How to Use Chlorine in a Spa: The Ultimate Guide
By Jennifer Rhodes · Updated
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Chlorine is a chemical that helps kill bacteria and other contaminants in your hot tub. When you add chlorine to the water, it has a sanitizing effect that can last for days—but it will eventually dissipate, and then you'll need to add more to keep the hot tub clean and safe.
In this article, I'll be discussing exactly how to add chlorine to your hot tub, some of the benefits of using chlorine as a sanitizer, and also some reasons why you might choose not to use it. I'll also mention some alternative methods for maintaining cleanliness in your hot tub.
What are the different types of chlorine?
There are several types of chlorine, but not all of them can be used in hot tubs. It's important to understand the difference so you know which ones are safe, and which ones can cause damage.
There are also many different terms you might hear, and it can be confusing trying to figure out how they all relate to each other. I will try to explain the common ones in simple terms here.
Dichlor vs trichlor
Dichlor and trichlor are both types of chlorine, used for spa and pool sanitation respectively.
- Dichlor is a granular chlorine that contains both chlorine and cyanuric acid or CYA (stabilizer) in the same compound. It is cost effective, easy to use, and dissolves quickly in water. Dichlor is safe to use in hot tubs, both as a shock treatment and also as a regular sanitizer.
- Trichlor is a slow-dissolving chlorine tablet, also containing chlorine and cyanuric acid. Trichlor is used primarily as a long-term sanitizer and is designed for pools, but not recommended for use in hot tubs due to its low pH. The advantage of trichlor is that it releases chlorine over a longer period of time, which helps maintain a more consistent chlorine level in the water.
Both dichlor and trichlor are effective sanitizers when used correctly and for the right purpose.
What's the difference between free, combined, and total chlorine?
Knowing these terms will help you understand a bit more about how chlorine works to sanitize your hot tub.
- Free chlorine is the active form of chlorine that is available to kill bacteria and other pathogens in the water. It is the chlorine that is not yet combined with other substances.
- Combined chlorine is the form of chlorine that has reacted with other substances, such as sweat, urine, or other organic matter, and is no longer available to kill bacteria and pathogens. This chlorine is "combined" with the other substances and is no longer active.
- Total chlorine refers to both the free chlorine and combined chlorine together. It is the total amount of chlorine present in the water.
So, think of it like this: when there's a lot of free chlorine, your spa is clean and capable of catching any nasties. Over time, this chlorine encounters contaminants and turns to combined chlorine. When most of the free chlorine has been used up (or combined), then you need to shock your spa. This will kill the bacteria, and turn that combined chlorine back into free chlorine so it will be able to do its job once more.
What is the best chlorine for a hot tub?
You have a couple of different options when it comes to sanitizing your spa with chlorine.
Most popular: granular chlorine
The safest and most effective form of chlorine for hot tubs is concentrated granules specifically designed for spas, like this Chlorinating Concentrate from SpaGuard:
This kind of granular chlorine is readily available online or at most pool or spa stores, and it's what most people will recommend you use.
Lower maintenance: a dispensing cartridge system
One of the downsides of chlorine is that you have top it up every couple of days. One way to get around this is by using a slow-release cartridge, like the FROG® @Ease® Floating Sanitizing System:
This is basically a chlorine and mineral cartridge, which floats in the spa and releases a constant low level of spa-safe 'SmartChlor' chlorine into the hot tub, eliminating the need for daily adjustments and maintenance. This makes it a great option for hot tubs with minimal use or for those who travel a lot.
The FROG® @Ease® Floating Sanitizing System works well for those with lower hot tub usage, who don't mind spending a little more for the convenience of not having to add granules daily.
Another advantage of the FROG system is that it does not contain CYA (Cyanuric Acid) which means that you will not have to drain and refill the hot tub as frequently due to CYA buildup. However, this advantage won't be as significant in a high-use hot tub as you'll need to replace the water frequently anyway.
On the downside, this system is more costly than granules, with the initial cost of the cartridge, and then replacement of the chlorine part every 3-4 weeks. In addition, if you have high hot tub usage, the amount of chlorine released by the FROG system may not be enough to handle the load, so you might have to supplement with granules as well in this case.
How much chlorine do you put in a hot tub?
It's important to know how much chlorine to add and how often because adding too much could make your tub unsafe or irritate sensitive skin. The right amount will depend on the size of your tub and the potency of the specific chlorine you add.
So, firstly you need to know the volume of your hot tub. This is the total amount of water in gallons. You can find this by checking your spa's manual, or looking up the make and model online.
