- 1). Determine the capacity of your spa in gallons. For a rectangular spa, use the formula: Length x Width x Average Depth x 7.5. To find the volume of a circular spa, multiply Diameter x Diameter x Average Depth x 5.9. For an oval spa, use this formula: Long Diameter x Short Diameter x Average Depth x 5.9.
- 2). Determine how long you are willing to wait for the spa to warm up 30 degrees. The less you are willing to wait, the larger the heater you are going to need.
- 3). Use a heater input chart to determine the capacity of heater that you need. These charts are available online. According to the chart at Poolandspa.com, if you have a 300-gallon pool and are content waiting 32 minutes for the temperature to go up 30 degrees, you need a heater with a capacity of at least 175,000 Btu per hour.
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