How to clean hot tub plumbing: a step-by-step guide

Your hot tub water can be crystal clear while the inside of the pipes tells a very different story. Behind the jets run tens of metres of pipe, and there, out of sight, a slimy deposit called biofilm builds up over time. This guide explains what it is, how to recognise it and how to clean the plumbing so that fresh water actually stays fresh. If you have recently bought a used hot tub, this is one of the most important first steps you can take.

What is biofilm, and why does it form in the pipes?

Biofilm is a slimy, sticky layer on the inner walls of the pipes, jet housings and pumps, in which bacteria live inside a protective coating they produce themselves. It feeds on skin cells, sweat, lotions and cosmetic residues that end up in the water, and warm water is the ideal environment for the bacteria to multiply.

The key point: sanitizer does not reach the inside of the biofilm. Chlorine or bromine kills the bacteria floating in the water, but the layer stuck to the pipe wall protects the bacteria living in it. That is why the water can be clear and correctly balanced while contamination keeps living in the pipes. Biofilm thrives where water stands still: in rarely used branches, in the air lines, and above all in a hot tub that has been sitting for weeks or months.

Draining does not solve the problem either. The water flows out, the biofilm stays on the pipe walls and starts contaminating the new water too.

How can you tell there is biofilm in the pipes?

The most common sign of biofilm is recurring water problems that normal water care does not fix for long. Typical signs:

• Recurring cloudiness. The water clears up after shocking, then turns cloudy again within a few days.
• Foam that always comes back. Anti-foam only treats the symptom, and the foam reappears again and again.
• A musty or unpleasant smell, especially when the jets are switched on.
• White, grey or brownish flakes and slimy bits coming out of the jets, especially if the hot tub has not run for a day or two.
• Rising sanitizer demand. The chlorine or bromine level drops unusually fast, and you need more and more chemicals for the same result.
• pH that is hard to balance, and even freshly filled water quickly goes off.

Why is it a must on a used hot tub?

On a used hot tub, cleaning the plumbing is one of the very first steps of getting it running, because you cannot know what is in the pipes. The previous owner may have looked after it carefully, but the hot tub may also have stood half drained for months, or not had a line flush for years. Standing water and water left in the pipes are exactly what biofilm likes best.

If you skip this step, you can easily end up filling the hot tub with clean water, balancing the chemicals and finding cloudy, foaming water a few days later. In that case the water is not the problem: it is being recontaminated from the pipes.

On a neglected hot tub, one flush may not be enough. If flakes, foam or brownish dirt still come out of the jets after the second fill, repeat the process.

When should you clean the plumbing?

Most line flush manufacturers recommend flushing the plumbing before every drain and refill, in the old water. This is the simplest habit: the water is due for a change anyway, and this is exactly when the product can do its work.

Never skip it in these three situations:

• when getting a used hot tub running, before the first fill,
• if you notice any of the signs above,
• if the hot tub has been standing for a longer period, or you are restarting it after winterising.

Some manufacturers consider it enough less often, every few water changes or once a year. Your own hot tub's owner's manual and the label of the product you choose are what count.

How do you clean the plumbing step by step?

Clean the plumbing with a purpose-made line flush product (you may also see it sold as pipe cleaner or spa purge; tablet versions exist too, for example AquaFinesse SpaClean), always in the old water still in the hot tub, before draining. The product reaches the pipes with the circulating water, which is why the hot tub needs to be full. The exact dose, run time and details are set by the product label and vary from product to product. The general process is as follows:

1. Read the label. Check the dose for your hot tub's volume (in litres), the run and soak times, and whether the filter needs to be removed and the air controls kept open or closed.
2. Prepare the hot tub. The water should be at its normal level and operating temperature. Take off the cover. Many products ask you to remove the filter cartridge, because the dirt that is released would clog it. Set the diverter valves to the middle position and open every adjustable jet, so the product reaches every branch of the plumbing.
3. Add the product. Pour the dose stated on the label into the water, usually near the filter housing, with the pumps running.
4. Run the jets. Switch on all pumps on high speed and, if you have one, the blower. The run time is typically between a quarter of an hour and an hour depending on the product, and some products include a soak period or even an overnight soak.
5. Watch the foam. Foaming is normal and shows that the product is working. Brownish, dark flakes often appear on the foam and on the water surface too: that is what has been sitting in the pipes. If the foam threatens to overflow, close the air controls or stop the pumps for a short while.
6. Switch off the power and drain the water. Before draining, cut the power to the hot tub at the circuit breaker or the residual current device (RCD). While it drains, rinse the walls with a garden hose so the released dirt does not dry onto the shell.
7. Clean the shell and the filter housing. Wipe the hot tub with a soft cloth and a cleaner approved by the manufacturer, then rinse with clean water. Remove the dirty water left in the footwell with a sponge or a wet and dry vacuum. Rinse out the filter housing and the skimmer as well.
8. Clean or replace the filter. Soak it in filter cleaning solution as the label describes, then rinse it thoroughly. If the filter is old, damaged, or you do not know how old it is, replace it with a new one.
9. Refill the hot tub. If the manufacturer says so, fill through the filter housing or the filter standpipe, which reduces the chance of an air lock in the pipes. If the jets do not give a strong, even stream after filling, bleed the air from the system as the owner's manual describes.
10. Rebalance the water chemistry. First the alkalinity (TA), then the pH, and finally the sanitizer level, to the ranges given by the manufacturer.

