Can I Use Salt Water In My Hot Tub?
If you want to know if you can convert your hot tub to a salt water system the answer will depend on a few different aspects of the spa that you have. Before we can start to look at these specific points first I need to start at the beginning and make sure that you understand that salt water systems are not an alternative to chemical sanitizers in any way. Even if you have used salt water pools and spas and they feel different or better to you it is important to understand this is a function of chemical balance and salinity. When you go with salt water for a pool or hot tub you are not choosing an alternative to mainstream chemical treatments...you are simply changing where you are getting your sanitizer chemical from.
In traditional pools and spas chlorine (or bromine) are bought as a finished consumer product. Simply add the liquid, or pucks, or granules to your pool or spa and now you have more chemical sanitizer in the water. With a salt system, whether for a pool or hot tub application, you are no longer buying finished product chemicals. You are buying an electrolytic cell and the base ingredient that chemical sanitizer comes from - salt. Sodium chloride specifically for generating your own chlorine, and sodium bromide for generating your own bromine. With a salt system you do not have an alternative to chlorine or bromine, you have a chlorine (or bromine) generation station built right into your pool or spa. Like chlorine on demand. Just turn up the dial and generate more sanitizer as you need it. If you find this interesting and want to know more about how salt systems in pools and spas work you can read this article which will help you get informed about salt water.
Now that you understand how salt systems work in pools and spas we can begin to answer your actual question of how (or if) you can convert a hot tub to use a salt water system. In short, yes salt systems exist which are specifically made for hot tubs. The concern is that your hot tub might not be suitable for use with a salt system. The more expensive your hot tub is the more likely it will be comprised of high quality materials able to withstand the potential for damage from adding thousands of parts per million of salt to your spa. If you have an entry level hot tub it is unlikely that you would be able to convert to salt water without a very high likelihood of damage. So what, specifically, are the points of damage to be concerned about?
Damage to heating elements - While there are a number of concerns for adding a few thousand ppm of salt to your hot tub, easily the largest concern is for your heating element. Every hot tub has a heating element and it will be something that is expensive enough that you do not want to be replacing it every year due to damage from salt water. The concern is that there is a very high chance for your water to be in a "scaling state" which indicates (according to the Langelier saturation index) that your water will experience precipitate scale forming. This means that materials dissolved within the water will accumulate and the places where it accumulates will likely experience damage due to adverse pH of the scale itself combined with uneven heat distribution on the heating element. The scale will eventually coat the element to such an extent that it can not heat the passing water properly and the buildup of heat damages the internal resistance of the element until it eventually ceases to function at all.
Scaling state water - In pool and spa water systems the measurement of the inclination of the water to form scale is called saturation index. This is the measurement of the balance between calcium, pH and alkalinity primarily, but also the measurement of the total dissolved solids within the water and water temperature. When you add salt to the water you are increasing the total dissolved solids count. The pH of the water with salt systems is chronically high. This is one of the main disadvantages of these systems. In a hot tub the problems associated with scale formation are even more prevalent than with colder pool water systems. Just look at the heating element in a kettle used with hard water and you will see how heat, calcium and scale interact.
Water balance is important and it is possible that you could maintain your spa water meticulously enough that the saturation index would never be in a scaling state. You could also use scale inhibiting products like Natural Chemistry Scale Free to help prevent the accumulation of scale on surfaces within your spa system. You could use borates which would help to further buffer the movement of your pH (tricks for salt water pool maintenance) and at least in theory the damaging effects from salt would largely mitigated on your heating element, however this does not factor in whether you have a high quality or low quality heating element to begin with. Precious metals like titanium are used in heating elements and so there is a wide quality (and price) gap between low end heating elements and high end heating elements. Higher quality and thicker metals will simply last longer.
There are other concerns as well, such as failing pump shaft seals due to accumulation of scale which are also a concern but for sure the biggest concern is for the heating element. These are already a weak link with hot tub systems and can experience early failure from a host of reasons such as an air leak / bubbles that run through the system and past the heater. High sanitizer levels is also specifically destructive to metals like your heating element, and low pH combined with high sanitizer is probably the worst chemical situation to be in for the health of your heater. Provided that you intend to keep your pH in the 7.4 to 7.6 range, you will use scale inhibiting products, and worst case you will replace your heating element earlier than you might have otherwise, then you can convert your spa to salt water and consider yourself fairly well informed about the risks of adding so much salt to your hot tub.
Drop in style salt water chlorine system for hot tubs up to 1000 gallons in size
Drop in style salt water system with chlorine detection for hot tubs up to 1000 gallons in size
Saltron Mini Salt System
This is a drop in style spa salt chlorine system for hot tubs up to 2000 gallons in size
Blu Fusion Bromine Generator
This is a salt system that generates bromine suitable for spas up to 1500 gallons in size
Using a salt system is your hot tub requires that you are more careful about how well you balance your water chemistry. Hot tubs are already notoriously challenging as they are a relatively small body of water that can experience large changes in water chemistry fairly quickly. You would not have to look far to find articles telling you that salt water spas are easier to take care of then regular (bromine or chlorine) spas...and this is fundamentally untrue. It is even more important than traditional spas to monitor and balance your water chemistry including total alkalinity, pH, calcium hardness, sanitizer levels and the level of total dissolved solids. That is assuming you want to avoid damaging your hot tub and have parts like your heater last as long as possible.
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