When Should You Close A Pool For The Winter?
If you are not lucky enough to live in a warm climate area where you can swim year round then you will need to close and winterize your pool for the off-season...but when should you close the pool? To clarify this article is for people with swimming pools that experience freezing winter climates. In some geographic areas it might not be warm enough to swim all year long but it never really gets all that cold either and as such there is no risk for freezing plumbing lines. In these places you can get away with modifying the chemical maintenance for your pool as the only step to making it through the winter season. In places with sub-zero winter temperatures extra preventative steps must be taken to eliminate the risk for damage to the pool from freezing water.
If you live in an area that experiences freezing winter temperatures then you already know you need to close and winterize the pool but when is the optimal time to close your pool for the year? There are a few different points to consider here relating to how much you use the pool, how much you are willing to spend to heat the pool to swimmable temperatures during the cooler shoulder seasons, as well as the type of pool cover system that you use. The right time to close your pool will depend on which of these is a priority for you and how much you are willing to spend to keep your pool going once the weather turns colder.
Water Temperature - The first problem with keeping your pool open later in the swimming season is that the water temperature drops drastically. When overnight low temperatures are sometimes near freezing you can expect the relatively stable water temperature in your pool to plummet. While you might hold 75 to 80 degrees Fahrenheit during the swimming season even without a heater, once the cool nights around September arrive you will find the near 80F water temp slips all the way into the 60's. You must either be an avid swimmer or be part polar bear if you like swimming in water that is 60-something degrees. For most pool owners this would be out of the question especially when the weather itself is not all that hot anymore. Using a pool heater sounds easy enough to rectify this problem. The only concern with this is cost. Electric heat pumps and rooftop solar heaters are primarily used to improve in-season swimming temperatures and once the cold nights have started these pool heat forms usually lack the BTU power to keep your water warm. If you want to swim when the weather is cooler you need a natural gas or propane heater. It will work to heat your pool water any day of the year if you feed enough fuel into it. Unfortunately these costs can get very high very fast heating large volumes of water to a warm swimmable temperature when the ambient daytime and night temperatures are not all that conducive to swimming. Keeping your pool water warm from burning gas will easily cost you in the hundreds of dollars per month, and can be thousands per month in extreme cases.
95% of heat energy in a swimming pool is lost through radiation, convection and evaporation through the surface of the pool water. Using a solar blanket at night can reduce heat energy losses by up to 95%. If you want to heat your pool during colder months the use of a solar blanket nightly is critically important.
Falling Leaves - If you have an endless budget to pay for heating the water plus forearms like Popeye for scooping leaves every day then I suppose it is possible to keep your pool open and operating until there is a serious risk for freezing temperatures. What most pool owners learn is that during the autumn season swimming pools turn into the "gutterball lane" for every falling leaf in your neighborhood. Literally on a daily basis you will need to be scooping, vacuuming and checking chemicals in your pool due to the sheer volume of organic debris that ends up in the water from falling leaves. If you live in a new development area without a lot of old growth trees this might pose only a minor inconvenience. If you live in a developed area with a lot of old growth trees then you can have clean and clear water one day and the next day you can not even see the water through the blanket of leaves that has settled into your pool. For many pool owners the best time to close is the period of time where the nights are getting colder but before autumn leaves start to fall from the trees. This gives you the longest possible swimming season that allows for the least workload to maintain the pool.
Freezing Temperatures - At the heart of the question "when should you close and winterize your pool" is the risk for damage to the pool from freezing water. If it were not for this danger to the pool you would not close the pool at all. Such is the case in temperate climates where pools remain open and operating rear round even though the actual swimming season is much shorter than this. Which looks better, your beautiful crystal clear swimming pool or a giant black garbage bag with stagnant water standing on top of it? If you have the opportunity to leave your pool open year round then definitely this is the way to go. However if you deal with consistently sub-zero winter temperatures then you need to take steps to protect and minimize damage to your pool from freezing water. If the weather will be too cold for too long then you need to drain the pool below the lowest return port (conventionally speaking) and blow out all pipes in the pool. As well as evacuating and plugging the pipes to and from the pool you also drain and winterize each component of the filtration and sanitation equipment. For more information you can watch this multi-part series breakdown on how to winterize a pool. Without any water remaining within the pipes or pool equipment there is minimal risk for damage from freezing. Cover the pool to keep it protected and isolated from debris and wait for warmer weather to arrive.
Cold But Not Freezing - Many areas of the west coast and southern USA experience weather that is colder than most would agree to be "swimming weather" however periods of time below freezing are few and far between. In these areas the drastic steps to drain the lines and winterize the equipment is not required. Instead it is common practice to maintain operating water levels in the pool as well as maintaining the water chemistry as per normal. In these colder months the water has very little activity in terms of algae growth and bacteria proliferation and as a result the chemical maintenance becomes very manageable compared with the more active summer temperatures. During overnight lows below the freezing point, and throughout any stretches of days where temperatures are at or below freezing you need to run your filtration and waterfall equipment continuously. Moving water does not have as much risk for freezing as stagnant water. The main body of the pool volume helps to keep the overall water temperature above freezing and the movement of water helps to prevent exposed and risk sections of pipe from freezing and bursting.
If you live in an area with seasonal swimming and consistently sub-zero winter temperatures then you might find that the best time to open and close your pool follows the schooling schedule for children. Once the kids have gone back to school in the late summer and early fall the pool likely does not get as much use as it did when school was out. The few weeks each year in between when the kids go back to school and before the majority of the leaves have started to turn and fall is the best time to close. The only exception to this would be using any form of non-isolating pool cover, most specifically mesh safety covers.
If you have a mesh safety cover, the style that allows rain and melt water into the pool, then you want to close the pool as late in the year as possible and then open as early in the spring as possible. These covers have a tendency to allow the pool water to turn green, often vibrant green, due to organic debris leeching water and tannins through the cover and into the pool. If you close right before the water turns freezing cold, and open as soon as the weather is above freezing every night, then you will avoid having green water. You do however have more maintenance work than other pools since once the pool is opened is must be started up, cleaned and chemically maintained whether you can swim yet or not. It is more work, but at least you get to look at your pool instead of a pool cover!
How to close a pool (step by step)
Common mistakes you might make closing a pool
Should you drain your pool for the winter?
How to winterize a pool heater
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