What should you not use for cleaning?

Clean the plumbing and the shell only with products made for hot tubs and approved by the manufacturer. Household solutions are either ineffective or harmful, and in the worst case dangerous.

• Household cleaners. Most of them harm the acrylic shell, and according to manufacturers, chemical damage to the shell is not covered by the warranty. Never use abrasive or solvent-based products.
• Washing-up liquid, soap. They cause heavy foaming, and the usual chemicals do not break them down in the water. They do not dissolve biofilm either, they just create a new problem.
• Household bleach. It can damage the acrylic surface, and on its own it is no solution against biofilm stuck inside the pipes.
• Mixing chemicals. Never mix two chemicals, and do not pour them into the water at the same time. When chlorine meets acidic products, toxic chlorine gas can be released. Dose each product separately, as the label says, and wait until the previous one has mixed in.
• Home remedies. A vinegar or baking soda flush does not break down biofilm. Vinegar has a role at most in removing scale, and even then only as the manufacturer describes.

How do you clean the jets and the filter housing?

Clean the jets and the filter housing in the drained hot tub with a soft brush and a hot tub cleaner, and take out the removable inserts. The line flush product treats the inside of the pipes, but the rims of the jets and the corners of the filter housing and skimmer need cleaning by hand.

Jets

• On many hot tubs the jet inserts can be unscrewed or popped out. Check in the owner's manual which ones are removable, and how.
• Soak the removed inserts in lukewarm water with hot tub cleaner and clean them with a soft brush. Brush the rims of the fixed jets as well.
• If a rotating jet turns more slowly or sticks, there may be scale in its bearing. Some manufacturers prescribe a vinegar soak for this, but only do it if your hot tub's manual recommends it too.

Filter housing and skimmer

• Take out the filter basket and the floating weir (if there is one) and clean off the deposits.
• A slimy layer often sits on the wall of the filter housing: wipe it off with a soft brush, then rinse.
• Manufacturers typically recommend soaking the filter cartridge in filter cleaner once a month and replacing it roughly every one to two years, depending on use. On a used hot tub, a new filter is a cheap and safe start.

What should you watch out for with the air controls and diverter valves?

The air controls and diverter valves are part of the plumbing, so during cleaning their position decides whether the product reaches every branch. The diverter valve controls which group of jets the water goes to: in the middle position it flows in both directions, so that is the right setting during cleaning. The air controls set how much air is mixed into the jets.

• Air controls during cleaning. Products differ here: some ask for open air controls, others for closed ones. With them open, the foaming is stronger. Follow the label.
• A stiff diverter valve. If the diverter valve is hard to turn, sand or dirt has often got into it. According to many manufacturers' instructions, after cutting the power the valve can be taken apart and cleaned, and the O-rings lubricated with a waterproof lubricant. If you are unsure, leave it to a professional.
• Air lines. Water and dirt can also settle in the air channels of blower systems. If you have a blower, switch it on during cleaning so the product reaches there too.

How often should you change the hot tub water?

With chlorine or bromine water care, leading manufacturers typically recommend a full water change every three to four months, more often with heavy use. Water also wears out because dissolved substances and residues of soap and body products build up in it, which chemicals do not remove. For salt water systems, some manufacturers consider less frequent changes, even once a year, enough. Your own hot tub's manual is what counts.

A rule of thumb based on the number of bathers helps you tailor it. The formula was originally developed for public spas, but it is a good starting point for a home hot tub too:

Days between water changes ≈ hot tub volume in litres ÷ average bathers per day ÷ 12

"Bathers per day" is an average: if two of you use the hot tub three times a week, that is about 0.9 people per day. The formula is only a guide. If the water turns cloudy, foams or feels unpleasant on the skin sooner than that, change it earlier, and do not skip the line flush when you do.

How can you stop it from coming back?

You cannot prevent biofilm completely, but regular, simple habits slow down its return:

• Keep the sanitizer level within the manufacturer's range at all times, and test the water regularly.
• Clean the filter regularly, and replace it when it is worn out.
• Shower before bathing and rinse off shower gel thoroughly. After washing your swimwear, rinse it in clean water as well, because leftover detergent also causes foaming.
• Run the jets regularly even when you are not using the hot tub: standing water favours biofilm.

The key is not a miracle product but consistency: regular testing, a clean filter, water changed on time, and a thorough line flush before every water change.