Solar cover only (no heater)?

[nostaljake]

I'm planning to renovate my pool soon and need to come up with a heating plan!

As part of the renovation, I'm hoping to move the equipment pad down closer to the pool. But space is at a premium and I'd rather not have a big honking heater there if it's not necessary. My current heater is also old, so buying a new heater and running the gas line could put me back several thousand dollars.

Can I get by with a solar cover only? I'm in southern Maryland just outside DC, so I won't need any heat from June through September. But it would be nice to have something to help make the pool swimmable earlier in the spring. I just opened a few days ago to 60 degree water.

I'm imagining using the solar cover in April when I open, leaving it on 24/7 to minimize extra work, and then taking it off for the season once the pool gets up to like 77 degrees. Is this a reasonable plan?

 

[Newdude]

The solar cover will retain about 5 degrees over where the pool would have been without it, which is an average of the last several days hig/low temps.

When it's 60 degrees, it's a moot point that the pool is 65 and the solar cover is working great to its ability.

For use without a heater, it's entirely dependent on the weather if it matters or not and the window for extending the season is small.

Those with heaters need it to keep more of that heat that they paid through the nose for, in the pool.

 

[nostaljake]

For use without a heater, it's entirely dependent on the weather if it matters or not and the window for extending the season is small.

Hmm, I've seen so many positive things about solar covers here that I assumed it could make a difference. I know there are lots of factors, but could I get an extra 2-3 weeks of swimmable water in the spring? If not, seems like the hassle wouldn't be worth it.

I'd be curious to hear from anyone who uses a solar cover only without a heater!

 

[Donldson]

Hmm, I've seen so many positive things about solar covers here that I assumed it could make a difference.

Of course it makes a difference, but it's not going to let a pool stay at 80 degrees when the air temperature is consistently below that. It's entirely passive, the amount of sun light and warm air heating the water is the same with or without one. It just reduces heat loss when the air temperature is cooler than the pool.

No, you will not get 2-3 extra weeks of swimming in the spring from it. Think more that you'll lose fewer days during the summer.

 

[Newdude]

I've seen so many positive things about solar covers here that I assumed it could make a difference

It totally can if the weather lines up that sesson. But it performs no miracles. If it's unseasonably cool, it's a moot point the pool is 68 instead of the 62 it should have been. (Etc etc).

If weather conditions are favorable, say unusually warm that year, it might hang in the upper 70s to take the edge off the lower 70s you would have had with no cover.

 

[mas985]

In my experience, you should be able to get close to 10F or more bump with a solar cover on the pool depending on your local conditions. Over the years I have done multiple experiments on my own pool plus I have a heat transfer model that will give you some approximations of pool water temperature under different conditions. I don't have all of the detailed information of your own pool but here are a few assumptions I made.

- Pool Surface area 371 sq-ft.
- Partial sun surface exposure 10am-3pm
- Yesterday's temperature data (assumes identical temperatures for multiple day convergence)
- 90% pool cover
- 25% cloud cover

Pool Heat Transfer Tools v022 - nostaljake

ReadMe Version:,v022 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Spreadsheet License,By using this spreadsheet you agree to the following conditions: This spreadsheet is for personal use only and shall not be used for any commercial business without the ...

docs.google.com

The column B is the baseline without any cover and column C is with a cover. The average pool temperature is shown in lines 115/116 but if you want the morning and/or evening temperatures, those are in lines 181/182.

So the baseline without a cover would be around 68F and with a cover 77F. So even under these conditions you might experience close to a 10F gain. But again there are a lot of factors that come into play.

With 75% cloud cover, the numbers drop to 65F without cover and 70F with cloud cover or a 5F bump. Air temperature becomes more important in this scenario.

 

[kul]

I use a solar cover to retain heat, decrease evaporation, and decrease chlorine dissipation. It works. I have a heater and it’s nice to have the solar cover as well so you can get the most bang for your buck after heating. When I heat the pool, I can usually do it one time and then use the solar cover to keep it at a great temp for 3-4 days.

That being said, I’m not sure who said they are great. They are a pain. But effective.

 

[Newdude]

I’m not sure who said they are great.

They *work* great.

Nobody ever said they *are* great. We all hate them something fierce but they're a necessary evil for anyone with a heater.


And I LOVE this point you raised :

can usually do it one time and then use the solar cover to keep it at a great temp for 3-4 days.

If it's cold one day, the solar cover will probably keep the pool warm for that one day. But after 3 or 4 days of cold, you need a heater or warm weather to return. Lots of times in the fall it got 'seasonable' for 2 weeks and just the solar cover wasn't enough.

