Overview
Chlorine is both a sanitizer and an oxidizer and makes it an ideal choice for making sure your hot tub is ready for a soak. Two important parameters for hot tub owners to track are free chlorine (FC) and combined chlorine (CC).
Why does this matter?
Sanitizers kill harmful microorganisms like bacteria, viruses, and algae, making the water safe for users. Oxidizers break down non-living contaminants in the water, such as oils, lotions, sweat, and organic debris, which can cause cloudy water and unpleasant odors.
When chlorine is added to the water, a residual level of free chlorine (FC) is established. As it interacts with nasties in your water, it will become inactive. Combined chlorine is responsible for the strong chlorine smell you might associate with pools and spas. Combined chlorine can be removed through oxidation using active chlorine, often by shocking.
How do I measure FC and CC?
The FAS-DPD drop test is the most accurate way for a hot tub owner to track FC and CC. I recommend doing this test daily, sometimes more in the beginning while you’re learning your water and the impact sanitizer has on your water.
What is a good FC and CC for my hot tub?
Let’s start with FC. FC heavily depends on your CYA level. As your CYA level increases, so does the amount of chlorine needed for proper sanitization. Your minimum target should be 7.5% of your CYA. So if your CYA is 30, 2 would be the absolute minimum you’d want your FC at.
I strongly recommend reading the following article to learn more about the CYA and chlorine relationship: CYA Chlorine Relationship – Further Reading.
As far as CC is concerned, the best number for that is zero. 0.5 is acceptable. Anything more, and there’s likely something going on in the water that needs to be taken care of soon. One word of caution: CC can elevate directly after soaking — so it’s best to check CC a few hours after using the tub to make sure it’s accurate.
Non-chlorine shock can also cause CC to show high, so make sure to take that into account if you’re using that in your program.
How do I change FC and CC in my hot tub?
You can raise FC by using a form of chlorine such as Dichlor or liquid chlorine (bleach). Remember, Dichlor adds CYA so it shouldn’t be the sole form of chlorine for the life of the water unless your usage is low and you plan to change your water frequently.
You can reduce CC by shocking (super-chlorinating) the water until it reads 0.5 or lower.