Once you know how many gallons of water your hot tub holds, you need to measure out the appropriate amount of granules according to the directions on the packaging.
For example, if it's recommended that you add 1 oz. for every 200 gallons in hot tub capacity, this means a 220-gallon tub should get about 1.1 oz. of granules, and a 400-gallon one should get 2 oz.
As a rough guide, you might expect a couple of tablespoons per dose.
How to add chlorine to a hot tub
Now you know what type to use and how to measure out the correct amount, I'll go over the process of actually adding the chlorine to your hot tub.
- Turn your spa pump on so the water is circulating. This helps to distribute the granules so they can dissolve evenly.
- Slowly pour the chlorine granules into your hot tub as the pump is running.
- Let the water circulate until the chlorine has had time to dissolve, usually around 15-20 minutes.
- Test the water to ensure it is in the appropriate range for safety and comfort. The recommended level is 1-4 ppm.
Chlorine can be toxic if handled incorrectly, so it's important that you follow these instructions carefully. You should wear protective gloves, avoid mixing the chlorine with anything besides adding it to your spa water, and not store chlorine directly next to other chemicals (especially bromine) as this could create a fire hazard.
When (and how often) should I add chlorine to my hot tub?
You should generally add chlorine to your hot tub every 1-3 days depending on usage. A typical use of one person taking a 15-minute soak once a week might only require adding chlorine every 2-3 days, while more frequent use can mean topping up the chlorine every day.
Another factor that contributes to how often you need to add chlorine is the total amount of bathers in your hot tub at one time. If there are more people using the spa then it will become dirtier faster and need more frequent sanitizing since the effectiveness of the chlorine becomes diluted with each person added.
Essentially, you need to measure how much chlorine is in your spa daily using test strips like these ones on Amazon so you'll know how often to put more in.
It doesn't really matter what time of day you add chlorine, but just know you'll want to wait around 30 minutes before getting in the hot tub after you do.
Generally, I'd recommend adding chlorine right after using your hot tub so it can get to work eliminating any bacteria or other potential contaminants that were introduced. It's also a good idea to avoid adding it in direct sunlight, because UV rays can drastically reduce chlorine's effectiveness.
How to shock a chlorine hot tub
Shocking a hot tub, also known as oxidizing, is the process of adding an extra chemical to kill any bacteria or other contaminants that might have entered the water since you last treated it.
To shock your hot tub, the easiest thing to do is usually to just add a "shock" dose of the same chlorine granules you use to sanitize your spa (the SpaGuard ones I linked above are a sanitizer and an oxidizer in one).
This should be done at least weekly, or more often depending on how much your tub is used. Remember, just like regular chlorine use, the number of bathers in your hot tub will help to determine the right frequency.
What happens if you put too much chlorine in a hot tub?
If your chlorine level gets too high, it can irritate your skin, eyes, and respiratory system. It can also damage your hot tub by corroding its components, or bleaching the shell.
Since you can't exactly remove chlorine from a hot tub once it's been added, you might find it easiest to just wait until the level lowers naturally—chlorine dissipates on its own pretty quickly.
But if you're impatient, there are a couple of things you can do to speed up the process:
- Leave your cover off for couple hours: This will allow more water (and therefore chlorine) to evaporate into the air.
- Replace some of the water: You don't need to drain all of the water—just remove enough to allow for a couple inches of fresh water to be added. This should be enough to get your chlorine levels down.
Whether you decide to try these suggestions or just wait it out, you should avoid using the hot tub until the chlorine levels have come down enough to be safe.
Did you know? Chlorine should have barely any odor if used correctly. 'Swimming pool smell' is actually the result of chloramines—chemical compounds released when the sanitizer mixes with unwanted contaminants.
If you ever experience a chemical odor from your chlorine spa that smells overpowering, it's a sign you need to shock your water. This will kill the contaminants your chlorine has encountered, and convert it back to 'free chlorine' so it can be effective again.
Why does my hot tub use so much chlorine?
There are a number of things that can cause a hot tub to use a lot of chlorine. These include: the size of your hot tub, how often you use it, whether you have kids or parties in your hot tub, and even the age of the water. Let's take a closer look at each of these things.
Spa size
If you have a large hot tub, it makes sense that it will use more chlorine because there is simply more water that needs to be chemically treated. Accepting that you will go through chemicals faster is just the price you pay for owning a big, luxurious spa.
