Why Does My Pool Lose Water In The Winter?
When you open your pool in the spring after being closed for the winter it is very important to note the height of the water in your pool. This is your first, and sometimes your only visual symptom that you may have a problem. A leaking pool is never a good thing. A leaking pool in the winter can be a real disaster if you live in a cold climate area. There could be a lot of reasons to be asking "why does my pool lose water in the winter?" and as much as it might seem counterintuitive, it is technically possible to lose water in the winter when the pool is closed, but not really as much when the pool is open and operating.
The first point to understand here is that pool leaks can be very elusive. The rate of water loss can increase, slow down and even periodically stop depending on environmental conditions. In a situation where your pool loses water more in the winter than the summer this could be due to a crack in a suction line. When the pool is running and the pump is sucking the crack does not really leak water out so much as it just allows air to pull in. Once the pump is disconnected for the winter the water can now more readily escape the crack. It could also be that saturated ground conditions from water tables or persistent leaks slow active leaks in the pool, but during the winter the water table exposes the leak location which increases the rate of water loss. It could be that you have an automatic fill valve on your pool that gets turned off in the winter and as a result you notice a leak which is actually always there.
Despite the possible, but less common causes for your pool losing water in the winter, by far the most common answer to this question is simply that the pool loses water in the summer also, and you either simply did not notice before how much water you were losing, or the leak is new. In either case if you open your pool for the spring and the water level is lower than where it should be, you very likely have a leak on your hands. So just how high is the water level in your pool supposed to be anyway? That would depend on the type of cover system that you use.
Tarp cover - Tarp covers are an isolation style cover where the water in the pool and the water on the cover are meant to be isolated from one another. This means you can pump filthy brown water from the top of the cover, and yet the pool water under the cover can (and should) be crystal clear. In theory this is a great cover system but as many pool owners know if you get any holes in the cover then the pool water under the cover will just end up looking like the gross water on top of the cover. Assuming the cover is in perfect condition the water that accumulates on top of the cover will displace water in the pool. This means as you scoop leaves and pump dirty water from on top of the cover, the water level in the pool goes down. By the time you remove the tarp from your pool the water level in the pool will be well down below the mouth of the skimmer. This necessitates to allow time for the pool to fill before the filtration system can be started up since the water level must be above the mouth of the skimmer on most pools. The normal water level in a tarp covered pool after the winter would depend on how much snow and rain accumulated on top of the cover, but typically anywhere from a few inches to a foot or more of space in between the bottom of the skimmer mouth and the water level is normal. This would leave, at minimum, around two feet of water in the shallow end of most pools, or more, and if you have less than that you probably have a leak in your pool somewhere.
Lock in cover - Somewhat common for vinyl liner pools are lock in covers which connect to coping tracks the same way that vinyl liners themselves do. This cover system requires no deck space at all, but other than this one feature they are largely difficult to work with, install, store, and move. Both the vinyl type as well as the polyweave type of lock in covers are both designed to be isolation covers similar to how the tarp covers are. This means that the water level in the pool will be displaced by the water on top of the cover and as you pump that dirty water off, the water level in the pool will be going down. Again, once you remove a lock in cover from a pool you should have anywhere from a few inches to a foot or more of space in between the water level and the bottom of the skimmer mouth. If your water level is much lower than this you could have a leak in your pool. It is also worth noting that you could have a leak in the cover itself. If you had a tear in an isolation cover like this, and you have been pumping water from on top of the cover, then you have also been pumping down pool water. It might be that the pool is not leaking at all, and that you isolation cover is leaking and you accidentally have been pumping off the cover and the pool.
