The Surface-Area Method
The surface-area method of sizing a pool heater is based on the surface area (in square feet) of the pool and the temperature difference (in degrees Fahrenheit) between the desired water temperature and the average air temperature. The procedure is as follows:
1. Determine the mean average air temperature for the coldest month in which the pool is to be used.
2. Determine the desired pool water temperature.
3. Find the difference between the air temperature (step 1) and the water temperature (step 2).
4. Calculate the pool surface area in square feet.
5. In Table 5-1, find the surface area closest to your pool size.
Move horizontally in a straight line across to the column that represents the temperature difference for your pool. The point at which the horizontal line and the temperature column intersect will be the required Btu per hour input rating for your pool heater.
The surface-area method can be illustrated with a simple example. Let’s suppose that you have a small 15-foot × 30-foot pool with a 17,000-gallon capacity. You want to maintain a pool water temperature of 70°F (step 2), and the mean average air temperature for the coldest month in which the pool is to be used is 50°F (step 1). What will be the required input rating of your pool heater?
Your temperature difference required for sizing the heater is 20° (70° – 50°F). The pool surface area is 450 square feet (15 × 30 feet). Moving horizontally across the top line in Table 5-1, you arrive at the 20° column and learn that the required input rating for a pool heater meeting these criteria is 150,000 Btu per hour.
Another, less precise form of the surface-area method is to multi- ply the surface area of the pool by 15 and then by the temperature difference between the pool water and the air. Using the same data as before, the following results are obtained:
The Time-Rise Method
The first step in the time-rise method of sizing a pool heater is to determine the pool capacity in gallons of water (pool gallonage). This only needs to be an approximate figure and is commonly rounded off to the nearest thousand (for example, 17,000 gal, 20,000 gal, 23,000 gal). In Table 5-1, the extreme right-hand column lists pool heaters recommended for different pool capacities. The sizing in this chart is based on the number of Btu required to raise the pool temperature approximately 1°F per hour.
Sizing Indoor Pool Heaters
If the swimming pool is located inside a heated building, the surface temperature of the water is naturally not affected by the colder out- door air temperatures. A simple rule-of-thumb method for sizing an indoor pool heater is as follows:
1. Determine the surface area of the pool (for example, 30 ft X 40 ft = 1200 ft2).
2. Multiply the surface area by 125 Btu (1200 ft2 X 125 Btu = 150,000 Btu per hour input).