In-Depth Hot Tub Water Chemistry

In this guide we will talk about water chemistry in depth. Understanding the foundations and fundamentals not only makes it easier to understand and manage your hot tub water chemistry but also makes it far easier to troubleshoot any water issues which may come about. When you understand the basics of chemistry as it relates to water you will often be able to look at your hot tub water and instinctively know what the problem is and how to solve it.

There are actually three stages of hot tub water chemistry that come into play. They are:




First off is water balancing. Hot tub water must be properly balanced for the sanitizer to do an effective job of keeping your water clean. In addition, proper water balance is far healthier not only for the people who use the hot tub but also for the hot tub components themselves. There are three things which should be balanced in order for your hot tub water to be considered in balance but only two of those relate to the effectiveness of your sanitizer and only those two need to be adjusted on a regular basis.

First though, let’s talk about calcium hardness as, while this is not important for the sanitizer to be effective, it is vitally important for the health of your hot tub. Water that is too low in calcium will try to “steal” calcium from the surfaces it comes into contact with. This means your hot tub shell, your plumbing and your pumps. Acrylic can actually be damaged over time with water that is low in calcium. On the other side of the equation, water that is too high in calcium will cause scaling your equipment (in particular on the heater element) shortening it’s life and can cause water to be cloudy. If your calcium levels are over 400 it is time to drain and refill your hot tub. There are products for stain and scale prevention that will counteract somewhat the effect of high calcium levels but we don’t recommend them because you are simply putting a band-aid on the underlying problem. To increase your calcium levels simple add a calcium booster to the tub at the levels specified on the container.

Now for the two most important components of water balance - alkalinity and pH. One thing to be clear about here is that your sanitizer, whether it be chlorine or bromine, doesn’t care one iota about your alkalinity level. The only thing that matters to the sanitizer is the pH level. So why do we worry about the alkalinity level at all? Well, without a proper level of alkalinity in your water, your pH will bounce, meaning that it will vary widely over even a short period of time. Since a sanitizer only works in a narrow pH range, we need to stabilize the pH level and that is what alkalinity does.

Don’t confuse total alkalinity with alkaline water. While they are related, they are two very different things - alkaline water is simply water that has a high pH level, that is it is more alkaline (or basic) than acidic. Total alkalinity on the other hand is a measure of the waters ability to neutralize acid. Water with a high total alkalinity will typically have a higher pH level because they are related but this is not always the case.

To stabilize the pH level of the water, you need to have the total alkalinity between 80 and 120 parts per million. This is where it gets a tiny bit tricky though. Adding alkalinity booster will also raise the pH and it will need to be lowered. Lowering the pH will also somewhat lower the total alkalinity so it’s a balancing act. The best way to overcome this is to test your water and note the pH and alkalinity levels. If the pH is normal to high, add enough alkalinity increaser to bring the total alkalinity to between 120 and 150. Then when you reduce the pH, the alkalinity will fall into the desired range. If the pH is low, on the other hand, add only enough alkalinity booster to raise the level to the desired 80-120 range.

Next up is pH and you want to bring the pH level of your water to between 7.2 and 7.6. Most conventional wisdom suggests a range of 7.2 to 7.8 but over 7.6 sanitizers will lose some of their effectiveness so I suggest keeping it below 7.6. To bring it into balance add enough pH reducer (or, very rarely, pH increaser) to bring the pH level to between 7.2 and 7.6.

Now your water is properly balanced and ready for your sanitizer!

There are two primary types of sanitizers that are used in hot tubs, namely chlorine and bromine. There are only minor differences between them and both are effective sanitizers. There is a MYTH that bromine is more effective in hot water than chlorine and it is just that - a myth! The only differences between chlorine and bromine are as follows:

  1. Bromine can’t be stabilized. Both bromine and chlorine have a half-life of 15-20 minutes meaning that when exposed to UV radiation they will break down by 50% every 20 minutes. A stabilizer increases this half life to hours which is why stabilizer is so important in swimming pools. Chlorine can have a stabilizer added to the compound but bromine cannot be stabilized.
  2. Bromine is slower acting than chlorine which is why hot tubs need a higher level of bromine than chlorine to properly sanitize the water. There is a school of thought that this slow steady process is somehow better for the water (and you) but there is no empirical evidence whatsoever to back this up.

