How to Figure Pool Heater Size Needed

Written by mark kennan
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Choosing the right size heater for your swimming pool is very important to make sure that the pool will be warm enough and that you minimise energy costs. A larger heater will result in larger utility bills, but a smaller heater will wear out quickly because it is overworked. The size you need depends on the surface area of your pool, the water temperature you want and the temperature of the air. The size is measured in British thermal units per hour.

Skill level:
Moderately Easy

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  1. 1

    Calculate the surface area of your pool. For a rectangular pool, multiply the length times the with. For a circular pool, square the radius and multiply by pi (about 3.14).

  2. 2

    Decide on the temperature that you want your pool to be set at. Most pools are set around 26.7 degrees Celsius, but you might want it to be a few degrees warmer if you are elderly or have young children.

  3. 3

    Determine the coldest air temperature your pool will be exposed to while in use. For example, if you plan to leave the pool open year round, use the coldest temperature your area experiences. However, if you only have it open from March through October, use the coldest temperature for just those months.

  4. 4

    Subtract the coldest air temperature found in step three from the desired pool temperature found in step two. For example, if you wanted to keep your pool at 80 degrees and the coldest the air temperature was 50 degrees, the difference would be 30 degrees.

  5. 5

    Multiply the surface area of the pool times the differences in temperatures times 12 to give you the size pool heater you need. For example, if you have a 30-degree difference and a pool area of 300 square feet, you would need a capacity of 90,000 Btu per hour.

Tips and warnings

  • Make sure you calculate the surface area, not the volume, of your pool for step one.

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