Solar Cover Question

Johnny-M

LifeTime Supporter
May 11, 2012
123
Altoona, Pa
Does pool water get warmer on a sunny day with the solar cover on or off? I always wondered. I know the cover prevents CL burn off during the day and and prevents evaporation and heat loss during the cooler nights, but for simple heat transfer to the water does it reflect heat as well as UV rays?
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,238
Tucson, AZ
With no wind and relatively high humidity, you would get more heat when the pool was in the sun with the cover off.
The covers generally are more for maintaining heat by reducing heat loss through evaporation/radiation ... solar cover is a really bad name for them.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
It depends on the clarity of the cover. If it's a very clear cover, it will let a significant amount of sunlight through, both visible and some infrared and some UV. Such covers do truly aid heating of the pool during the day. For an opaque cover, if it's white or reflective then there will be little heating of the water, but if it's a dark opaque cover then there will be some heat transfer for thin covers such as electric safety covers. For bubble-type covers, there won't be much sunlight absorption and heat transfer unless the top part of the cover is clear and only the bottom part touching the water is black or dark.

So in order of heating, it is the following:

Clear bubble-type cover -- passes through the most sunlight, retains heat well
Clear-on-top, dark-on-bottom, bubble-type cover -- dark bottom heats up from sunlight and passes heat to the water, retains heat well
Dark opaque thin cover -- dark cover absorbs heat and passes it to the water, but the thin cover doesn't retain as much heat
White or reflective bubble-type cover -- does not absorb heat but retains heat well
White or reflective thin cover -- does not absorb heat and the thin cover doesn't retain as much heat

The main trade-off is that the clear covers also pass through UV so while you get heating from sunlight passing through, you also get breakdown of chlorine.

With no cover at all, there is substantial absorption of sunlight which heats the pool where a white plaster pool with 4.5 foot average depth absorbs 60% of the sun's light energy and heats the pool at around 0.7ºF per hour with noontime sun. Competing with this is water evaporation where the same pool would lose about 5ºF if 1/4" of water evaporated. Evaporation depends not only on the water and air temperature (except for perfectly dry air), but also depends on the humidity and especially on wind at the pool water's surface. See the thread Water Absorption and Heating from Sunlight for more technical details.
 

Johnny-M

LifeTime Supporter
May 11, 2012
123
Altoona, Pa
Chem Geek I knew you would have a scientific explanation of my question. Which is exactly what I was looking for. Thank you very much. I appreciate the education. I knew there was a trade-off on heating vs CL burn-off, but wanted to make sure. I am just getting everything ready to open the pool tomorrow and wanted to get the water as warm as I could for the kids. I always keep the cover on while not in use to prevent CL loss. so I may compromise a bit this year on both.
 

mmdhar

Well-known member
Jul 26, 2013
47
Chandler, AZ
Hi, I am also looking to buy a solar cover for my pool. I ve read several suggestions to buy the lightest I can find. But, no body mentioned about 5-MIL cover. Is it ok for me to buy 5-MIL? or is it too thin? I know solar covers don't sell them. But, found on ebay vey inexpnsive. But, good reviews for the seller.

Thanks,
Murali
 

Kiss4aFrog

LifeTime Supporter
TFP Guide
May 22, 2013
2,725
Hudson, WI
Why did they suggest buying the lightest ??

I'm in the market as mine is disintegrating and is in no condition to even go on the pool this year.
I'm surprised to find out that a clear one would gain the most heat. Being "north" gaining and keeping heat is an important consideration.
 

Shelley N

Well-known member
May 1, 2007
199
Denver, Colorado
This year we are using a new, dark blue, 8ml cover (solarcovers.com $53.99 tax, tag and delivery). I would call it clear dark blue. My findings are it is protecting the chlorine in a BIG way. I have almost NO measurable CYA as I just used what was left in the bottle from last year and never got around to getting more. Might not now. Water temp is has been around 80-85 this year, which I'm also going to say may be a tad warmer than other years this early in the season. We haven't used the pool much at all with it being a cooler spring, so the cover hasn't been off much at all which also helps, but I've been through years like this and don't believe I've noticed such a dramatic saving of chlorine with thinner, clear and light blue covers. Just my $.02.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
If you have no CYA in the water, then the active chlorine level may be too strong and will degrade the cover faster and oxidize your swimsuits, skin, and hair faster when you use the pool. CYA is not just for protecting chlorine from sunlight, but also significantly moderates chlorine's strength so that it is not too strong.
 

Shelley N

Well-known member
May 1, 2007
199
Denver, Colorado
Well, we took the cover off over the weekend and it took about two seconds for the CL to burn off! Dramatic. In fact, the sides got slimy in an afternoon!

I have a 15 x 30 above ground and follow the chart. Chem geek - You are right, my levels may have gotten up to the 3ppmish occasionally with the low CYA. I hope I didn't damage anything.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
So 3 ppm FC with no CYA will be around 50 times higher in active chlorine than the minimum FC for CYA level recommended here. That just means that every day your cover is exposed to this chlorine, it could degrade as much as around 2 months of exposure at a more suitable lower level. In practice, it's not quite this bad since diffusion limits the rate, but it's still noticeably higher. So since covers last 2-3 years, it would take at least 2 weeks of exposure to "ruin" the cover and probably more like 1 month of such exposure. Also realize that chlorine degradation is only part of cover degradation -- UV is also a factor and that part doesn't change.

So I suggest getting some CYA into your pool as soon as possible. It will also prevent the chlorine from degrading so quickly from the UV in sunlight.
 

Shelley N

Well-known member
May 1, 2007
199
Denver, Colorado
Agreed, chem geek. I'm at around 30 for CYA now, keeping CL at around 4 as evenly as I can and am STILL amazed at the protection this cover seems to be providing. It's 8ml dark blue for me from her on out.
 

guytimes

LifeTime Supporter
May 17, 2015
56
Louisville, KY
So in order of heating, it is the following:

Clear bubble-type cover -- passes through the most sunlight, retains heat well
Clear-on-top, dark-on-bottom, bubble-type cover -- dark bottom heats up from sunlight and passes heat to the water, retains heat well
Dark opaque thin cover -- dark cover absorbs heat and passes it to the water, but the thin cover doesn't retain as much heat
White or reflective bubble-type cover -- does not absorb heat but retains heat well
White or reflective thin cover -- does not absorb heat and the thin cover doesn't retain as much heat

The main trade-off is that the clear covers also pass through UV so while you get heating from sunlight passing through, you also get breakdown of chlorine.
Here's a good place for covers. Solar Covers
Just wanted to say thanks for the info, all. Based on this info I bought the 16mil 18x36 crystal clear cover. Can't wait to get it. At night my pool is around 86, and in the morning it has dropped to 82. Hoping the cover will help.
 

sumter

New member
May 17, 2016
1
Clinton, NJ
I have an L-shaped pool and covering the smaller part is real hassle, so I've just been covering the long part which is about 75%. Is it worth using the solar cover if it only covers 75% of the surface area, or is it like leaving the windows open with the heat on?