Best time to remove safety cover in spring for fastest water warmup

JohnGn

Active member
Sep 28, 2019
31
St. Louis, MO
When is it most beneficial to remove the (green) safety cover and put down the (blue bubble) solar cover to get the maximum benefit of the increasingly warm solar effect from the sun? Average air temps are still in the mid-50's this time of year, but of course there are super nice sunny days that get near 70. I haven't tested my water temp yet, but I will be tracking it. Just wondering if anyone else along this latitude or north of me has experience with this. Thanks!
 

Homebrewale

Well-known member
Apr 21, 2020
400
Apex, NC
For me, it changes from year to year. Some years are really cool in the spring and others are hot. I try to open sometime in April. I look at the long range weather forecasts. I am still working so the weekend is the only time I can do the work. I won't remove the cover on a particular weekend if the forecast for Monday through Friday after the weekend is for cold temperatures and rain.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
24,030
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
I don’t think which cover you use will make a big difference. Do whatever makes you feel good.
 

RTamanda

Gold Supporter
Jul 25, 2011
43
Chester, VA
Pool Size
23000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
I tried solar cover for many years. I never could tell that it did much of anything for my temps. Seemed like way more of a hassle that what it was worth. I leave my safety cover on until I open, usually in early/mid April depending on water temps, then just totally leave it uncovered.
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
6,488
NY
Hey John !! You’ve bought into the bogus manufactures claims too much. The solar cover can’t perform any miracles. What is does spectacularly is greatly reduce overnight evaporation and heat loss. So you’ll see perceived ‘gains’ by not losing. But it doesn’t create any actual warmth on its own.

Where it really shines is if you have warm weather and the next day or two cools off. The pool will still be mostly as warm as it was before the air temps dropped. After 2+ days of cool weather the pool will start to reflect it also and although the 60 degree water is improved from the 50 degrees it would have been on its own, it’s of no consolation that it ‘helped’ being too cool to swim anyway.

If solar covers did even half of what everyone seems to think they do, nobody would ever need to burn tons of energy using heaters. Nor would they buy one for $3k-$5k.
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
6,488
NY
Also, for full disclosure and some insight into why people believe the solar cover hype, whenever I pulled my clear cover, the top 1/2 inch of water was literally smoking hot. But the other 89.5 inches of water laughed at it and it was like ‘spitting in the ocean’.
 

JohnGn

Active member
Sep 28, 2019
31
St. Louis, MO
Thanks for the replies, everyone.

I don't think I am expecting miracles out of the solar cover. In fact, the main reasons for using it have always been less evaporation, retention of heat gained during the day, and a lowering of my liquid chlorine requirements when in use.

However, that said, I have to argue in favor of the solar cover actually having a genuine effect on temperature gain. My company makes equipment that is normally used to track concrete temperatures (every 10 minutes) and I have two years of temperature records for the deep end of my pool, one year without a solar cover and one year with the new solar cover. There is absolutely no question that the solar cover makes the water temperature rise faster in the spring over time than not having one. Over the course of 30 days, for example, between Apr 15 and May 15, the uncovered pool gained 0.5 degrees per day on average, and with the cover gained 1.1 degree per day. Ambient temperatures and solar availability were about the same for each time period. I ran the pump at various times during each time period also, and it should be noted that running the pump had a significant effect on water temperature gain rate. Until I installed my pump timer, I would have to manually turn it on and off. The stunning rise in temperature when the pump had been running for an hour or so after not having been run was amazing. Thermal stratification exists in both scenarios, but is vastly higher with the cover on. When that 6-8" of hot water on top gets circulated through the pool, it most definitely has an effect.

My pool water is currently a brisk 42F in the deep end right now, with my safety cover still on and the pool still closed, so no circulation.
 
