Determine your pool’s surface area in square feet.
Area = LxW Area = RxRx3.14 Area = (A+B)xLx.45
Determine your spa capacity in U.S. gallons(surface area x average depth x 7½).
The reference tables list the time required in minutes to raise the temperature of the spa/hot tub by 30°F. In the table below, locate the spa/hot tub size in U.S. gallons. Select the desired time to raise the spa/hot tub temperature 30°F, read to the left and select the appropriate heater.
This guide can be adjusted for other temperature rises. For example, if you desire a 15°F increase in temperature, simply divide the time for 30°F rise by the 15°F increase (30/15=2).
Note: Heat losses and/or heat absorbed by spa walls or other objects will add to heat-up time.
Spa sizing is based on an insulated and covered spa. Always cover your spa or hot tub when not in use to minimize heat loss and evaporation.
Heater Sizing Chart
Shaded columns indicate sizing with a 0 MPH wind. Example: Rectangular pool, 20' x 40', with desired temperature rise of 25°F: 20 x 40 = 800 square foot surface area, the recommended heater is a 250.
Example: To heat a 400 gallon spa 30°F in 30 minutes, the recommended heater is a 250.