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Many people in the northern climates would like to extend their swimming season beyond the normal period. A pool heater will allow an additional 6 to 8 weeks of comfortable pool usage. A heater can also help increase the temperature to a more comfortable level in pools that receive little direct sunlight.


There are basically two types of heaters for above or in-ground pools. The best known is the gas-fired heater, fueled by propane or natural gas. Installation is simple: a plumber runs a gas line to the installation site, which is adjacent to the pool filter area. The return line from the filter is attached to the input of the heater, and the output from the heater is attached to the pool at the return fitting. That's all there is to it! Ignition is accomplished by a built in "microvolt" system; similar ignition systems are found on expensive butane cigarette lighters. In the case of propane, a separate tank is needed. Depending on your township, the propane tank may be buried. Because there is little or no retrofitting done, the heater may be installed at the same time as the pool, or at a later date convenient to the owner.


The other method of heating a pool is the solar heater. This is a passive system that consists of one or more 4' by 20' black flexible plastic panels that contain hundreds of feet of tubing. The water is pumped from the filter, through the solar panels where it absorbs heat from the sun, and is then returned to the pool 10 to 15 degrees hotter. There are no moving parts, and the force of the filter's pump circulates the water. The benefit of solar vs. gas is obviously cost. The heater itself runs about $200 per panel, and the actual cost to heat the water is zero. The major drawbacks are the space required for the panels (they may be roof mounted if space is not available), and the fact that overcast days will not heat the water as well as sunny days will. While solar heaters will usually not extend the season as long as a gas heater will, they are a good alternative for budget conscience buyers.

Heat Pumps

The newest technology in pool heaters is the Heat Pump.  Utilizing the ambient temperature from the surrounding atmosphere, a heat pump “extracts” the heat from the air and uses it to warm your pool.  Heat pumps are electric, eliminating the need for large propane tanks.  They are also a good choice if you have natural gas and your pump and filter are located far from the house because they do not require long and costly plumbing runs.

While initially more expensive than natural or propane gas pool heaters, heat pumps are substantially more efficient.  The additional cost will generally be recovered in three to four seasons of use due to energy bill savings.  For example, a typical propane heater used from May to September in Long Island, New York costs approximately $2034 to run.  Natural gas in the same environment will cost approximately $1037, and the heat pump, $401. 

Also, because of their efficiency, heat pumps are designed to operate continuously so your pool is always at the temperature you desire.  Set it and forget it… you won’t have to worry about remembering to turn your heater on Thursday in order for it to reach swimming temperature by Saturday!  And because a heat pump is used to maintain a pool’s temperature rather than cycling through on and off periods, savings are also obtained because heat pumps need less BTU’s.

And there are actually models that will cool your pool water in the really hot summer months!  So when the air temperature hits triple digits, your pool will be refreshingly cool.

The following formulas are provided to help determine the correct size heater needed to heat and maintain a comfortable temperature in a given size pool.


Note: Average depth is determined by adding the depth of the shallow end to the depth of the deep end and dividing by 2.

For Round Pools:

    Gals. = Diameter x Diameter x Avg. Depth x 5.9

For Oval Pools:

    Gals. = Length x Width x Avg. Depth x 6.7

For Rectangular Pools:

    Gals. = Length x Width x Avg. Depth x 7.5

For Kidney Shaped Pools:

    Gals. = Length x Avg. Width x Avg. Depth x 7.0 





    30,000 gallon pool @ 55 degrees, heated to 75 degrees

    30,000 x 8.33 x 20 = 4,998,000 BTU'S / 24 hours = 208,250 Output BTU'S per hour

The example above shows that 20' x 40' pool with an average depth of 5 feet with a heater producing an output of approx. 210,000 BTU's would require 24 hours to raise the temperature of the pool 20 degrees; hence a filter with an output of 400,000 BTU's would do the same job in about 12 hours. Note the BTU's are shown as an output number. Many heaters are rated at input BTU's, meaning you need to adjust for heater efficiency. If you cannot determine the efficiency rating of a heater, an acceptable rule of thumb would be to use 85%. This means a 400,000 BTU heater would produce 340,000 output BTU's.

The above formula gives you the needed minimum BTU's needed to initially heat a pool to the desired temperature in 24 hours. This should be used as a minimum figure to consider when selecting a pool heater.


Once the pool has been heated to the desired temperature, the following formula will help you determine the output requirements of the heater to maintain that temperature.

Assuming an average wind speed of 3.5 mph, you will need about 10.5 BTU's per square foot of the waters' surface area for every degree above ambient (average air) temperature you wish to keep your pool.


    A 16' x 32' pool is heated to a temperature of 72 degrees. The air temperature is 57 degrees.

    16 x 32 = 512 sq. ft. x 10.5 BTU's = 5376 x 15 degrees = 80,640 BTU's per hour heat loss

This means that you must replace the lost heat to maintain the desired temperature. A 250,000 BTU heater would have to run about 20 minutes out of every hour in order to keep pace with the heat loss.


It takes about 1 gallon of propane for every 100,000 BTU's generated, at a current cost of about $1.20 per gallon. The cost to initially heat the 16' X 32' pool used in the above example is determined by the first formula; gallons (21,000) x 8.33 lbs/gal x 15 degrees = 2,623,950 BTU's, divided by 100,000 BTU's x $1.20 per gallon of propane = $31.49. To maintain the temperature after the initial heating, you need to replace the 80,640 BTU's per hour of heat loss as shown in the formula above. 80,640 divided by 100,000 times $1.20 equals $0.97 per hour. The use of a solar cover will reduce the heat loss about in half, or $0.48 per hour. If you use the heater for six weeks, your cost will be approx. $32 to initially heat it, plus an additional $490 to maintain it, for a total of $522.

If your pool is in an exposed area with higher winds, or if you're located at a higher altitude, your requirements will increase approximately 25 to 35%.

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