All pool heaters, 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 is usually used for fast (15 - 30 minutes) heat up of the spa.
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 smaller sizes of pool heaters 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|