Off-Season Water Care: Managing Pool Water Balance and Controlling Organics

In some parts of the U.S., pools are typically treated only once as the cover prevents outside debris and waste materials from entering the water.

Whether one calls it winterizing, closing, off-season, or end-of-season pool care—how pool professionals handle water care once there is not much pool activity, all boils down to what part of the country the pool is located. In all cases, the two variables that need to be addressed are (a) managing water balance, and (b) controlling organics. This article will focus on the various products that can be used to balance pool water, as well as those that can be used to control organics during the winter months. When both are tended to, the likelihood of a successful start-up next season will improve greatly.

Water balance

Water balance factors include pH, total alkalinity (TA), calcium hardness (CH), water temperature, and total dissolved solids (TDS). During the season, the goal is to maintain these water balance parameters to ‘ideal’ levels suggested by organizations such as the National Swimming Pool Foundation (NSPF). In these ideal conditions, whether at the lower or upper end of the scale, the water is close to being ‘balanced’ according to an equation that helps determine the potential for scaling. This equation is known typically as the Langlier Index or Saturation Index (LSI).

For example, the average water temperature for most pools during the season is approximately 27 C (82 F). As the peak of the season winds down, so do water temperatures and, as temperatures decrease, so does the indicating number in the LSI. This value describes the water’s potential to be more corrosive or aggressive. The chart (on the right) shows how the LSI number decreases as the water temperature decreases, no matter if the minimum or maximum ideal water care parameters are maintained. Certainly, the majority of pools are not maintained on these minimum and maximum levels, but the purpose is to demonstrate how important it is to make adjustments as the pool season winds down. By not responding appropriately, the risks of having a problem the following season become higher.

Regardless of geography, one of the ways to move the needle on LSI is adding an all-purpose sequestering treatment product prior to closing (or continuously) during the off-season. Sequestering products bond with soluble metals (such as calcium) to prevent the formation of scale. Essentially, this water treatment regimen moves the total LSI level to a higher, positive number. When traceable heavier metals are present, such as iron and copper, the use of all-purpose treatments helps to prevent those metals from creating stains and discoloring the water while the pool is not in use.


The other element of seasonal pool water care has to do with what grows in the water. Mould, algae, bacteria, viruses, and other micro-organisms all linger in the water regardless of the season. However, as the water temperature gets lower the activity of these micro-organisms also decrease, sometimes exponentially. So, similar to when the pool is open, there is as equal importance in eliminating them or at least keeping them under control, when it is closed for the season.

Universal in the approach to off-season water care is the use of a product that keeps algae growth under control—regardless if the pool is covered or left exposed to the elements. That said, there are essentially three types of algaecides to consider.

Algaecide can use quaternary ammonium compounds, or often referred to as a quat algaecides. These are inexpensive and have the potential of ridding the water of any existing algae. It is important to note, however, these algaecides—especially at higher concentrations—may cause foaming. Additionally, when administering high levels of chlorine to the water, the effectiveness of the quat-based algaecide is diminished.

Other algaecides commonly used contain poly dimethyliminio ethanediyl dichloride. These algaecides are usually referred to as poly algaecides. They are non-foaming and keep a greater residual in the water in the presence of chlorine.

Lastly, algaecides with the presence of copper are effective chemistries to combat algae in the long-term. These algae inhibitors are similar to poly algaecides as they are typically non-foaming and hold well in the water, even in the presence of chlorine.

Although these are the three basic types of algaecides used in pool winterization, there are other hybrid options available. Poly and copper hybrid algaecides is a growing market segment because they provide all the advantages of two separate algaecides in a single container.

The shocking truth

When closing the pool, shocking the water is a must to reduce the amount of contaminants. After the water is shocked, contaminant loads are reduced, which allows the disinfectant (sanitizer) to help control the other materials lingering in the water, either currently or later during the off-season. There are two different camps with respect to what works best. Some like to use chlorinated products, while others prefer to use non-chlorine oxidizing shocks.

Using chlorinating products to close a pool has its advantages. For instance, if chlorine levels are non-existent in the water then applying a chlorinating shock will simultaneously scourer the pool of algae, bacteria, and other contaminants, while leaving a residual in the water during the off-season. When unstabilized chlorines are used, the impact on the water’s pH is minimized.

