specifies, then sanitizer. The CDC advises at least 3 ppm free chlorine in a hot tub, or at least 4 ppm bromine if you use bromine, and no cyanuric acid, so don't use stabilized pool chlorine tablets. Add one chemical, run the jets for 20 minutes and retest before the next. Once the sanitizer is in, shock the fresh water and leave the cover open for an hour afterward.

When the job is done right, the water looks clear, smells of nothing and holds its numbers for a week. If it foams or clouds in the first few days, a skipped line flush or a trace of cleaner is the usual cause.

Frequently asked questions

How long does it take to drain and refill a hot tub?

With a submersible pump, draining a 400-gallon tub takes 15 to 30 minutes, and refilling takes 40 to 80 minutes at 5 to 10 gallons a minute. Add 30 to 45 minutes of cleaning and the heating time overnight, and the work itself fits in an afternoon.

Can I drain my hot tub into the yard?

In most places you can, once the sanitizer has dropped close to zero and the water has cooled, as long as you spread it over a lawn or gravel area away from the foundation. Rules on spa water discharge vary by town and some ban it from storm drains, so check locally.

Do I have to flush the lines every time?

Not strictly, but biofilm in the plumbing is a common reason a fresh fill foams or smells within a week. Flushing before every change, or at least every other change, saves chemicals and a second drain.

How long does a hot tub take to heat after refilling?

It depends on the heater, the outside temperature and the cover. Typical 240-volt spa heaters raise the water 3 to 6 °F an hour, so from a 55 °F tap to 102 °F, plan on 8 to 16 hours with the cover on.

Can I change hot tub water in winter?

Yes, with care. Work fast, don't leave the tub empty for long, blow the lines out with a shop vac if there's any delay, and refill with the warmest tap water you have. In below-freezing weather it's better to wait for a thaw if you can.

On city water with a tap within reach, use the hose. On a well, without a tap, or with a swim spa, tell us the ZIP and the gallons and we will say plainly whether a truck makes sense and send one delivered price if it does.