Gas Pool Heater Sizing

Gas Pool Heaters

Proper heater size is based on the time required to heat pool or spa to the desired temperature.

• If water is heated for each intended use and rapid heating is desired, a larger unit is required.
• If water is gradually heated to the desired temperature and then maintained at that temperature, a smaller heater may be acceptable.

First determine desired temperature rise per hour using the chart below:



Temperature rise per hour formula:
_____ desired water temperature (°F)
minus _____ average air temperature (°F)
equals _____ required temperature rise (°F)
divided _____ required hours to heat to desired water temp.
equals _____ required temperature rise per hour (°F)
Gas Pool Heater sizes are determined by their BTU* output. Use the chart below to determine BTU required to raise the water temperature depending on the pool size.
POOL SIZE - SQUARE FEET
sq. ft. >> 300 400 500 600 700 800 1000
BTUs Desired Temperature Rise in °F per hour
200,000 1-1/2° 1-1/4° 3/4° - -
250,000 1-3/4° 1-1/2° 1-1/2° 3/4° -
300,000 1-3/4° 1-1/4° 3/4°
400,000 3-1/2° 2-3/4° 2-1/2° 1-1/2° 1-1/4°
SPA/TUB SIZE - GALLONS
gallons >> 300 400 500 600 500 800 1000
BTUs Desired Temperature Rise in °F per hour
200,000 59 45 36 30 25 22 18
250,000 84 63 51 42 36 32 25
300,000 104 78 63 52 45 40 32
400,000 128 96 77 64 55 48 38

* BTU stands for British Thermal Unit. A BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.