Safety cover (mesh) - A mesh safety cover allows rain water (and some sunlight) to go through the cover. These lightweight covers are easy to work with and protect the pool well, but allowing water to pass through the cover means that the pool water is not isolated from outside factors. If you have a bunch of leaves on the cover then rain and melting snow will pull the tannins from the leaves and essentially turn your entire pool cover system into a teabag. This is why mesh style safety cover pools are supposed to close as late in the year as possible, and open as early as possible. If left too long the pool water will turn bad so the pool needs to be opened and started up before the warm weather arrives. In most areas a mesh safety cover pool will be essentially overflowing in the spring when you open the pool. This is an advantage for being able to start up the filtration system on the same day you remove the cover, but the pool is definitely more dirty than when an isolation cover is used. If you have a mesh safety cover and your pool is not full in the spring, then you almost certainly have a leak in the pool. Or you live in an area where is does not rain for the whole winter.
Safety cover (solid) - A solid safety cover is the isolation cover version of the mesh safety covers. These robust covers are the same system as the deck mounted and spring loaded mesh safety covers, but with much more heavy duty hardware and a thicker, non porous cover surface that does not allow water to pass through. These solid covers are heavy and hard to work with compared to the lighter mesh covers, and the solid covers require the use of a cover pump when the cover is on the pool. Since no water can pass through the cover, all rain will accumulate on top just like with tarp and lock in covers. The only difference is that the springs and straps of a safety cover can only stretch a small amount, and the weight of the water on top of the cover will exceed what they can support. You will regularly need to pump the water from on top of the cover to avoid damage from overloading. Since this is an isolation style cover this means that in the spring when you remove the cover, the water level in the pool will likely be down from the periods of time where water was on top of the cover and displaced water from inside the pool. Since a solid cover like this does not move or stretch like tarps and lock in covers, your water level should not be down a foot or more from the skimmer like with them. The water level in a pool with a solid safety cover should be a few inches down from the skimmer mouth at most. Lower than this indicates a leak in the pool somewhere and higher than this might mean that you have a hole in the cover itself. As with the other isolation style cover systems if you have a hole in the cover and you have been running a pump to pump off the cover, then you are effectively pumping down the pool level at the same time through the hole.
Automatic covers - An automatic cover is the exact same process as a solid safety cover except that instead of being installed into the deck of the pool with straps and springs, the cover is rolled up on a roller at one end of the pool, and installed by a pulley system that sends the cover down a set of tracks that run the length of the pool. Similar to solid covers, automatic covers need to have a cover pump to pump off water and snow melt to avoid stress and damage to the hardware and track system. Automatic covers are less able to absorb stress from water loading than a solid safety cover, and automatic covers are also much more expensive to repair if damage from water loading does happen. The water level in your pool when you open in the spring should be similar to solid safety covers in that the water on top of the cover will have displaced a small amount of water from inside the pool, but your level should not be down more than a few inches below the mouth of the skimmer or I would be concerned that there is a leak. Having a leak over the winter on a pool with an automatic safety cover (or a solid safety cover) could be a really bad situation as this might go unnoticed until the cover (or hardware) broke under the weight of the water on top of the cover. These cover systems require the water level in the pool to help support the weigh on top of the cover. If the water level in pool goes too low then this can result in a sudden failure of the cover system.
Now that you have a better idea as to where your water level should be you can determine whether you likely have a problem or not. If your water level is lower than it should be when you take the cover off after the winter then you should begin the leak detection process detailed in this article: how to find a pool leak. While a leak specialist may likely be required there are actually quite a few non-intrusive and largely non-technical steps that you can take on your own to determine if your pool is losing water, and approximately how much it is leaking. You might even be able to glean enough information from the symptoms that the pool is giving you to determine where in the pool system the leak is. Most importantly, as discussed in this article, you should never ignore a leak in your pool.
If your pool loses water in the winter, then it also loses water in the summer, or at least you could say that there is a deficiency with the pool somewhere. Whether you can notice a problem with water loss in the summer versus the winter or not you can rest assured that the problem is there during both seasons...it just might not be as easy to notice.
Why is my pool losing water?
Swimming pool leak detection
Why you should never ignore a leak in your pool
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