Go bigger than you think.

You probably need a larger heater than you expect. Most people undersize and end up disappointed when the pool stays cold on a breezy day.

Pool heating is slow. A heater that’s too small runs constantly but can’t keep up when the air temp drops or you want the water 10 degrees warmer. Rule of thumb: in a moderate climate, plan on roughly 100,000 BTUs per 10,000 gallons of water. So a 20,000-gallon pool gets a 200,000 BTU heater. Colder climate? Go up another size.

The only downside to a bigger heater is the upfront cost and maybe a larger gas line. But the comfort difference is worth it. A too-small heater costs almost as much to install and still leaves you cold.