Pool heater size calculator

How it works

output needed (BTU per hour) = surface area x (pool temperature - air temperature) x 12 x wind factor
wind factor = 1 at 3.5 mph, 1.25 at 5 mph, 2 at 10 mph
heat-up hours = gallons x 8.345 x degrees to raise / (heater input x efficiency)

This is the sizing method the US Department of Energy publishes for gas pool heaters. It sizes the heater by what the pool loses through its surface, which is why the surface area matters and the gallons do not. The answer is output; gas heaters are sold by input, so divide by the efficiency to compare it with a heater's rating.

Heating a cold pool at start-up takes much longer than holding it warm, and the heat-up figure leaves out what the surface loses on the way, so it runs short on a cool, windy day. A pool cover keeps most of that heat in.

Gas heater or heat pump

A gas heater warms a pool quickly in any weather. A heat pump costs less to run but works best with air above about 50 F and heats more slowly. Heat pumps are rated by output, often measured with 80 F air, so check the rating at the air temperature you expect.

Questions

What size heater does my pool need?

Multiply the surface area by the temperature rise and by 12, then by 1.25 for a 5 mph wind or 2 for a 10 mph wind. A 32 x 16 foot pool held 22 F above the air needs about 135,000 BTU per hour of output.

Why surface area and not gallons?

A heated pool loses most of its heat through the surface, to evaporation and the air. Gallons decide how long the first heat-up takes.

How long does it take to heat a pool?

Gallons times 8.345 times the degrees to raise, divided by the heater's output, plus time for what the surface loses meanwhile. A 400,000 BTU gas heater warms 18,000 gallons about 2 F an hour.

Is a bigger heater worse?

No. A larger gas heater warms the water faster and runs for less time. It costs more to buy and needs a bigger gas line.

More pool calculators

Pool volume calculatorPool chlorine and chemical calculator