Pool Heater Sizing

All of the swimming pool heaters at Poolcenter.com, from the small 100 (100,000 btu) unit to the largest 405's (405,000 btu's of gas usage per hour) will heat your pool. The smaller heaters are slower to heat, and will operate more often to keep up with the output of larger heaters. Larger heaters are a small bit more efficient than the smaller pool heater units. Use of a pool cover to retain heat and installing fences or other wind* obstructions can reduce demands on the pool heater.

If you are trying to heat a pool/spa combo, a 400 btu pool heater (the Big Boy) is usually used for fast (15 - 30 minutes) heat up of the spa. Using a spa solar blanket cover can reduce the heat up time by a few minutes. Custom made insulated spa tops can keep most of the heat inside the spa, and have it ready for the moment of use.

Ask yourself a question about your anticipated usage patterns. If you will be heating intermittently, for example, turning on the heater on Friday, and turning it off on Monday, then a quick heat up is required. Heaters only add 1 - 3 degrees of heat per hour to the average pool. A larger heater is best for this usage pattern. If you expect to be maintaining a temperature by setting the thermostat, and maybe adding a few degrees in the evening or on weekends, then a smaller size pool heater can be used.

Use the chart below as a guideline in sizing a heater to your pool size. Factor in the usage patterns and other considerations above and then consider going one size larger for good measure.


Size Heater

Gallons in Pool

Sq. Ft. Surface Area of Pool

100 - 200 BTU heaters

1,000 gals to 10,000 gals

up to 300 sq ft

200 - 300 BTU heaters

10,000 gals to 20,000 gals

up to 500 sq ft

300 - 400 BTU heaters

20,000 gals to 40,000 gals

up to 800 sq ft

400 BTU heaters

40,000 gals to 80,000 gals

up to 1200 sq ft


*wind is the largest heat thief around your pool, with the possible exception of teenagers. To prevent other household members from cranking up the heat, (which doesn't heat the water any faster) you can disconnect a wire inside the heater – such as a pressure switch wire. Then the pool heater only works when you want it to!