Your Pool Heater Is Only As Good As Your Cover
We sell heaters. We'd still tell you that if you're choosing between a heater and a solar cover, and you can only do one this season, buy the cover.
That isn't modesty. It's because heating a pool without a cover is close to heating a house with the windows open — the equipment works fine, and most of what it produces goes straight into the sky.
Here's why, and how to use both together properly.
Where pool heat actually goes
When people picture a pool losing heat, they picture the water cooling into the cold air, or heat soaking away into the ground. Those are real, and they're minor.
The dominant loss is evaporation.
Water molecules leaving the surface carry a disproportionate amount of energy with them, because it takes a great deal of heat to turn liquid water into vapor. Every gallon that evaporates off your pool takes a substantial quantity of heat away, and evaporation runs continuously — hardest at night, when the air is cool and dry and your warm water surface is exposed to an open sky.
That's why an uncovered pool can lose overnight much of what you paid to add during the day, and why people who run a heater without a cover conclude that their heater is undersized when it isn't.
Three things drive evaporation:
Exposed surface area. Fixed by your pool.
Wind across the surface. Enormous effect. A windy site can dramatically increase losses, which is why a fence or hedge on the prevailing-wind side is a genuine heating upgrade.
The temperature difference between water and air. This is the cruel part: the warmer you make your pool, the faster it loses heat. Every degree costs more than the last.
A cover addresses the first and second directly, and blunts the third.
What a cover does for a heater
Three things, all of which matter.
It stops evaporation while it's on. This is the main event and the reason for everything else on this list.
It adds heat of its own. A solar cover transmits sunlight into the water and traps the resulting warmth. On a sunny day, a covered pool gains temperature with the heater off.
It holds heat overnight. The single biggest practical win. Cover the pool at dusk and you start the next day close to where you finished, instead of starting over.
The compounding effect is what surprises people. Owners who start covering the pool between sessions commonly find their heat-up times drop substantially and their heater runs far fewer hours — which shows up directly on the fuel bill. Our run cost post covers the arithmetic, but the intuition is simple: you're no longer paying to replace the same heat every night.
How to actually use them together
Cover it whenever you're not swimming. Not just overnight. A midday cover on a breezy day retains more than you'd guess.
Cover it overnight, always, in heating season. This is where the cover earns its money. Skipping it undoes the day.
Bubble side down. The air pockets sit against the water. Getting this backwards is a common and quiet mistake.
Run the heater with the cover on. People sometimes assume the cover has to come off. It doesn't — heating a covered pool is the efficient way to do it.
Get a reel. The single reason people stop using their cover is that it's a hassle to move. A reel turns a two-person chore into a one-person minute. If your cover ends up in a heap beside the pool by July, the reel was the missing piece. We keep them in our liner and cover accessories collection.
Size it to your pool and trim to fit. A cover that overlaps awkwardly gets used less.
Take it off for heavy chemical treatment. After shocking, leave the pool uncovered until chlorine returns to normal levels — high chlorine degrades the cover material.
Our Solar Pool Covers 101 post covers selection and care in detail.
The buying order we'd actually recommend
If you're building toward a comfortable, longer season, spend in this order:
1. Solar cover and reel. Cheapest, biggest effect per dollar, and it works whether or not you ever buy a heater.
2. A windbreak, if your site is exposed. Fence, hedge or screen on the windward side. Often free if you were fencing anyway for barrier requirements.
3. The heater. Now sized against a pool that holds its heat, which may mean you need less heater than you thought — or that the one you're buying will comfortably do more.
Doing it in that order tends to save people money twice: once on the equipment, and again every month it runs.
A word on expectations
A cover is not a substitute for a heater. It can't hold a target temperature through a cold week, it won't rescue you in April in a northern climate, and it does nothing to heat a pool on demand. If you want to decide on Saturday morning that you're swimming Saturday afternoon, you need a heater — our guide to heating an above ground pool covers the choice between gas, heat pump and solar.
What a cover does is make whatever heat you have — from the sun or from a heater — actually stay in the pool. It multiplies your equipment rather than replacing it.
The two together are worth considerably more than the sum of the parts. That's the whole point.
The short version
- Most of a pool's heat loss is evaporation off the surface, and it runs hardest overnight.
- A solar cover stops evaporation, adds solar gain, and holds heat overnight.
- Running a heater without a cover means paying to replace the same heat every night.
- Cover it whenever you're not swimming, bubble side down, and leave the heater running with it on.
- Buy a reel. It's the difference between a cover you use and a cover in a heap.
- Take the cover off after shocking until chlorine drops back to normal.
- If you can only do one thing this season, do the cover.
