Updated January 2017. Replacing a defunct pool or spa heater for a customer on your route? Adding a heater to an equipment pad that previously lacked one? Welcome to the world of heater sizing and the art of finding the right unit to provide your customer with a cost- and energy-efficient source of heated pool or spa water.
Doing the job right means the water will heat to the desired temperature in the desired time frame. An undersized heater will heat too slowly and, although heater manufacturers advise that bigger is always better when choosing a heater, a significantly oversized unit will do the job, but also will boost the cost of the installation.

Choosing the right heater will please your customers and provide you a trouble-free heating system on your service route.

Warming up

Before selecting a heater, you must determine the customer’s needs. For example, is the heater going to warm a pool or a spa? Although many units can be used for either vessel separately, the thought process behind selection differs for pools and spas.

Take a pool: One of the primary factors used in calculating heater size is heat loss from the surface. In a spa, however, surface area is far less a factor because so many spas are covered, which greatly reduces surface heat loss. Instead, heat-up time relative to the spa’s gallonage is the critical factor.

Now find out if the customer plans to use the heater for maintenance heating or only occasionally for intermittent or demand heating. Although figuring the heater size for both types of heating strategies is the same, maintenance heating is typically calculated using surface area, while heaters used for intermittent heating often are sized by factoring total volume.

If your customer isn’t sure on this point, it breaks down to a question of usage: If the pool is used nearly every day during the swim season, maintenance heating is the best strategy. If the vessel is host to bathers only occasionally, it’s more cost-effective to heat the water only when needed.

Next, determine exactly which fossil fuel will be put to use — natural gas, liquid propane or heating oil. In some areas of the country, natural gas is either unavailable or excessively expensive, making one of the other fossil fuels more desirable.

Find out if there are codes in your area governing the type of pilot-ignition system used in heaters. In some states, continuous pilots (or millivolt systems) are banned for all new installations. Intermittent ignition systems requiring an electrical hookup are required in these jurisdictions.

Next, determine whether the available utility hookup provides adequate pressure to run the heater. Gas piping, meters and other delivery equipment must be sized correctly to ensure an adequate gas supply.

Defining the variables

To properly size a heater, you need to know the following information.

Surface area: The main job of a heater is to offset the heat that is lost from the water’s surface. This is especially true for maintenance-style heating. Here’s a rundown of the basic surface-area calculations:

Rectangular pool: length x width

Oval pool: 1/2 length x 1/2 width x 3.14

Rectangular pool with rounded ends:

length x width x .8

Kidney-shaped pool: length x width x .75

Things get a bit trickier with free-form pools. Here, you must carefully draw an image of the pool’s perimeter on standard 1/4-inch grid paper. Using a scale of 1/8-inch per foot, for example, means that each of the 1/4-inch squares on the grid will equal 2 feet on each side, giving each grid square an area of 4 square feet.

Now count the squares that fall entirely within the drawn perimeter of the pool. Then count all of the squares that fall approximately 3/4 within the surface area of the pool as 3 square feet, those with half in the pool surface area 2 square feet and so on. Add up the value of all of the squares.

Volume: For spas and pools in which a demand-heating technique is to be used, total water volume is used rather than surface area in calculating heater size.

To figure volume, use the surface-area data derived above and multiply it by the average depth, thus developing a cubic-foot measurement of the vessel. To determine total gallonage, multiply the pool’s or spa’s cubic water footage by 7.48 — the number of gallons in a cubic foot of water.

Temperature differential: In most sizing charts for pools, temperature differential is primary factor along with surface area or volume. Before you can determine temperature differential, however, you must first peg the desired water temperature. For pools, the American Red Cross recommends a range of 78- to 82 degrees Fahrenheit — a range that seems to satisfy most bathers. In a spa, the temperature should not exceed 104 degrees.

Once you know the desired temperature, you need to determine the average ambient air temperature. Most experts recommend taking the average daily temperature during the coldest month when the vessel will be used. When you subtract the average ambient air temperature from the desired temperature, you’ve found the necessary temperature differential.

Heater efficiency: Expressed as British thermal units (BTUs), the heater output is the energy that a heater transfers to the water. The heater input is the energy (again in BTUs) used to generate that heat. Heater efficiency (HE) is the ratio of the output to the input, expressed as a percentage. All modern pool heaters are above 80 percent efficient, experts say. Typically, 84 percent to 87 percent efficiency is common in the pool heater market.

Heater-sizing charts often express the required heater output necessary to achieve the desired temperature rise for the pool’s surface area or volume (see Chart 1 below for a generic example). Because heaters are rated by their input, however, you must know the heater efficiency to determine what size heater is required to do the job. In other words, if you divide the required output by its efficiency (for instance, .80), you’ll have the proper heater input rating.

Manufacturers do part of the work for you in their heating charts by replacing the required output with the appropriate heater model number for the desired temperature rise and surface area or pool volume.

