Calcium Scale in a Hot tub

Scale is a mineral based build up that happens in a hot tub when the water is out of balance. There are multiple factors that determine if your tub will develop hot tub scale. This mainly affects hot tub owners that live out of town and have to use well water to fill their hot tubs or if phosphates are present in your source water. 

Types of Hot Tub Scale

There are 4 types of scale that happen in spa water– calcium phosphate, calcium carbonate, calcium sulfate and barium sulfate. There are various methods of prevention and treatment for each type of scale. The best thing to prevent hot tub scale is to find a hot tub professional you trust that understands the different scale types, how they form, why they form and together you can work on a plan of attack for you and your unique water conditions.

Where Hot Tub Scale Comes From

Calcium hardness from well water contributes to scale because of the high amounts of calcium present in the water. First factor to look at is the pH in a hot tub. Ideally a hot tub’s pH should be between 7.4-7.6. A hot tub generally has a high temperature which mixed with a high pH level can cause calcium to build up on the surface of a person’s hot tub. Alkalinity can also affect scale build up in a hot tub. Totally alkalinity is a buffer for pH. It will help control a pH level that is in a hot tub. Generally if alkalinity is high in a hot tub, the pH will be high. If alkalinity is low in a hot tub, the pH will be low in a hot tub. There are exceptions of course where the alkalinity and pH may be on opposite ends of the spectrum.

scale in pipe

Other Sources of Scale

Another cause of scaling or calcium deposits in your hot tub is PHOSPHATES. These little guys cause demands on your sanitizer, are food for algae, and can bond with calcium ions to create scale. The best way to prevent this type of scale is to remove phosphates at a fresh fill and when you are topping up your water. You should also avoid adding spa care products that contain phosphates – just like high fructose corn syrup, phosphates are everywhere!

Why should I care about hot tub scale?

Scale build up in hot tubs is an issue because it can start to clog plumbing and build up on metal surfaces. This is also a clear indication that there is a water imbalance in the hot tub. When plumbing starts to become clogged, this prevents good circulation of water through the spa which can be hard on equipment. Scale will also start to build up on the acrylic surfaces of the hot tub. This leaves an uncomfortable feeling to sit on for hot tub users.

What should I do if I see scale in my hot tub?

In order to remove scale from surfaces, hot tub owner can take a cloth to remove the grainy particles. Work with your local spa water specialist if the scale is ‘stuck’ and they can analyze your water and give you treatment recommendations.

In order to prevent the scale from coming back, hot tub owners must balance their water to unsure they are in the recommended ranges for pH, alkalinity and calcium hardness. For people that have well water it is hard to bring down the calcium level in hot tubs because their source water generally has high calcium. The recommendation is usually to fill half of their hot tub with softened water and half with hard well water. This generally brings the calcium levels into the range that is recommended for hot tubs. Don’t forget that you should also be applying a stain and scale preventer. This is a chemical that prevents that formation of scale in hot tub water. In order to remove scale build-up from plumbing, use a hot tub flush product when you drain and refill their hot tubs. This helps remove scale from pipping and heater surfaces in order to prevent the plumbing from getting clogged.

The take away about scale in a hot tub is that it takes time to develop and it takes time to get rid of it. Like most things in life, the best prescription is prevention.


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