Many spa hot tub users suddenly discover a strange phenomenon during routine maintenance: the originally smooth surface of the tub suddenly becomes rough to the touch, even somewhat like sandpaper. This not only affects the feel and comfort but may also damage the internal system of the whirlpool spa hot tub. The most common cause of this phenomenon is—scale.
This article will delve into the core question, "Why does my whirlpool spa hot tub feel like sandpaper?", explaining its causes, identification characteristics, hazards, and removal and prevention methods to help you solve the problems caused by scale at their root.
Why does the surface of a spa hot tub become like sandpaper?
1. The real source of the roughness: Calcium carbonate deposits
The rough feel of a spa hot tub is usually caused by scale buildup. Scale is mainly composed of calcium carbonate (CaCO₃) or magnesium carbonate (MgCO₃), minerals that come from tap water.
When groundwater or tap water has high hardness (i.e., high concentration of calcium and magnesium ions), its solubility decreases after heating, causing minerals to precipitate on the bathtub walls, nozzles, and heating element surfaces, gradually forming a hard crystalline layer.
These tiny crystals adhere to the inner wall of the whirlpool spa hot tub, making the surface rough, dry, and sandpaper-like.
2. Why does heating exacerbate scale formation?
Spa hot tubs are continuously heated during use, typically maintained at 35℃～40℃. As the temperature rises, the solubility of calcium and magnesium ions in the water decreases, making them more prone to combining and forming crystals. This phenomenon is called thermally induced precipitation.
In other words, the higher the temperature and the harder the water, the easier it is for scale to form.
3. The Influence of Water Circulation and pH Value
Spa hot tubs typically use a circulation system to maintain water cleanliness. However, if the pH value is too high (above 7.8), the alkaline environment promotes the precipitation of carbonates, further accelerating scale formation.
If the pH is not properly controlled, even with a normally functioning water filtration system, a sandpaper-like surface may still appear.
What are the early signs of limescale buildup in a spa hot tub?
When limescale first forms, it may be difficult to see with the naked eye, but you can identify it by the following signs:
• Rougher surface texture: You can feel fine particles when you touch the bottom of the tub or the edge of the nozzle.
• Difficult water flow from the nozzle: Limescale clogs the nozzle orifices, causing uneven water flow or reduced spray power.
• Cloudy or turbid water: Tiny calcium particles suspended in the water make it appear "milky" or cloudy.
• Shortened filter life: Limescale buildup on the filter cartridge reduces filtration efficiency and increases maintenance frequency.
• Reduced heating efficiency: When the heating element is covered in limescale, heat transfer is hindered, increasing energy consumption and slowing down heating.
These are all early signs that your spa hot tub will have a "sandpaper-like" feel.
How does limescale damage a spa hot tub?
1. Permanent Wear and Tear on Surface Materials
Limescale buildup not only changes the feel but also damages the smooth layer on acrylic or fiberglass surfaces.
If left untreated for an extended period, limescale particles can scratch the surface during cleaning, leading to loss of shine that is difficult to restore.
2. Pipe Blockage and Abnormal Water Flow
The internal circulation system of a whirlpool spa hot tub is very complex. If limescale accumulates in the nozzles, heaters, or filter tubes, it can reduce flow and even cause pipe blockage.
In severe cases, the circulation pump needs to operate at higher power, increasing energy consumption and shortening motor lifespan.
3. Heating System Overload and Increased Energy Consumption
Limescale has extremely poor thermal conductivity. When a thick layer of carbonate forms on the heating element surface, heat transfer efficiency decreases, requiring longer heating times for the same volume of water, resulting in increased energy consumption and temperature control malfunction.
4. Impact on Skin and Equipment
Rough surfaces easily trap dirt and grime, allowing bacteria and fungi to multiply faster. Long-term contact may cause skin itching or dryness. Limescale also affects the overall appearance and lifespan of the spa hot tub.
Why is limescale so common in spa hot tubs?
1. Sustained High Temperatures Accelerate Deposition
Regular bathtubs are used for shorter periods and don't maintain a constant temperature, while whirlpool spa hot tubs require prolonged periods at a constant temperature, leading to more frequent carbonate precipitation.
2. Closed Systems Cause Increased Ion Concentration
Spa hot tubs typically don't have frequent water changes, operating on a recirculating system. With evaporation and replenishment, dissolved salts accumulate, gradually increasing hardness and creating conditions conducive to scale formation.
3. Improper pH and Chemical Use
Many people use inappropriate amounts of chlorine or disinfectants when maintaining spa hot tubs, causing pH imbalance. An alkaline environment (pH > 7.8) most easily causes carbonate precipitation, thus promoting scale formation.
How to Effectively Remove Scale from Spa Hot Tubs?
1. Use Specialized Descaling Agents
Specialized descaling products for spa hot tubs are available on the market, dissolving calcium and magnesium deposits without damaging the inner wall.
Follow these steps when using:
• Turn off the power and circulation system;
• Add an appropriate amount of descaling agent;
• Let it stand for a while, then scrub the surface;
• Rinse thoroughly and refill with water.
2. Adjust pH to the normal range
Regularly use pH test strips to check the water quality and ensure the pH remains between 7.2 and 7.8. If it is too high, use a pH lowering agent to adjust it to prevent minerals from precipitating again.
3. Clean the filtration system
After descaling, the filter cartridge must be cleaned or replaced; otherwise, residual crystals will re-enter the circulation system.
It is recommended to clean the filter every two weeks and replace it every 3-4 months.
4. Use a water softener system
Installing a water softener is one of the most effective ways to prevent limescale buildup. It removes calcium and magnesium ions from the water, reducing limescale formation at its source. This is especially important for users in hard water areas.
How to prevent the spa hot tub from feeling sandy again?
1. Regularly test water hardness
Use a hardness test kit to ensure the water hardness is below 150 ppm. If this value is exceeded, adjust or add water softener promptly.
2. Weekly Maintenance and Water Change Schedule
Maintain a regular drainage and cleaning schedule—generally change all water every 1-2 months to prevent mineral buildup.
3. Control Chemical Additive Ratio
Excessive chlorine, bromine, or alkaline detergents can disrupt the water's chemical balance. Strictly follow the instructions to avoid pH fluctuations.
4. Keep the Heating System Clean
Perform a "circulation cleaning" every 3 months. Use descaling solution to circulate through the system and remove deposits inside the pipes.
Summary of the Essence of Spa Hot Tub Scale and Sandpaper Feel Problems
The sandpaper-like feel of a whirlpool spa hot tub is essentially a result of mineral deposits and chemical imbalance. It's not a manufacturing defect, but a natural phenomenon caused by maintenance negligence.
Long-term neglect of scale not only affects the user experience but also leads to:
• Wasted heat energy and reduced heating efficiency;
• Clogged nozzles and weakened water pressure;
• Permanent surface wear;
• Potential for hygiene and skin problems.
The key to success lies in regular testing, timely cleaning, and maintaining the water's chemical balance. With proper maintenance, a whirlpool spa hot tub can remain smooth, clean, and perform consistently.
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Innovation is a core principle at LOVIA SPA. Our R&D team continually studies ergonomic seating, water therapy trends, insulation technology, and digital control advancements. Each year, we release new spa and swim spa models with updated features to help buyers stand out in competitive markets. Wholesale customers gain early access to new releases and exclusive pricing. When you buy from LOVIA SPA, you’re purchasing high-quality products created with modern design and technological improvements—ideal for expanding your brand’s market appeal.
