Draining a hot tub: what to do with the filter (line flush, refill and restart)

Last reviewed November 2, 2026

Take the cartridge out of its housing before you add a line flush, clean it (or replace it) while the tub drains, and have it seated again before the power comes back on. None of the four flush labels we read says to leave the filter in its housing during the flush^{[54][55][56][57]}. Refill through the filter compartment: Hot Spring says always^[9], Jacuzzi says it helps prevent airlocks^[52]. Sundance says its footwell usually works and to use the filter inlet if you get an airlock^[12]. Whether the filter is in or out while the water runs in depends on whose instructions you follow (table below).

Owners ask this in the same few ways: “do I leave the filter in when I use Ahh-Some?”, “filters in or out while the line flush runs?”, “do I put the filter back before I refill?”. The answers below come from the drain-and-refill sections of a 2026 Hot Spring manual, a 2024 Sundance manual, two Jacuzzi owner articles and the printed directions for four line-flush products. Your own spa’s manual still wins where it says something different.

Drain day, in the order the filter is touched

1. Before the flush: filter out. Leisure Time’s Jet Clean directions start with removing the filter cartridge and cleaning it separately with a filter cleaner while the purge runs^[56]. SpaGuard’s System Flush label does the same: remove the cartridge and clean it with SpaGuard Filter Cleaner, then add the flush^[57]. Ahh-Some’s UK site says to take out all filters before the purge so loosened debris is not pulled back into them^[55]. The US Ahh-Some sheet is the odd one out (next section).
2. Run the flush per its label, then power off and drain. The label times differ: SpaGuard circulates for 15 minutes^[57]; Jet Clean runs the jets 15 minutes, sits for an hour, then runs jets and blower another 15 minutes^[56]; Ahh-Some says up to 30 minutes^[54]; Jacuzzi’s cleaning article says 15–20 minutes on high^[53]. Hot Spring says to disconnect power before draining^[9]. Sundance says to turn the power off at the breaker before draining and not to turn it back on until the spa is refilled^[12].
3. While it drains: clean or replace. Hot Spring’s drain steps say to install a new filter or clean the existing one with filter cleaner before you refill^[9]. Jacuzzi says to clean the filters while the water drains, and to replace any that are more than one to two years old, damaged, or will not come clean^[52]. The soak itself is in how to clean a hot tub filter. A cartridge still wet with cleaner should be rinsed before it goes back. Jacuzzi says to reinstall it only once it is free of cleaning solution residue^[53].
4. Refill through the filter compartment. Details and the in-or-out question are below.
5. Filter seated, then power. Jacuzzi’s refill steps put the clean filters back before power is restored at the breaker^[52]. Hot Spring says not to use the spa with the filter cartridge removed^[9].

The Ahh-Some exception: filter in the tub water

Ahh-Some’s US instruction sheet does not put the filter aside. It says to remove and rinse the filters and place them inside the hot tub water while the purge runs, then hose them off after draining and put them back in the filter cavity^[54]. Ahh-Some’s UK site says the opposite: take all filters out before the purge and rinse or soak them separately^[55]. In an r/hottub thread asking exactly this, owners split the same way. Some said they float the filter in the tub as the US sheet says. Others pulled it and soaked it in a bucket after seeing what the flush brought out. Neither version leaves the cartridge in its housing. If you use the US method, the cartridge will need its normal cleaning before it goes back.

Filter in or out while you refill?

Jacuzzi’s two articles disagree with each other, so there is no single Jacuzzi answer here. What all four agree on is that power stays off until the spa is full and the filter is back in. If you fill with the cartridge out, it must be in before you switch on.

Refill details that touch the filter area

• Water level. Hot Spring says one inch above the highest jet^[9]. Sundance says until the water covers all jets but does not touch the bottom of the lowest headrest^[12].
• Fill water. Hot Spring sells the CLEAN SCREEN pre-filter for the hose and says not to fill from a swimming pool^[9]. Sundance says never to fill from a water softener. With very hard water it suggests filling half with hard water and the rest with softened water^[12].
• Temperature. Hot Spring says refill water must be 50–70°F (10–21°C) to avoid high-limit tripping, and that refilling is not recommended when the air is below 50°F. If tap water below 50°F trips the high limit, it says to disconnect power and warm the heater sensors and tubing with a hair dryer for about 10 minutes, or to blend in warm water while filling^[9]. Jacuzzi’s cleaning article says you can refill with water up to 104°F (40°C)^[53]. Those two do not agree. Follow the manual for your spa.
• “COOL” on the display. Sundance says a COOL message is common on first fills and refills, because tap water is often very cold, and needs no action^[12].

