Pool temperature

Most people arrive at this subject from one specific direction: the pool is colder than they want, and they want to know whether that is normal and what to do about it. So start with the target. The Department of Energy’s published recommendation is 78 to 80°F for active swimming and 82 to 84°F for recreational use — and those are two genuinely different numbers, because a lap swimmer generating heat wants cooler water than a child standing still in it.

That gap is worth money. DOE also publishes that each additional degree of target temperature raises heating cost by 10 to 30% depending on location. Going from 80 to 84 is not a small adjustment; it is potentially a doubling of the bill. If the pool feels cold and the people using it are mostly standing in it, the honest answer might be a higher setpoint and a bigger bill — but it should be a decision, not an accident. The temperature page goes through it properly.

Where the heat goes

A pool is an enormous, uninsulated, open-topped tank of water, and it loses heat continuously by four routes: evaporation, radiation to the night sky, convection to the air, and conduction into the ground. Evaporation is about 70% of it, which is the single most useful fact in this whole subject.

Evaporation is expensive because water carries an enormous amount of energy when it changes phase — 1,048 BTU per pound of 80-degree water, or about 8,730 BTU per gallon. A pool that loses an eighth of an inch overnight has quietly shed hundreds of thousands of BTU. We work that through with real numbers and turn it into dollars.

Wind makes it dramatically worse. DOE’s published figure is that a 7 mph wind across the surface can increase energy consumption by 300% — four times the still-air rate. Seven miles an hour is a light breeze. If your pool sits in an open yard, a hedge or a fence on the prevailing side is doing more thermal work than most equipment upgrades.

Stretching the season

The concept this site is built around is not “get your pool to 90 degrees.” It is getting several more usable weeks at each end of the year, which is a much cheaper problem. In spring and autumn the air is cool, the nights are long, and the overnight loss is at its worst exactly when the daytime gain is at its weakest — so the interventions that pay off are the ones that attack the loss.

In rough order of value per dollar: a cover used nightly, then a windbreak, then running circulation during the warm part of the day rather than overnight, then heat. That order is not intuitive — equipment feels like the answer — but it follows directly from where the losses are. The season-extension guide works through the whole sequence, and the overnight page handles the specific case of a pool that is fine at 6pm and cold by morning.

If your pool is cold right now and you want the short version of what to do about it, start here — it is a decision tree rather than an essay.