Why Does the Chlorine Level Keep Dropping in My Hot Tub?

It’s pretty frustrating to see your chlorine levels just disappear without explanation. But the good news is that it’s a common problem that’s easily fixed. Here is a quick summary of the potential causes.

If your chlorine levels keep reducing suddenly, then it’s likely caused by one of the following: a water chemistry imbalance, contaminated water, dirty filters, frequent usage, high bather load, UV rays, evaporation, impurities in the water source, rain, and pollution.

At the end of this article, I’m going to show you how to ensure that your sanitizer levels always remain stable.

8 causes of chlorine demand

1. Water chemistry imbalance

Chlorine demand usually occurs when you don’t check and balance your water chemistry regularly. Maintaining proper alkalinity and pH levels is the bedrock of successful water chemistry in your hot tub. 

The pH levels affect the ability of chlorine to do its work properly. If the pH levels are too high, the chlorine will fail to clean the spa efficiently. Meanwhile, if they are too low, the chlorine will dissipate more quickly.

Therefore, you should always test your hot tub water to know how much chlorine or pH you need to add or remove. If the test shows a zero chlorine reading, it could mean that the chlorine level is too high and is bleaching the color out of the reagent on the test strip.

2. Contaminated water

Another cause of chlorine demand is contaminated water. Your water may contain contaminants like bacteria, algae, and fungi. This usually leads to the formation of biofilm on the surface of your hot tub. 

Biofilm is formed when bacteria or microorganisms stick themselves to a surface that is in contact with water. Biofilm is resistant to chlorine and will cause the chlorine to disappear from the spa water. This is because the chlorine is continuously working to eliminate the biofilm.

3. Dirty filter

Dirty filters can also lead to chlorine demand. If your filter is dirty, it won’t function effectively. This will cause the chlorine in your hot tub to work overtime since it isn’t being filtered out. 

4. Frequent use and high bather loads

If you use your hot tub frequently or a large number of bathers use the spa at the same time, this affects the chlorine levels significantly. Activities such as a hot tub party can boost bacteria and oxidizable compounds in the spa water to unusually high levels. 

This is especially true if the bathers enter the spa with many products on their bodies, such as sunscreen, deodorants, oils, lotions, and moisturizers. 

These products can contaminate your spa water, and as the chlorine works to kill these contaminants, the chlorine levels in the hot tub drop, leading to chlorine demand. 

You can prevent this by requiring bathers to shower before entering the hot tub. Additionally, you can shock the spa immediately after parties or other events where many people have used the hot tub.

5. Sunlight

The UV rays from sunlight can break down the chlorine in your hot tub, causing the chlorine levels to drop very quickly. 

Unless you’re in the process of removing algae from your hot tub, it is advisable you add shock to your spa water only in the evening time. This is because adding shock during sunlight hours would make the chlorine less effective.

6. Evaporation

The temperature of your hot tub water can affect the effectiveness of the chlorine present. If your hot tub is not in use and the water temperature exceeds 100 degrees, the chlorine can quickly evaporate if there is no cover. 

7. Source water

If you’re wondering why your chlorine level disappears so fast, it could be because your water source is contaminated with metals, algae, or other pollutants.

For instance, hard water contains high levels of calcium which can damage your spa and prevent sanitizers from working properly. 

Another source of water that you should take into consideration is well water. Although it is safe for filling hot tubs, it contains a high mineral content that must be treated using chemicals.

Whether you’re using hard water or well water, It is advisable to use an inline pre-filter when filling your hot tub. Pre-filters remove up to 98 percent of the impurities in the water. This makes it easier to balance the water chemistry after filling your hot tub.

8. Rain and pollution

Another reason your chlorine level is dropping so quickly is because of the pollution from heavy rain. Heavy rain dilutes the chlorine in your hot tub and affects the water’s pH. 

Therefore, it is important to test your chemical levels after rainfall to see if you need to adjust the pH or add more chlorine.

However, this case is more common among open pools and shouldn’t be a problem for hot tubs, especially if you use a spa cover.

How long should chlorine last in a hot tub?

Generally, it should take between 1 hour to 48 hours for the chlorine in your hot tub to dissipate completely. You should add chlorine to your hot tub water at least once every week. 

Some hot tub owners prefer to use a small amount after each use. Some hot tub owners also prefer to use bromine instead of chlorine since it can do the same job with less odor.

However, the number of times you should add chlorine (or bromine) to your hot tub depends on how often people use it and the number of people that use it. 

How to fix chlorine levels and keep them stable

Test your water before filling your hot tub

One of the most important things to do to ensure stable chlorine levels is to test your fill water (water from your hose) before filling your hot tub. 

You can take a sample of your fill water to your local hot tub dealer to help determine whether it contains high levels of metals, calcium, and/or phosphates.

You don’t want your hot tub water to contain any of these contaminants because they can contribute to algae growth and reduce the effectiveness of your chlorine sanitizers.

Therefore, ensure you filter out these substances during the filling process. Metals like iron and manganese should be below 0.5 ppm, while copper should be below 0.2 ppm.

Test the chemical levels of your hot tub water regularly

Once your hot tub is up and running, ensure you use test strips to check the levels of chlorine, pH, alkalinity, calcium, and phosphates regularly. 

The type of test strip you use depends on your water care system. Your local dealer can also help you determine the appropriate strip to use for your spa. 

When you dip a test strip into your hot tub water, you’ll see if your water’s chemical levels and overall condition are within acceptable levels. 

Shock your hot tub water 

Shocking your hot tub water allows you to raise chlorine levels above the recommended level for a short period. This will help kill bacteria, remove chloramines and bromamines, and increase free chlorine in the hot tub.

Get rid of biofilm 

Biofilm can be found on the hot tub shell and in the plumbing of your spa. Using a plumbing cleaner, you can get rid of the biofilm in your plumbing and pipes. 

As the cleaner gets to work, it will cause scum to form on the water’s surface. This indicates that the chemical is doing its job well. However, you’ll need to drain the water afterward. 

Once you’ve drained the tub, use an enzyme product or hot tub surface cleaner to clean the hot tub shell and eliminate the biofilm on the surface. After cleaning, refill the tub and keep on top of water maintenance.

Add some shade

You should consider adding an offset umbrella or sunshade to help filter the sun. Additionally, keep your spa cover securely latched when the hot tub is not in use. 

Lower the temperature

You should consider lowering your spa water temperature below 100 degrees when you’re not using the hot tub. Not only will this help to maintain proper water chemistry, but it could also reduce your energy usage.

Rinse and clean the filter

Ideally, you want to rinse your filter with fresh water every week and with chemicals every month. In addition, you should place it in a chemical soak every three months. 

However, if you’ve been using your spa more frequently than normal, you may need to clean the filter sooner than expected.

Joshua Milton

Joshua Milton is a seasoned hot tub enthusiast. With many years of experience in the industry, he offers valuable insights on hot tub maintenance, health benefits, and relaxation techniques.

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