Non-chlorine oxidizing shocks, relative to chlorine, are still new in the recreational water maintenance industry. Since most are comfortable with the ability of chlorine, most do not look at non-chlorine as an effective tool—especially when it comes to closing a pool. However, just because the product does not have chlorine present, it does not make it any less of an oxidizer. Non-chlorine oxidizers have the ability to scour more contaminants in the water than its chlorine counterpart. Further, these products will not combine with chlorine and possibly form any disinfection byproducts. Most importantly, its lack of chlorine is better for the long-term appearance of the pool’s surface. Regardless of its effectiveness as an oxidizer, a chlorine presence during the off-season is still a must.

Additional treatments

Depending where the pool is located (climate region), the amount of products used to close and/or maintain a pool over the winter, as well as the frequency of application, may vary. In the northern U.S., pools are typically treated only once as the cover prevents outside debris and waste materials from entering the water. In areas where pools are covered (or use mesh covers) that experience mid-winter thaws, additional algaecide treatments are important to keep the growth of micro-organisms under control.

In southern and southern-coastal communities, a pool cover is not generally used. The lack of a cover means the pool water can be exposed to outside debris and, of course, the sun, even though the temperature is cold. Due to the pool’s exposure, the water will need to be treated more often than a pool located in the north, but certainly less than when the pool is open for the season.


Enzymatic treatments are also a good option when winterizing pools. One of the benefits of using these types of products is their ability to prevent a waterline ring around the pool during the closing. Another important function of these treatment products is that they will breakdown complicated structures in the water, leaving chlorine (and algaecides) to effectively do their job.

Borate-based water treatment products also offer superior protection to surfaces as they inhibit corrosion and scale. However, the use of these products does not excuse one from having to balance the water first. That said, its use at closing time is an added insurance policy that the water balance from pool closing to opening will be identical. These products are beneficial for those pools with covers that allow rain and melted snow to enter the pool. Borate’s natural clarifying ability can help maintain water clarity during the off-season as well.

Anywhere there is exposed water during off-season care, evaporation is likely to occur. Placing a solar cover on the water or using a specialty liquid formula that mimics a solar cover can help reduce evaporation. The benefit of either one (or both) will reduce the amount of time or at least the amount of water one would have to replace to top off the pool when it is opened.


In northern regions, it is more than likely for a pool to experience prolonged freezing conditions. Therefore, in these harsh conditions, it would be wise for maintenance professionals to vacate the water from everywhere but the pool itself. This includes the pump, filter, heater, skimmer, and effluent and influent line.

After evacuating the plumbing lines, placing an anti-freeze solution in the skimmer and plumbing is a cheap insurance policy in the event water retreats into these areas. To provide pool owners even more protection, maintenance professionals should add a sacrificial plug to the skimmer in the event it gets flooded with water.

That said, equipment plugs should be removed and drained on pools in northern regions. Pressure gauges and switches from various components should also be removed as they typically have a small amount of water remaining in them. If there is a multi-port valve attached in the equipment system, it should be moved to the closed position, or at least in-between valve positions so the gasket does not rest in one position for the entire off-season.

Other helpful winterizing devices include: snap-on skimmer closures which can eliminate costly water draining in the winter. These devices are easy to install and help protect the skimmer from freezing in the winter while conserving valuable, treated pool water. Additionally, many pool professionals use a specialized ‘blow out plug’ for pool winterization. These plugs allow air and water to flow freely when pressure is applied and seal with an O-ring. They can even function as a hydrostatic relief valve or a multi-directional return fitting.

If the pool is not covered, as is the case in the southern states, the water must be circulated to maintain its appearance. Fortunately, since the activity in and around the pool is significantly less, resulting in slower growth of micro-organisms in the pool, the water can be circulated less than it needs to be during the season.

Water circulation is especially important for those pools that are not completely shut down for the season to avoid water from freezing in the plumbing and equipment. Pools can also be equipped with ‘freeze-guards’ which will automatically turn on the pump in the event the water temperature drops to a certain temperature.

This article was written by Kevin Vlieststra and originally appeared on Pool & Spa Marketing [link].