How to heat a pool

Start with the number, not the equipment

Almost every version of this question gets answered with a product recommendation, which is backwards. The first thing to establish is how much energy your pool actually needs, because that number decides everything after it and it is a two-minute calculation.

Do not know your gallons? Length x width x average depth x 7.48 for a rectangular pool. Then the heating time calculator will do the rest.

Step one: stop losing what you have

Before adding heat, it is worth knowing where the heat you already paid for is going. The Department of Energy puts about 70% of a pool’s heat loss down to evaporation, and evaporation is expensive because water carries enormous energy when it changes phase — 1,048 BTU per pound, about 8,730 BTU per gallon.

A pool that drops 3°F overnight has lost around 500,000 BTU, and roughly 350,000 of that went to evaporation. That is a fifth of the entire heat-up figure above, gone every single night. Against that, a hundred-dollar cover is not an accessory — it is the highest-return item on this page, and DOE puts the saving from covering a pool when it is not in use at 50 to 70% of heating cost.

Three loss-side moves, in order of value:

1. A cover, used nightly. Attacks the 70%. Cheapest fix available.
2. A windbreak. DOE’s published figure is that a 7 mph wind across the surface can raise energy consumption by 300%. Seven miles an hour is a light breeze. A hedge or fence on the prevailing side does more thermal work than most equipment upgrades.
3. Lower your target. Each extra degree of setpoint costs 10 to 30% more, per DOE. Going from 84 to 80 is not a small adjustment. What to aim for.

Step two: the five ways to add heat, priced

Here is the comparison the category does not publish. Fuel price alone is misleading — electricity looks ruinous per unit of energy bought and natural gas looks cheap — and the ranking inverts completely once you divide through by what the equipment actually delivers.

The heat pump number is the one that surprises people. It costs less than half what natural gas costs and about a fifth of electric resistance, on the same electricity, because it moves heat rather than making it — delivering roughly five units for every one it draws.

Step three: pick on the trade that matters to you

Running cost is not the only axis, and pretending it is would be its own kind of dishonesty. Three genuine trades:

Speed against cost

A 400,000 BTU gas heater will lift a 20,000-gallon pool 10 degrees in about five hours. A 140,000 BTU heat pump takes about twelve. If you heat a weekend pool on Friday afternoon rather than holding a temperature all season, gas is the right answer despite costing more per BTU. The full comparison.

Control against free

Solar panels cost nothing to run, forever. What you give up is the thermostat: you get what the sun delivered that week, and on a cold cloudy stretch — exactly when you want heat — they deliver very little.

Climate

A heat pump’s efficiency falls as the air gets colder, which is precisely when you want it in the shoulder season. Gas does not care about air temperature. If your season-extension ambition is April and October rather than June and September, that matters.

The order to actually do this in

1. Measure your overnight drop. Thermometer at dusk, thermometer at dawn. That single number tells you how much of your problem is loss rather than lack of heat.
2. Buy a cover, and a reel if you need one to make yourself actually use it.
3. Fix the wind if your pool sits in the open.
4. Re-measure. A lot of pools stop being a problem here.
5. Then buy a heater, sized to the gap that is left rather than to the whole problem — which is usually a smaller and cheaper heater than you would otherwise have bought.

If step five still looks expensive, the cheapest-way page works through the options at each budget, and doing it with no heater at all is a real strategy rather than a consolation prize.