 

[nostaljake]

In my experience, you should be able to get close to 10F or more bump with a solar cover on the pool depending on your local conditions. Over the years I have done multiple experiments on my own pool plus I have a heat transfer model that will give you some approximations of pool water temperature under different conditions. I don't have all of the detailed information of your own pool but here are a few assumptions I made.

- Pool Surface area 371 sq-ft.
- Partial sun surface exposure 10am-3pm
- Yesterday's temperature data (assumes identical temperatures for multiple day convergence)
- 90% pool cover
- 25% cloud cover

Dude this is amazing, thank you so much! I'm a math nerd so you're speaking my love language.

Your assumptions actually look very accurate. The only question I have about the model is with the the multiple day convergence thing... how many previous days of data do you need for a good estimate? And does the model have a starting temperature input? For example, my water was 60 degrees when I opened last Sunday so obviously it wasn't getting up to 68 or 77 with that one warm day.

Is there a way to use the model to estimate, say, the first day on which the water temperature would hit 75 with (and without) a solar cover? Maybe by using last year's temperature data? That would be very useful from a practical standpoint. I know that could be a bunch of work so no worries if it's not easy to plug into the model.

 

[nostaljake]

Of course it makes a difference, but it's not going to let a pool stay at 80 degrees when the air temperature is consistently below that. It's entirely passive, the amount of sun light and warm air heating the water is the same with or without one. It just reduces heat loss when the air temperature is cooler than the pool.

No, you will not get 2-3 extra weeks of swimming in the spring from it. Think more that you'll lose fewer days during the summer.

Thankfully I don't really lose any days here in the summer. It's pretty consistently hot and humid all summer.

I totally get what you and Newdude are saying about it not helping when the temperature is consistently lower. But temperatures in the spring obviously vary quite a bit... a quick look at the past two years tells me the high in May can be anywhere between 55 and 95 here. So I guess the main question is how well the cover can "hold" those higher temperature days. If we're talking a ~10 degree difference like mas985 is saying then it seems reasonable to me that you could get a couple extra weeks of swimmable water.

 

[Newdude]

So I guess the main question is how well the cover can "hold" those higher temperature days

You should remain 5 to 7 degrees above the average of the last 4 to 5 days highs and lows. (What I got fairly near you)

Or thereabouts.

If we're talking a ~10 degree difference like mas985 is saying

His experience in sunny CA is different than what we have going on in the northeast. You're south northeast, which can only help.

 

[mas985]

Dude this is amazing, thank you so much! I'm a math nerd so you're speaking my love language.

Your assumptions actually look very accurate. The only question I have about the model is with the the multiple day convergence thing... how many previous days of data do you need for a good estimate? And does the model have a starting temperature input? For example, my water was 60 degrees when I opened last Sunday so obviously it wasn't getting up to 68 or 77 with that one warm day.

It is an solver loop so effectively it is infinite. It just uses a single days data for an infinite series.

However, you can run it for a single day with either setting morning water temperature (recommended) or evening temperature. It will then calculate the other. This is what I used to calibrate the model for my pool.

Is there a way to use the model to estimate, say, the first day on which the water temperature would hit 75 with (and without) a solar cover? Maybe by using last year's temperature data? That would be very useful from a practical standpoint. I know that could be a bunch of work so no worries if it's not easy to plug into the model.

My first model did just that but I found that it really didn't make that much of a difference to the end results. The most impactful day is the one that proceeds the analysis day. If the previous days were a lot warmer, it might be better to use two or three day averages instead of a single day. But the further away in time you get, the less the impact is to the final results.

 

[mas985]

His experience in sunny CA is different than what we have going on in the northeast. You're south northeast, which can only help.

This is definitely a true statement but the temperature gains I gave earlier were for Bethesda, MD on 4/18/2024. the 10F gain was for a 25% cloud cover case, 5F for a 75% cloud cover case. The standard deviation on gains with a solar cover are actually quite large depending on conditions. In the summer, I can get almost a 15F gain. I believe others on the forum have suggested that as well so not impossible but the conditions have to be right. Heat gain for a solar cover are higher for dryer climates where evaporation is worse and lower for more humid climates.

 

[Newdude]

Heat gain for a solar cover are higher for dryer climates where evaporation is worse and lower for more humid climates.

I see 2 ironies here.

1) The solar cover works best where they don't need it as the dry climates tend to be hot.

2) They need it anyway as after getting acclimated to 112 degree air temps, 82 degree pool water is 'cold'.

 

[mas985]

I like many others have stopped using my cover because it is such a pain. If I had to do it all over again, I would have gone with a pool design that is conducive to an automatic cover. Live and learn.