Heat-up time: For spas, in particular, the time required to heat the water to the desired temperature is important when sizing the heater. In fact, many spa-heater sizing charts use required heat-up time as a primary factor and assume a given temperature differential.

For intermittent heating, heat-up time also can be very important, although many sizing charts assume a 24-hour heat-up time.

Plugging in the numbers

Once you’ve determined these factors, selecting a heater is a matter of plugging the numbers into sizing charts. Although they are typically easy to use, the charts are formatted in varying ways. Some plot the temperature differential on one axis with the pool volume on the other. In that case, you would cross-reference these two key factors to determine the proper heater output, which is listed in columns across the chart.

Other charts, most of them provided by manufacturers, list model numbers on one axis, with the temperature differential on the other. Cross-referencing the heater model with temperature rise then leads to the pool sizes listed in columns on the chart.

Finally, some manufacturers offer easy-to-use sizing slide rules. In that case, select the pool volume and temperature rise to determine the model heater.

For spas, heat-up time often is critical. In these applications, sizing charts typically assume an increase in water temperature — say, 30 degrees — with models (or input ratings) listed on one axis and spa gallonage listed on the other (see Chart 2, below). Simply pick the spa volume and the desired heat-up time to find the appropriate heater model or rating.

To determine the heater model on a chart that lists required heater output, divide the output by the efficiency to find the heater input. (All heaters list their heater input ratings on their faceplates and in specification manuals. You can use this simple, generic calculation to double-check the accuracy of the manufacturer’s charts.)

Tips for do-it-yourselfers

For those who prefer to size the heater based on their own calculations, the following formula is the basis for most heater-sizing charts used in the industry and can be easily applied to either pools or spas:

Number of gallons x 8.33 (pounds per gallon) x the temperature rise = the number of BTUs required to heat the vessel.

For example, consider a 40-degree temperature rise in a 400-gallon spa — that is, 400 x 8.33 x 40 = 133,280 BTUs.

This number can either be divided by the desired heat-up time to give you the required heater output, or it can be divided by the heater capacity to give you the heat-up time a given model will provide.

Let’s assume, continuing the previous example, that you have a heater with an output of 266,000 BTUs. Here, 133,280 divided by 266,000 yields a heat-up time of .5 hours, or 30 minutes. Conversely, if the customer has a specific heating time in mind — say, 30 minutes — the formula works thusly: 133,280 ÷ .5 = 266,000 BTUs.

In other words, a 400-gallon spa would need a heater with an output of 266,000 BTUs to heat the water in 30 minutes.

Remember the ‘intangibles’

When it comes to sizing a heater, it’s not all numbers and formulas: Variables thrown in by Mother Nature and the location of the vessel need to be factored in. Following are issues that can affect heater size.

Wind: The wind can dramatically increase the surface heat loss from a pool or spa. By making waves across the water, the wind effectively increases the surface area of the pool. The basic rule: In a pool with an 11-mph wind, you need to increase the heater size by 25 percent.

Altitude: Where the vessel is located is another factor that calls for a bigger heater. For each 1,000 feet above sea level, the heater needs to be 4 percent larger.

Shade: If the pool is in the shade, you may need to upsize the heater. Although there’s no precise rule, you should throw in a small “fudge factor” to be on the safe side.

In this and other areas, it pays to contact heater manufacturers or your local supplier for expert guidance. After all, customer satisfaction is at stake.

Typical Spa Heater Sizing Chart
Heater Input (BTU/hr): 125,000 175,000 250,000 325,000 400,000
Spa Volume (gal.) Minutes required for each 30° temperature rise
200 30 21 15 12 9
300 45 32 23 17 14
400 60 43 30 23 19
500 75 54 38 29 23
600 90 64 45 35 28
700 105 75 53 40 33
800 120 86 60 46 37
900 135 96 68 52 42
1,000 150 107 75 58 47
Chart 2: Typical spa heater sizing — intermittent heating, volume method, temperature rise of 30°F
Generic Heater Sizing Chart: Maintenance Heating Method
Temp Rise 10° 15° 20° 25° 30°
Pool Size (sq. ft.) Heat loss = Required heater output (BTUs/hour)
200 21,000 31,500 42,000 52,500 63,000
300 31,500 47,300 73,000 78,800 94,500
400 42,000 63,000 84,000 105,000 126,000
500 52,500 78,800 105,000 131,000 157,000
600 63,000 94,500 126,000 157,000 189,000
700 73,500 110,000 147,000 184,000 220,000
800 84,000 126,000 168,000 210,000 252,000
900 94,000 142,000 189,000 236,000 284,000
1,000 105,000 157,000 210,000 263,000 315,000
Chart 1: The heat loss/output figures are based on an assumed wind velocity of 3 1/2 mph; for a velocity of 5 mph, multiply these losses by 1.25; and for 10 mph, multiply by 2.0. These numbers need to be divided by the heater efficiency to find the required input rating.