Pool Heater Sizing Guide

Two heater types dominate residential pools: gas heaters (natural gas or propane) and heat pumps. They work on different principles, so sizing them correctly requires different math.

How each type works

A gas heater burns fuel to heat water directly — fast, effective in any climate, and ideal for quick warm-ups.

A heat pump extracts heat from the air and transfers it to the water — slower to warm, but far cheaper to run and best in moderate climates above 50°F (10°C).

Sizing a gas heater

Gas heaters are rated in BTU/hr. The common rule: 50,000 BTU per 10,000 gallons of pool water for a moderate warm-up. For faster heating or colder start temperatures, use 100,000 BTU per 10,000 gallons.

A 20,000-gallon pool typically needs a 200,000–400,000 BTU gas heater.

Sizing a heat pump

Heat pumps are rated in BTU/hr too, but their output depends on air temperature and humidity. Size by the rule of roughly 100,000 BTU per 10,000 gallons for warm climates, and 125,000+ BTU per 10,000 gallons in cooler regions.

Because heat pumps work gradually, oversizing slightly shortens warm-up time without raising running cost much.

Choose based on use pattern

• Swim daily, want fast warm-ups? Gas heater.
• Swim most of the season in a mild climate and want low running cost? Heat pump.
• Want the best of both? Many pool owners pair a heat pump for steady season-long warmth with a gas heater for on-demand boosts.

See the Illuminex AquaHeat inverter heat pumps for efficient, app-controlled heating.