Last edited:
  • Like
Reactions: Newdude

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
6,488
NY
At first glance mine performed with similar results to yours. But with further education it opened up even more variables like the wind on the warm days. With no cover I wasn’t warming/retaining the daily highs on the days that it was windy, which is most of the spring. So the cover appeared to perform more magic than it did. Some of it was real, some imagined, but wherever that scientific line actually is, I’m glad you’ve seen the results you have. :)
 
  • Like
Reactions: JohnGn

magiteck

Gold Supporter
May 20, 2020
331
Neenah, Wisconsin
Pool Size
13600
Surface
Vinyl
Chlorine
Liquid Chlorine
I've definitely also seen a positive impact of using a solar cover- although I do feel it's much more so the insulation factor than any special "sun absorption". It can cool down substantially in the evenings here, and without the cover the temperature drops a lot faster. Over time, that greatly slows/limits the overall temperature gain.

One thought on the question at hand here, is when it might make sense in the early spring. To me, given the chief benefit seems to be insulation/reduced evaporation, my unscientific feeling would be that you wouldn't want to start using a solar cover until the water temperature is warmer than the air temperature.

If the pool is 40 -- and the air is 60 -- using a solar cover could almost insulate the cold pool from the warm air above, and slow down the warming -- almost like a beverage cooler.

Hopefully one of the scientists on the forum can tell me if I'm crazy. :)
 
  • Like
Reactions: Newdude

JohnGn

Active member
Sep 28, 2019
31
St. Louis, MO
I like your line of reasoning, Magiteck, and that's kind of where my thinking was, but it seems that by keeping the safety cover on the pool, it is blocking both solar AND ambient air temps from impacting the large mass of cold water. I suppose the easiest thing for me to do is to just pull the safety cover off without actually "opening" the pool, cover it with the solar cover, and continue my testing to see if it makes a difference this early in the season without the aid of circulation. Net impact should be for the water temp to rise more quickly than it is now at least. I'll post my scientific testing results in 30 days or so...
 
  • Like
Reactions: magiteck

Enjoying this content?

Support TFP with a donation.

Give Support

bmoreswim

Mod Squad
Gold Supporter
Platinum Supporter
LifeTime Supporter
Jul 16, 2012
6,801
Central MD
Pool Size
27000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
I open in early-mid April (then use my autocover) at a similar latitude because that's when I feel like being outside more often. I have this unscientific feeling that my pool water temp benefits from the sun similar to the way I do.
 
  • Like
Reactions: JohnGn

Homebrewale

Well-known member
Apr 21, 2020
400
Apex, NC
Hey John !! You’ve bought into the bogus manufactures claims too much. The solar cover can’t perform any miracles. What is does spectacularly is greatly reduce overnight evaporation and heat loss. So you’ll see perceived ‘gains’ by not losing. But it doesn’t create any actual warmth on its own.

Where it really shines is if you have warm weather and the next day or two cools off. The pool will still be mostly as warm as it was before the air temps dropped. After 2+ days of cool weather the pool will start to reflect it also and although the 60 degree water is improved from the 50 degrees it would have been on its own, it’s of no consolation that it ‘helped’ being too cool to swim anyway.

If solar covers did even half of what everyone seems to think they do, nobody would ever need to burn tons of energy using heaters. Nor would they buy one for $3k-$5k.

A pool gains much of its energy from solar radiation during the day. US Dept. of Energy states that pools absorb about 75-85% of the solar energy striking a pool surface. Most of a pool's energy losses comes from evaporative cooling with some losses from convection and radiation losses. There are minor conduction losses not worth talking about. An uncovered pool will have evaporative losses, both day and night. A covered pool will reduce the evaporative losses. A covered pool will also reduce the solar energy gain by 5%-40% depending on how opaque the cover is. A cover will reduce convection losses. The energy gain or loss depends on if the solar energy gain exceeds the energy losses from evaporative cooling and convection. While a pool with a solar cover may have less solar energy gain than an uncovered pool, it will have a lot less energy losses from evaporative cooling and convection. A solar cover doesn't heat a pool. A solar cover reduces the losses to below the gains, therefore the water can increase in temperature.

I have actually seen temperature rises with a solar cover. My heat pump is set for 87F. On days where the air temperature is in the mid 80s, the pool is not heating up from any gains from air temperature. I have seen the temperature rise from 87F in the morning to 92F in the afternoon. I don't see those type of temperature rises when I leave the solar cover off which does occur on the weekends. I saw this repeatedly last year as I would check the pool each day while working from home.
 
  • Like
Reactions: Newdude