Air after a refill, and why the filter gets blamed

Hot Spring lists draining without disconnecting power, and an air lock in the plumbing, as causes of the heater pressure switch opening and of the circulation pump’s thermal cut-off tripping^[9]. Sundance’s 780 runs a blow-out cycle at start-up to purge air left by draining and filling: jets pump 1 and the circulation pump run for a minute, then jets pump 2 for a minute^[12]. If an FL--1 still shows up, Sundance’s fix uses the filter well. Turn the power off, remove the cartridge, hold the garden hose over the filter wall fitting with a rag sealed around it, run water for 30 seconds, then reinstall the cartridge^[12]. A flow message right after drain day is more often air than a bad new filter. The code-by-code list is in hot tub flow errors and your filter.

Is drain day the day to replace the filter?

Drain intervals line up roughly with short-life inserts, and not with pleated cartridges. Hot Spring says to change the water every three to four months, or up to twelve months with its salt system^[9]. Jacuzzi says every three to four months, or at least three times a year^[52]. Sundance’s manual says about every 3 months, though the water refresh timer on the 780 defaults to 180 days^[12]. CDC’s formula for water age is (spa gallons ÷ 3) ÷ average bathers per day, as days between changes^[14].

• Inserts and microfilters: often yes. Sundance says to throw away the small MicroClean cartridge every 3 months^[12], which is about one drain cycle. Jacuzzi notes that some of its models use microfilters that cannot be cleaned and must be replaced every three to four months^[52].
• Pleated cartridges: only if it is due. Jacuzzi’s two articles give 12–18 months^[53] and one to two years^[52]. Hot Spring leaves it open at each drain: new filter, or clean the one you have^[9]. Other makers’ intervals are in how often to replace a hot tub filter, and the replace and clean calculator counts them from a date.

If you do replace it, the cartridge is out and the part number is readable on the end cap, which makes this a good time to look it up. Use the part-number search, or the model-year lookups for Hot Spring (including Hot Spot and Solana), Sundance and Jacuzzi. Common cartridges include Hot Spring 71825 / C-6430 and Sundance 6540-488 / C-8326.

After a winterizing drain (antifreeze)

Hot Spring’s winterizing steps remove the cartridge, clean it and store it dry. When the spa is reopened, its manual says not to install the filter during the first fill, so the cartridge is not exposed to the antifreeze. That water is superchlorinated with three teaspoons of sodium dichlor per 250 gallons, put in through the filter compartment, then drained. Only after the spa has completely drained does the filter go back in, before the normal refill^[9]. We do not publish a winterizing procedure for other brands. See winter spa care.

If the tub was contaminated

CDC’s Legionella toolkit is written for public hot tubs. Its steps after a positive Legionella test include draining, scrubbing surfaces, and replacing filters or filter media before refilling and hyperchlorinating. For lapses without illness, it says to clean and service filters per the manufacturer^[14]. For a home spa, the takeaway is narrower: after a real contamination event, a cleaned cartridge is not a fresh start. CDC also says to scrub all surfaces each time the tub is drained^[14]. Product choices for the flush step are in foam, cloudy water and jet cleaners.

Questions

Sources

9. Hot Spring Hot Spot Collection Owner’s Manual (2026)
10. Jacuzzi: Hot Tub Filter Cleaning Made Easy
12. Sundance Spas 780 Series Owner’s Manual (2024)
14. CDC: Controlling Legionella in Hot Tubs
52. Jacuzzi: How to Drain a Spa: Step-by-Step Guide (filter care while drained; refilling)
53. Jacuzzi: How to Clean a Hot Tub (plumbing flush, filters, refill)
54. Ahh-Some: Hot Tub/Jetted Bath Plumbing & Jet Cleaner instructions (US, PDF)
55. Ahh!Some Watercare UK: How to purge your hot tub (step by step)
56. Leisure Time Inc.: Leisure Time Jet Clean (directions for use)
57. BioGuard: SpaGuard Spa System Flush technical bulletin with label directions (copy hosted by The Pool and Spa House)

This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.