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We have a pool with an odd shape that makes it difficult to use a pool cover. Specifically, the sides of the pool change elevation in several places which make it impossible to use a roller to easily retract the pool cover when not in use. There is usually only one person around when the cover needs to be retracted, and it usually takes about half an hour and is very tiring.

If the pool cover was cut into smaller pieces it would make an easier job of it, as each piece could be rolled up separately; the weight of each piece would be managable, and the need to run back and forth across a 30 foot length to try to keep both ends rolling up evenly would be eliminated.

However, my concern is how much of the benefit of a pool cover would be lost. The actual gaps caused by having multiple pieces would potentially allow some heat to escape, but of more concern is the fact that water would more easily get on top of the individual pieces where it could evaporate and carry off heat. Reducing water usage is one motivation for using the cover in the summer; there is a lot of shade during the day so overheating the pool isn't a big concern, but I assume that since most of the heat loss is through evaporation the two goals mess well.

The cover is the silver/gray plastic type with bubbles that sit on the bottom when it floats in the pool, so it is also likely there would be a strip along the cut where the bubbles are destroyed. Obviously once I cut I can't undo the process so I don't want to do so until I have considered all the ramifications.

Is this something that I should do? If so, how much efficiency can I expect to lose? (For instance, if the cover was heating the pool by 15 degrees, how much can I expect when the cover in pieces?) Is there a particular pattern of pieces that would be more efficient? For instance, perhaps cutting stripes across the width of the pool would prevent the cover from overlapping itself and exposing gaps when the pump is on.

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It's crazy complicated. But:

If you are concerned with heat-loss due to evaporation, then reducing the exposed surface area will help. The rate of evaporation is proportional to surface area.

If you are likewise concerned with maximizing heat-gain from the sun, then that too is proportional to surface area.

Finally, heat loss due to convention is also proportional to surface area.

So everything you care about is strongly affected by surface area, thus maximizing the amount of water that is covered is a good goal.

It's impossible to estimate how important this will all be as far as actual water and heat loss/gain - you haven't given enough information in your question to judge that.

The unanswered question is how much heat and water loss your pool is experiencing, and what is preventing it from losing more. You might alter the rate of cooling/water-loss without actually affecting the ultimate final temperature and total water-loss.

Example: If I have a leaky bucket that I fill up with water once, it doesn't mater how quickly the bucket drains because once it's empty there isn't any more water to lose. If I am constantly topping up the bucket, then fixing those leaks is more important.

If I were you, I'd conduct an experiment. Leave the cover totally off for a few days and measure whatever you care about (water level, temperature, pool-heater-energy-consumption). Then do the same with the cover on, and perhaps again with the cover 1/2 on. That should give you enough information to make a gut decision on how much the pool cover is helping.

  • Well, I know that when I took the cover off the temperature dropped by 15 degrees in 2 days. But I didn't measure cloudiness or ambient air temperature, so a cold snap could have caused it to drop some of that even with the cover on. In general I have seen at least 8-10 degrees between on and off, but how does covering up with a single cover compare to covering up with the cover e.g. split into two pieces? how much will wind up exposed between the two (same total area, but they may slide over each other a bit) and how much extra water will get on top what percentage of the time to... – Michael Jun 10 '15 at 18:45
  • ...effectively increase the exposed surface area. That is probably the way it has to be calculated. I may wind up conducting an experiment where I just cut the cover in half, and if I can cut into 4ths for optimal heat retention versus ease of use then that's a good start, but if thirds is better then I can't do that once it's in half. Obviously the length of the pole segments which connect to roll the cover up plays a role too, as if splitting the cover in half results in insufficient pole it won't work. – Michael Jun 10 '15 at 18:47
  • And to make it even more complicated, after I wrote this question it also occurred to me that during part of the year I also want to address a different problem - excessive water loss due to evaporation - without gaining too much heat and making the pool overly hot. My initial thought was maybe leave the cover on during the day and take it off at night when cooling due to evaporation is lower and presumably due to conduction is higher, but... well, the initial question is complicated enough I'm not sure it would be useful to post another similar question... – Michael Jun 10 '15 at 18:49

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