How to keep a pool warm

Keeping is cheaper than adding

Most of the money spent on pool temperature goes on equipment that adds heat. That is the second half of the problem. The first half is that the pool is losing heat continuously, and a degree you never lost costs nothing to put back.

This page is only about the keeping half. Adding heat is a separate question with a separate answer, and the order matters: retention first, because it makes every heating decision afterwards smaller and cheaper.

Where the heat actually goes

A pool loses heat four ways, and they are nowhere near equal in size. The Department of Energy puts about 70% of the total loss on evaporation alone.

That distribution is the whole strategy. One path is worth more than everything else combined, and there is exactly one product that addresses it.

1. Cover it, and cover it consistently

DOE puts the heating-cost saving from covering a pool when it is not in use at 50 to 70%. Nothing else on this page is in that range, and nothing else is close.

The word doing the work in that sentence is consistently. The saving is not a property of owning a cover, it is a property of the cover being on the water. A blanket that goes on four nights out of seven delivers a bit over half of what the figure promises, and the commonest failure in this whole subject is a cover that stopped going on in week three because deploying it alone was a fight.

Which is why a reel is not an accessory. On any pool big enough that the blanket is a two-person job, it is the purchase that decides whether the cover was worth buying at all.

On thickness: it is a service-life decision, not a heat one. The only published hands-on comparison in this category measured the same temperature change across every cover in its group. The cost-per-year comparison is the way to choose.

2. Deal with the wind

Wind does not cool a pool directly in any meaningful amount. What it does is strip the humid layer sitting on the surface and let evaporation run much harder, which is why it belongs with the first row rather than as its own effect.

The DOE figure here is startling and worth quoting exactly: a surface wind of just 7 mph can raise a pool’s energy consumption by 300% — four times the still-air figure. Seven miles an hour is a light breeze.

A windbreak has to be at the water surface to do anything. A tall fence twenty feet away does very little; hedging or fencing close to the pool edge does a lot. If your pool sits in the open and you have never considered this, it may be the largest uncosted item on your heating bill.

The complete version of a windbreak is a structure over the water rather than beside it. Our roundup of pool domes and enclosures ranks what you can buy, and separates the transparent domes that warm the water from the opaque ones sold under the same search term that cannot.

3. Choose a lower target temperature

This one is free and people dismiss it, but the DOE number behind it is large: every extra degree of target temperature costs 10 to 30% more to hold.

DOE puts active swimming at 78 to 80°F and recreational use at 82 to 84°F. Those are genuinely different activities with genuinely different comfort requirements, and a household that swims lengths does not need the temperature a household that floats and chats does. What each target actually costs works it through.

Dropping a target from 84 to 82 is not a small saving. On the DOE range it is somewhere between a fifth and half your heating bill, for two degrees most people cannot reliably feel.

4. Run the pump when it helps, not when it hurts

Circulation itself does not add or remove much heat. But the water going through an exposed pipe run, an uncovered filter, or a solar collector at the wrong time of day does.

• Run filtration in the warmest part of the day rather than overnight where you can. Water moving through above-ground plumbing at 3am is losing heat to the cold air around it.
• If you have solar collectors, stop circulating through them after dark. A panel on a roof at night is a radiator pointed at the sky, and it will actively cool the pool if you keep pushing water through it.
• Do not chase this too far. Filtration exists to keep the water safe. Cutting run time below what the pool needs to stay clear is not a heat strategy, it is a chemistry problem waiting to happen.

5. Liquid covers and rings, for the pools where a blanket will not happen

Both of these are worse than a blanket on the physics and better than a blanket on the behavior, and for a lot of households that is the trade that matters.

A liquid cover — Coverfree is the one we look at closely — is a molecule-thick alcohol film that spreads across the surface and slows evaporation. It does nothing for daytime solar gain, it is disturbed by swimmers and wind, and it is unquestionably less effective than a physical blanket. It is also self-dispensing, so it is on the water every night without anyone deciding to put it there.

Solar rings give you roughly the fraction of the surface you actually covered — evaporation carries on through every gap, in proportion. Their argument is the same: several discs dropped on one at a time gets done, where a full blanket does not.

What does not work

• Black hosepipe coiled on the deck as a retention measure. It is a heat-adding trick, not a keeping one, and the honest version of it needs far more tubing than people expect.
• A winter cover left on in the shoulder season. It is opaque, so it blocks the sun that would warm the water. The two covers are not interchangeable.
• Insulating the pool walls on an existing pool. Conduction is the smallest of the four paths and retrofitting it is not practical.

The order to do it in

1. Measure the overnight drop, so you know whether this is your problem.
2. Buy a cover, and a reel if the pool is large enough to need one.
3. Deal with wind at the water surface if the pool sits exposed.
4. Drop the target temperature a degree or two and see if anyone notices.
5. Move filtration into daylight hours.
6. Re-measure. Then, and only then, size a heater to whatever gap is left — which will be a smaller and cheaper heater than the one you would have bought first.

For the shoulder months specifically, stretching the swim season takes the same measures and ranks them for April and October rather than July.