Frequency of use
Whenever you use your spa, you introduce new contaminants into the water in the form of oils, make-up, lotions, or sweat. These need to be neutralized by chlorine so that they can't grow into mold or algae and cause the water to get cloudy. If you use your spa frequently, it's going to need more chlorine to keep these contaminants in check than if you only used it on occasion.
Kids
As much as we love them, kids can make a mess of a hot tub if they've been playing elsewhere in the yard, or if they bring snacks or drinks into the spa which could get spilled. These things can introduce a lot of dirt into the water that needs to be killed with chlorine, so you can expect to be adding sanitizer more frequently.
If you have kids, it's a good idea to keep an eye on them in the hot tub anyway. Why not try setting some rules like not eating or drinking in the spa, or getting in the habit of having everyone shower beforehand?
Number of bathers
If you have a party tub, you can expect to use lots of chlorine simply due to high usage. Once you've had a busy weekend with lots of folks over, it's going to be time for an extra dose of sanitizer since there are almost certainly some residual contaminants in the water that will take more chlorine to kill.
Age of water
If none of these other things apply and you're still having trouble maintaining your chlorine levels, it might be time to drain and clean your hot tub. The older the water gets, the harder it becomes to maintain the balance of the levels in a spa, because there are more dissolved solids in the water.
You can expect to do a full drain-and-refill every 3 to 6 months, depending on the size of your spa and how often you use it. You'll need to rebalance the water chemistry every time you drain and refill anyway, so if something is out of whack, think of this as a fresh start.
Is there a difference between pool chlorine and hot tub chlorine?
Unfortunately, pool chlorine and hot tub chlorine are not interchangeable. Liquid chlorine designed for pools should never be applied to a hot tub.
Here's why:
- Pools have a large volume of water so they can withstand a dose of concentrated liquid chlorine because there's much more water to dilute it. Hot tubs are much smaller, which means liquid chlorine can actually cause damage.
- Hot tubs are generally hotter than pools (100-104°F versus around 80°F), which causes liquid chlorine to dissolve much more quickly and unevenly.
- Pool chlorine contains certain additives to stabilize it. The stabilizers found in pool chlorine can actually affect your hot tub's water chemistry, especially its pH, and cause it to drift out of the hot tub's safe range.
Can I use chlorine tablets in my hot tub?
It's generally not a good idea to use chlorine tablets in your spa. This is because chlorine tablets are made with Trichlor, which is acidic and can cause your pH to drop. Chlorine granules are a better option because they are closer to pH neutral.
However, if you have an inflatable or other type of non-acrylic spa, chlorine tabs are okay to use because there won't be the same risk of bleaching an unsightly line around the shell as there is with acrylic spas.
If you do decide to use chlorine tablets, you'll need to keep a close eye on your pH and alkalinity levels. You can expect these to read lower than the ideal range more frequently than if you just used granules.
Alternatives to chlorine in a hot tub
If you want to avoid using chlorine altogether, there are other alternatives like bromine, biguanide, and even salt water that can be used instead. Let's take a brief look at each of these options.
Bromine
After chlorine, bromine is the next most common sanitizer used in spas. I wrote a full guide to using bromine in hot tubs which goes into a lot more detail on how to use it, but here's a brief comparison of the two:
| Chlorine | Bromine |
|---|---|
| Chlorine is a more effective sanitizer which means it's faster to kill contaminants than bromine | Bromine is gentler on the skin, whereas chlorine is more drying and can be irritating to some people, causing redness, itchiness or rashes |
| Chlorine is cheaper on average | Bromine has a lower pH, which can make your water balance easier to maintain |
| Bromine is more stable, which means the levels stay more consistent, so it lasts longer and doesn't need topping up as often as chlorine |
Biguanide
Biguanide is a less popular but great all-around sanitizer which also kills bacteria effectively. It's a non-chlorine sanitizer, which means it needs to be used less frequently than chlorine or bromine. It's also odorless, and can enhance the feel of the water making it silky smooth.
The downside of biguanide is that it's usually more expensive than chlorine or bromine, and can have negative effects of certain spa parts. It's a good idea to check with your hot tub manufacturer before using biguanide to make sure you're not risking any damage to your spa.
Salt water
Salt water sanitizing sounds like a great option to many people, because it sounds natural. However, you need a special system installed—you can't just pour salt into any hot tub.
Aside from this, what most people don't realize is that salt water hot tubs don't actually mean your spa is chemical-free. This is because salt systems actually process the salt to produce chlorine, so you end up with a chlorine-sanitized spa anyway.
Salt is cheap, but the hassle of converting a spa to use it often isn't worth the hassle. For most people, it's easier to just stick to using plain old chlorine granules.
