Hot Tub Stabilizer: What to Do When Cyanuric Acid Is Too High or Low

Jennifer Rhodes

By Jennifer Rhodes · Updated

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Having adequate cyanuric acid (CYA) levels is essential to keeping certain chlorine hot tubs clean and safe for soaking. But hot tub stabilizer is a confusing topic for spa owners—specifically, what to do when your CYA levels are too high or too low.

In this post, we will discuss what CYA is, why it's important, and what you can do to make sure your CYA levels stay in the optimal range.

What is the role of CYA in a hot tub?

The purpose of cyanuric acid in a hot tub is twofold:

  1. It helps protect chlorine from being broken down by the sun's UV rays (this is more relevant for outdoor pools where this is more of a problem than for hot tubs, which spend most of their time covered).
  2. At the same time, it is also used to moderate the strength of chlorine, allowing you to more easily maintain an ideal chlorine level.

4 ppm of free chlorine (FC) with 30 ppm of CYA in a hot tub is only equivalent to about 0.6 ppm FC with no CYA present. So, it's easy to be running your spa with too little chlorine and not realize it.

It's important to keep CYA within the optimal range for the best effect. Too much CYA can prevent chlorine from working properly, while too little can mean that chlorine levels can spin out of control.

How to measure CYA in a hot tub

Test strips are the most economical and easy-to-use way to measure the levels of chemicals in hot tubs. To test your cyanuric acid levels, use a test strip that measures for cyanuric acid like these:

Premium Pool and Spa Test Strips - 100 ct - 7 Way Accurate Testing Strip for Pool + Hot Tub | Chlorine Bromine Alkalinity pH Hardness & Cyanuric Acid | Water Quality Testing Kit for Water Maintenance
Premium Pool and Spa Test Strips - 100 ct - 7 Way Accurate Testing Strip for Pool + Hot Tub | Chlorine Bromine Alkalinity pH Hardness & Cyanuric Acid | Water Quality Testing Kit for Water Maintenance
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There are also more advanced liquid test kits or test kits that use the titration method which can give a more accurate reading. I wrote a post comparing two that I have personally. I know the LaMotte ColorQ is able to test for CYA.

The level of cyanuric acid in your spa should be between 30-50 ppm (parts per million). Below 30 ppm, your chlorine may be too unstable, and above 50 ppm, it may not work as effectively.

Monitoring and maintaining the correct levels of cyanuric acid in your hot tub is important for ensuring clean, clear, and balanced water. With a quick and easy test strip, you can easily check your cyanuric acid levels and keep your hot tub balanced.

What to do when your stabilizer is too high

Too much CYA can mean that your chlorine's effectiveness in sanitizing your water decreases—leaving you exposed to bacteria growing in the water.

The key indicator of too much CYA is that the chlorine in your hot tub seems too weak, despite multiple attempts to shock it. In this case, the chlorine is unable to adequately oxidize bather waste and can be too weak to kill certain pathogens fast enough.

Fortunately, the fix for high CYA levels is quite simple. To lower CYA in a spa, you have a couple of options:

1. Dilute with fresh water

The simplest method is partial draining and refilling of the tub. The amount you need to replace will depend on just how high your stabilizer is. Don't forget to test and rebalance everything else once you've topped up with fresh water too.

2. Add CYA reducer

Another option is to try a CYA reducer product:

Bio-Active BA-CAR-08 Non Polluting 100-Percent Cyanuric Acid Reducer Powder for Commercial and Residential Swimming Pools, 8 Ounces.
Bio-Active BA-CAR-08 Non Polluting 100-Percent Cyanuric Acid Reducer Powder for Commercial and Residential Swimming Pools, 8 Ounces.
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These are really made for pools though, so be sure to calculate the quantities very carefully if you decide to try this.

In order to prevent any further issues, it's also important to keep an eye on the chlorine and CYA levels going forward. That way, you'll be able to detect any issues early and take steps to address them as needed.

What about CYA buildup?

CYA buildup occurs when too much cyanuric acid is added to a hot tub over time. This often happens due to long-term use of a stabilized chlorine like dichlor—which many spa owners are not even aware contains CYA.

They add the chlorine granules, and then over a couple of months, start to notice their spa water getting dirty or smelling bad. The common advice is to shock the hot tub, so they add yet more granules.

However, the reason the chlorine isn't working is due to CYA buildup, so adding more is only going to make the problem worse! After 2-3 months, the only way to fix the problem is to fully drain and replace the water.

The dichlor/bleach method

One solution to this is called the dichlor-then-bleach method. This is a two-step process for maintaining the chlorine levels in a hot tub:

  1. Starting with a freshly filled and balanced hot tub, you begin by using dichlor, which is a CYA-containing form of chlorine. You add this normally until the CYA level reaches 30 ppm. This usually takes about a week.
  2. Then, you switch to using a 6% unscented liquid bleach to shock the water instead. This will also work as an oxidizer to kill contaminants and replenish free chlorine—but without adding any more CYA.

This is a popular method used in chlorine hot tubs, as it allows for more time between water changes (up to 6 months rather than just 2 or 3!).

Over time, the CYA will drop gradually. This isn't a problem though; just check the level every couple of months. When it falls, simply shock with dichlor again until it has recovered to around 30 ppm, then switch back to the bleach.

What to do when your stabilizer is too low

A low CYA level can make chlorine too strong in certain cases, resulting in a reaction with swimsuits, skin, and hair, as well as making it outgas more quickly—which can in turn damage your hot tub's cover.

If your cyanuric acid levels are too low, you can add cyanuric acid stabilizer or "conditioner" to raise the levels. However, there's a chance you won't need to do this.

If you are using a stabilized chlorine such as dichlor, it's important to note that you usually should not need to add extra CYA.

It all depends on your chlorine measurement. Test your chlorine before you do anything else. If it's in range (1-3 ppm), then you don't need to do anything—even if your stabilizer is reading zero.

If you are not using stabilized chlorine however, then you will likely need to add CYA separately, as the chlorine may be too strong otherwise.

Closing thoughts

In summary, restoring the balance in your hot tub if your stabilizer is too high can be done by replacing some or all of the water. If instead you have a too-low level of CYA, this is not normally a problem as long as your chlorine is in range. If it isn't, add a stabilized chlorine like dichlor until the CYA reaches 30 ppm.

If you are constantly running into problems with high stabilizer and weak chlorine, you may have CYA buildup. You can either change your water every couple of months, or try the dichlor-then-bleach method which will allow you to go up to 6 months between water changes.

As with any sanitizing system, testing your water regularly is still essential to make sure all levels remain balanced.

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