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Hard water and limescale in an inflatable hot tub

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In short

Much of England has hard water, rich in calcium. Heated to hot tub temperatures, especially with high pH, that calcium comes out as limescale on the heater, filter and liner, which slows heating and can shorten the pump's life. Keep pH in range, use an anti-scale product as general guidance and descale the pump as the maker's manual describes.

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If you live in a hard water area, you already know what limescale does to a kettle. A hot tub heater faces the same problem, but holds hundreds of litres at close to 40 °C for weeks at a time. Scale builds up on the heater element, clogs the filter and leaves a rough film on the liner. This guide explains why scale forms, how to tell if it is building up, and how anti-scale treatment, pH control and descaling fit into a normal routine. Follow your tub maker's manual and the chemical maker's instructions for every product.

Why scale forms in hot tubs

Hard water contains dissolved calcium and magnesium. As water warms, and as pH and alkalinity rise, less calcium can stay dissolved, so it comes out as solid calcium carbonate: limescale. The hottest surface in the tub, the heater element, is where it forms first. Air bubbles also push pH up, which adds to the effect.

Much of the south and east of England has hard water, while Scotland, Wales and the north west tend to have softer water. Your water company can tell you how hard your supply is.

pH

Target band: 7.2 to 7.6 (scale 6.8 to 8.0)

too low: pH increasertoo high: pH reducer

Total alkalinity

Target band: 80 to 120 ppm (scale 40 to 200 ppm)

too low: alkalinity increasertoo high: pH reducer

Free chlorine or bromine

Target band: as on the product label (scale 0 to 10 ppm)

too low: add sanitisertoo high: cover off, wait

Common guidance only. Always follow the ranges and safety instructions on your own test strips and chemicals.

Keep each reading inside its target band. Correct alkalinity first, then pH, then sanitiser.

Signs of scale

Scale on the heater cannot be seen without opening the pump, which you should not do. Slower heating and flow errors are the usual signs.

  • A white or grey crust on the liner at the waterline.
  • Rough patches on the floor or walls.
  • A filter that turns hard and crusty and does not rinse clean.
  • Milky water, especially after heating.
  • Slower heating than before, or flow errors from the pump.
  • White deposits around the inlet and outlet fittings.

Preventing scale

Some tubs list a built-in hard water treatment system, such as several Intex PureSpa models and the BANGBANGJI tubs in our 6 person guide. These help, but do not replace balanced water and clean filters.

  1. 1

    Find out your water hardness from your water company or a test strip with a hardness pad.

  2. 2

    Add an anti-scale or scale inhibitor product after filling, as the label directs, if your water is hard.

  3. 3

    Keep pH and alkalinity in the ranges on your test kit; high pH is the main trigger for scale.

  4. 4

    Rinse the filter every few days and soak it in filter cleaner when it feels crusty.

  5. 5

    Top up anti-scale when you add fresh water, as the label directs.

  6. 6

    Drain and refill as often as the manual suggests, since minerals build up over time.

Descaling the pump and heater

Many makers, including Bestway for Lay-Z-Spa, sell a descaler for the pump and heater, or describe a descaling routine in the manual. It usually involves running a descaling solution through the pump in a small amount of water, then flushing. The exact method varies by model, so follow your manual or the descaler's label.

How often depends on your water hardness and how long the tub runs. In hard water areas, descaling at the end of each season, or when heating slows, is a common habit.

Note:Unplug the pump before connecting or disconnecting hoses for descaling. Do not open the pump housing. Wear gloves if the descaler label advises it, and follow the chemical maker's instructions.

Removing scale from the liner

When the tub is drained, wipe light scale off the liner with a soft cloth and a cleaner made for hot tubs or a mild acid cleaner allowed by your manual. Do not use abrasive pads or scrapers, which can damage the liner. Rinse well so no cleaner is left in the tub before refilling.

ProblemFirst fix
Crust at waterlineWipe during drain, keep pH in range
Crusty filterSoak in filter cleaner or replace
Milky water after heatingLower pH and alkalinity, add anti-scale
Slow heating or flow errorsClean filter, then descale as the manual describes
Scale problems and first fixes

Soft water and a water softener

If your home has a water softener, softened water can be used for filling in many cases, but very soft water can foam and may need calcium added to reach the range on your test kit. Check your manual and the chemical labels. Our foaming article explains low calcium foam.

See our hot tub shortlist →

Frequently asked questions

Do I need anti-scale in a hot tub? In hard water areas it is widely used as general guidance. Follow the label and your manual.

How do I descale a Lay-Z-Spa pump? Bestway describes a method and sells a descaler. Follow the manual for your pump model.

Can limescale damage the heater? Scale insulates the heater and slows heating, and it can contribute to faults, so prevention matters.

Will a hard water filter on the hose help? A pre-filter can reduce some minerals and debris. Check its label for what it removes.

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