A saltwater pool uses a chlorinator system to electrically convert regular salt (NaCl) into a form of chlorine that is milder than the chemical chlorine typically used in pools. Chlorine is used as a sanitizing agent to kill bacteria, prevent algae growth and ensure that the pool is clean for humans to swim in.
Modern salt chlorinator systems often have a built in shut off feature that reduces or outright stops chlorine production when the temperature drops below 50° F (10° C). Salt cells don’t work well below this temperature as salt synthesis becomes difficult. It is also more taxing on the chlorinator cell which may decrease its lifespan. However, it is often recommended that salt water pool owners wait until temperatures have fallen to or below 60° F to close their pools. At this point, if you choose to delay pool closing for whatever reason, you may have to manually add chemical chlorine to your pool to prevent algae growth if the salt cell does shut down and chlorine is no longer being automatically produced.
Salt Water Pool Chemistry
Salt water pools only require a relatively small amount of salt to remain sanitized. Parts per million (ppm) is the measurement unit used and a level of 2,700 ppm – 3,400 ppm salt content is required. Actual chlorine content for a pool is in the 1 – 3 ppm range. By comparison an ocean has 35,000 ppm salt content and 4 ppm chlorine content is considered the maximum level for safe drinking water.
Algae Growth
One of the biggest and most common problems for any pool owner is algae growth. Algae is typically green and slimy but can also be yellow or black. If you see slimy green growth on the walls or floor of your pool, it’s likely algae.
Algae generally occurs because of a lack of chlorine. The moment you notice algae growth, you need to immediately check your chlorine level as it’s likely low and outside the desired and necessary 1 – 3 ppm range. Your chlorine reading might be 0 if your salt cell has stopped working and if you’ve been using the pool and getting hot weather since the cell stopped producing chlorine.
If you experience black algae growth, the method of getting rid of it is different than if the algae is green or yellow/mustard.
Preventing Algae Growth
Algae typically won’t grow and will become dormant once the temperature drops below 40° F (4° C). But if you closed your pool when the temperature dropped to 60° as mentioned above, you may have several weeks or more where the temperature is within the 40° – 60° range (or higher if the weather picks up) where your pool is closed, no chlorine is being produced, and algae is allowed to take root.
When closing your pool in the autumn, it’s recommended to shock the pool with a high level of chlorine just before shutting the pump off for the last time in the season, and adding a preventive algaecide too. By adding liquid chlorine manually, you are bumping up the chlorine level to kill bacteria and prevent algae growth while the temperatures are still above 40° since chlorine is no longer being produced as the system is shut down. Adding the algaecide is an extra measure to specifically prevent algae growth.
Your pool requires less chlorine as the temperature drops and since the pool isn’t being used once it is closed, the chlorine required decreases further. And as mentioned above, algae growth slows as temperatures drop and stops at about 40°.
Smart Pool Closing Tips
Once the summer temperatures start to drop, kids go back to school and the pool is just sitting there not being used, you might think about closing the pool. If you’re not using it, why keep it open with the pump running? You have to keep the pool vacuumed and maintained. It’s easier to close the pool and not have to worry about it, right?
While you should try to keep the pool open and the pump running to produce chlorine until the temperature consistently drops below 60°, there may be occasions where you have to close the pool earlier. If so, you definitely want to first super chlorinate by adding liquid chlorine to the pool before closing and also adding the aforementioned algaecide to prevent algae growth. You may consider purchasing a winterization kit if you close your pool yourself or ensure that your pool company doesn’t forget these steps.
You should also use a winter pool cover (like the green LOOP LOC shown above) to help block the sun’s UV rays as it kills chlorine. This sort of winter cover is not cheap but serves several purposes.
First is the sun blocking ability that we just mentioned. UV rays from the sun speed up the destruction of chlorine which can lead to lack of sanitization in the pool which leads to algae growth and a green pea soup-colored pool come spring. Which means more work for you and more chemicals to get rid of it.
Second is the safety aspect. The pool cover is strong enough when installed correctly to support the weight of a person or animal to prevent them falling in. This is especially important if the pool does turn green from algae and you can’t see the bottom.
Third, this style of pool cover prevents leaves and other debris from falling into the pool and disintegrating and making a mess of the pool over the winter. Our LOOP LOC pool cover unfortunately doesn’t cover the entire pool area because we have a rock waterfall with plants and soil and the cover was designed to be strapped onto the rocks. Every winter, leaves from our bushes and trees fall into the pool, sit and rot over the winter. Every spring, I spend time vacuuming the rotting leaves out from the deep end of the pool as a result. If not for this design, the pool would probably be virtually clean come spring opening.
Looking for more advice on closing a pool come season end? Check out my post on 10 Tips To Winterize A Swimming Pool.