Do Solar Covers Work on Above Ground Pools?

I have been in the swimming pool business for approaching 35 years in Central Florida. There are only a couple of things in my industry that annoy me a little. The industry not stating that solar pool covers do not heat water is one of them.

A solar swimming pool cover will NOT heat the water any better than direct sunlight will (not using a cover). What it will do is help keep any heat that there is in the water. This is why solar covers have a volume of trapped air pockets built into them (like bubble wrap). They are insulator blankets and not heater blankets.

WHAT A SOLAR POOL COVER DOES, AND WHAT IT DOES NOT DO

I would love to just discuss and focus on what a solar cover can do and how you need one if you are heating your swimming pool. But I can’t avoid what people who buy them and the industry that sells them say and think they can do, but actually do not.

A solar cover CAN help retain any heat that is already in the pool water

Almost all of the heat in pool water will escape from the pool’s surface. Regardless of the type of swimming pool, pools have walls. And those walls will transfer a temperature difference at a slower rate than if there wasn’t a wall there.

To help explain this for an inground pool, if the pool water temperature is warmer than the earth on the other side of the wall of the pool, then those two temperature differences will try to become the same. As a result, the earth along the outside wall of the pool will be slightly warmer than the earth further away from the wall.

Simultaneously, the water directly next to the pool wall will be slightly cooler than the water further from the wall and towards the center of the pool.

Now, as long as there continues to be a temperature difference between these two areas(the earth outside the pool and the water inside the pool), there will be a transference of temperature attempting to become the same.

The winner of this temperature war will be the side that has the most energy. In the case of the earth versus the warm water in the winter, the winner will depend on how much heat enters the water as the earth will have unlimited energy to make and keep it colder.

Lastly, the battlefield here for this transference of energy(heat vs cold) is the wall of the pool. And it’s the wall’s make-up that determines the rate at which the energy(heat, cold) can transfer. A thicker concrete wall will transfer the energy at a lesser rate than say a thin metal wall of an above-ground pool.

For an above-ground swimming pool that is installed above the ground(not semi-inground), this transference of energy occurs in the same way. The rate may be different depending on the outside temperature as the battle in this case will be between the warm pool water versus the outside air.

The pool’s surface has no barrier, so the transference of heat is much more efficient.

Since the pool’s surface doesn’t have a wall of any kind, there is no insulator between the warm water and the cold outside air. This means that if there is more energy (cold) in the air than there is in the water (hot), then the cold will transfer into the water at a much faster rate. This is why most of the heat loss is out of the top of the pool and an insulator blanket is important for keeping the heat in.

So this is why a solar cover has hundreds of tiny pockets of trapped air. That trapped air slows down the rate of transference of energy in the form of heat loss in the pool and cold loss from the air. This is the ONLY function of the solar cover.

A solar cover CANNOT generate heat in any way

In this universe at least, the only way to bring heat somewhere that it is not, is to use energy. A solar cover has no energy and only a trace ability to store it. It, therefore, cannot generate heat.

Maybe you are reading this and just said “duh” to the above sentence. Maybe your logic is that the solar cover can somehow magnify the sun’s already incredible ability to transfer heat directly.

This is certainly logical, and with this logic, you would then have to assume that the sun’s ability to put heat in the pool water would be improved if there was an insulated cover over the surface against nothing at all. That a cover would in some way magnify the sun’s heat into the water by going through this thin barrier.

This theory is possible, but only if the solar cover material has an ability to retain heat(energy) within it. This would work if using something that absorbs heat well, like black plastic or steel metal. And you wouldn’t want them to have much in the way of an insulator because that would slow down the transference of the heat into the water.

So, a blue-colored plastic cover made with hundreds of bubbles of trapped air(which is about 99% of all pool solar covers) would not retain heat very well and is an insulator, so would not be an ideal enhancer for the sun transferring heat energy into the water.

I will agree with you(if you are still in disagreement with me) if you wanted to put a black plastic cover(with no insulation value) over a baby pool that is only 12 inches deep and say that it will heat the water faster. But not for a body of water that is four plus feet deep.

DIRECT SUNLIGHT DOES A GREAT JOB HEATING WATER

Swimming pools that have a lot of direct sunlight will have much warmer water than pools with partial or full shade. And it doesn’t matter if it’s 100 degrees outside. This is because direct sunlight can transfer its heat energy well into the water.

This has helped me consider that a solar cover heats water better than direct sunlight as a ridiculous notion. Here in Central Florida, the direct sun is intense in the summer. It’s very common here for people to have their pools in too much direct sunlight which makes the pool water too hot to be enjoyable.

To help remedy this, some will cover their pools during the heat of the day just to keep the water temperature down. That’s right. To keep their pools cooler, they cover them. Some will only have a solar cover and will use it for this purpose and it works well.

It’s funny to me that while many think a solar cover will somehow make their pool water hotter, some are using one to keep the water cooler.

WHY ARE DEEPER POOLS COOLER?

If you have been around pools a lot like I have, you might’ve noticed that deeper pools usually have cooler water temps than shallow pools. This again has to do with direct sunlight.

Deeper swimming pools are cooler because the ratio of gallons of water to surface area is greater. The sunlight does a great job of transferring heat into water, but only at the surface. It cannot penetrate its heat energy very deep into the pool water.

As a result, only so many gallons of water can gather heat energy from the sunlight. To compare, a 24’ round above ground swimming pool that is 4’ deep will have about 14,000 gallons of water. This pool’s water will be warmer if in the same spot than an inground with the same dimensions that has a deep end and has 20,000 gallons in it.

IF YOU ARE HEATING YOUR SWIMMING POOL, YOU MUST HAVE A SOLAR COVER

As explained above (in probably too great a detail), most of the heat loss of the pool’s water will be out from the pool’s surface. So, if you are heating your pool with something that actually generates heat into the water, then you will need a blanket of insulation over the top of the pool to help keep the heat in.

This blanket of insulation is what my industry calls a solar cover. And you absolutely need to be using one in unison with the heater. This will allow the water you heated (which was probably expensive) to stay in the pool longer.

A pool heater and set-up is expensive. A solar cover is not. If you are planning on heating your pool, just take my word for it and order one and have it before heating your pool.

USING A SOLAR COVER WHEN YOU DON’T HAVE A HEATER

Normally, it’s ridiculous to use a solar cover on your pool if you don’t have a heater to go along with it. But you can use it for one thing in some climates.

If you live in an area where there is a big temperature difference between daytime and nighttime, then you may want to use a solar cover at night.

If you live in one of these areas where your pool water heats up nicely during the day from a lot of direct sunlight, but then goes back to “too cold” by the next morning because of the night coolness, then in the late afternoon, throw your solar cover on the pool. This will help keep that heat in the water.

The next morning, remove the cover and allow the sun to do more heating and this will keep your water nice. It may be a pain to have to put the cover on and off every day, but warm water may be worth it.

You could experiment with leaving the cover on during the day too(if you’re not using the pool) and it may still gain some heat during the day. If that is enough to maintain the water temp you want, then great. If you need more heat though, taking the cover off during the heat of the day (especially on cloudless days) will heat the water better.

FINAL THOUGHTS ON SOLAR COVERS

It’s not cheap to heat a swimming pool. Even the correct amount of solar panels properly placed may not cost anything to heat the water, but it was expensive and laborious to set up.

To think that you can spend a hundred bucks on a bubble cover or that some bottle of liquid will heat your swimming pool is flawed. Just like many worthwhile things in life, there’s no shortcut to heating pool water.