So which should you use? It’s up to you but I recommend stabilized chlorine in granular form which brings us to the next point. Chlorine is available in two forms - dichlor and trichlor. Dichlor comes in granular form and trichlor is typically sold in puck form. You will hear that a dispenser with chlorine tablets is the best way to manage your chlorine (or bromine) but there is again a fundamental flaw with that line of thinking. A dispenser will add chlorine at a steady rate whether the hot tub needs it or not. If you have a period of low usage your chlorine levels will rise and if you use the tub heavily for a period you will not have enough sanitizer in the water. Additionally, trichlor is a very aggressive form of chlorine and will bleach out your jets, damage your pillows and damage any hard plastic surfaces it comes in contact with.

A far better alternative is to use dichlor chlorinating granules and it is actually easier than a dispenser when you get accustomed to it (which does not take long). When you test your water and the chlorines levels are low or zero, add enough granules to bring it to the proper level and here’s the kicker - every time you need to bring your chlorine levels up it will take the same amount of granules assuming you don’t have biofilm in your tub (that’s rare and another article). If you do have biofilm nothing will bring your levels up whether you are using a dispenser or tossing in granules. So once you know how much granules raise the level to where you need it, you know how much to add every time you need to raise your levels.

One thing we recommend for any hot tub owner is a good enzyme system. Using these systems means you can lower your chlorine levels by up to 80% so, if you have a sensitivity to chlorine, this is definitely the way to go. It’s much better than switching to bromine with even higher levels. Forget fancy UV tech and the like, enzymes are easy, effective and there is nothing to wear out! We have field tested over a dozen enzyme systems and we have found the best by far to be Spa Marvel. Just by adding a bottle every three months you can keep your chlorine levels below 1ppm so no smell or fumes and no skin issues (many people have reported that it has actually helped with skin conditions).

Now that your water is sanitized it is clean - until you use it. The next step is oxidization and it is, arguably, the most important step in clean water. To understand why, we need to explore how chlorine (or bromine) works. When the chlorine is added to the water it will attack bacteria and contaminants and form a molecule called chloramine. The contaminants are still in the water but they have been rendered inert by the chlorine so at this point you haven’t actually cleaned your water. Chloramines give off a strong odour so when you smell that “pool smell” you are actually smelling these molecules and the stronger the smell the more contaminants are in the water. To get rid of these, we use a process called shocking and it is simply adding a good quality shock to the water and leaving the cover open for a period of time which causes the chloramines to gas off out of the water.

There are two types of shock for hot tubs - chlorine based shock and non-chlorine shock. We recommend non-chlorine shock because it shocks the tub without overdosing the water with more chlorine. With a chlorine based shock it adds a large dose of concentrated chlorine to the water so the chlorine reaches it’s break-point. This is ten times the current chlorine level so if you are running 4ppm of chlorine the break point is 40ppm. When the levels get that high the chlorine starts attacking the chloramine, breaking ti apart into nitrogen gas and free chlorine with the gas escaping from the water. The main issue with this method is that you have to wait for the chlorine levels to come down before using the hot tub and it can be difficult to get the levels right afterwards.

A better alternative is a good quality non-chlorine shock. This has an ingredient such as sodium-monopersulphate which will also attack the chloramine molecule and gas off the nitrogen but without overdosing the water so you can get back in much more quickly (an hour) and also doesn’t throw your levels out of whack. Regardless of which shock you use, it is absolutely critical that you leave the cover open for at least 20 minutes after shocking it. Otherwise the gas will hit the damp underside of the cover and fall right back into the water again defeating the entire purpose of the shock.

Even using a non-chlorine shock there are times when you will want to super-chlorinate your hot tub and this is where granules are much more convenient. Super-chlorinating is not the same as shocking because you will raise your levels to about twice your normal level, not 10 times as much. If someone spills something in the tub or someone lets the dog jump in or you have a party or your water turns cloudy, you can simply add a double dose of chlorine, let the hot tub circulate for 4-6 hours and then add a double dose of shock.

So it’s easy to see that the best combination is stabilized chlorinating granules along with a good quality non-chlorine shock and combine it with Spa Marvel - you will have crystal clear water with no smell or fumes that is super gentle on your skin.