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Gas-fired pool heaters remain the most popular system for heating pools. Today you can find new gas-fired heater models with much higher efficiencies than older models. However, depending on your climate and pool use, they may not be the most cost-effective option when compared to heat pump and solar pool heaters.

How They Work

Gas pool heaters use either natural gas or propane. As the pump circulates the pool's water, the water drawn from the pool passes through a filter and then to the heater. The gas burns in the heater's combustion chamber, generating heat that transfers to the water that's returned to the pool.

They're ideal for quickly heating pools. Therefore, gas pool heaters can be a good choice for pools that are only used periodically. Unlike heat pump and solar pool heaters, gas pool heaters can maintain any desired temperature regardless of the weather or climate.

Selecting a Gas Pool Heater

When selecting a gas pool heater, you need to consider the following:

  • Size
  • Efficiency
  • Costs.

Sizing a Gas Pool Heater

You should have a trained pool professional perform a proper sizing analysis for your specific pool to determine pool heater size.

Sizing a gas pool heater involves many factors. Basically, a heater is sized according to the surface area of the pool and the difference between the pool and the average air temperatures. Other factors also affect the heating load for outdoor pools, such as wind exposure, humidity levels, and cool night temperatures. Therefore, pools located in areas with higher average wind speeds at the pool surface, lower humidity, and cool nights will require a larger heater.

Gas pool heaters are rated by Btu (British thermal unit) output. Outputs range from 75,000 Btu to 450,000 Btu.

To calculate an approximate heater size for an outdoor pool, follow these steps:

  1. Determine your desired pool temperature.
  2. Determine the average temperature for the coldest month of pool use.
  3. Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed.
  4. Calculate the pool surface area in square feet.
  5. Use the following formula to determine the Btu/hour output requirement of the heater:

Pool Area x Temperature Rise x 12

This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0.

Determining Efficiency of a Gas Pool Heater

Gas pool heaters have a standard test to determine their energy efficiency based on their Btu (British thermal unit) output.

Heater efficiency is the ratio of usable output to energy input. For example, an 80%-efficient heater uses $80 worth of useful heat for every $100 worth of fuel. Therefore, it wastes 20% of the fuel.

Most gas pool heaters feature their efficiency percentage on their nameplates. A pool heater's manufacturer can also provide its efficiency percentage.

Today, you'll find some gas pool heaters with 89%–95% efficiency. Table 1 shows how much you can save for every $1,000 in annual pool heating costs relative to a 55% efficient gas pool heater (baseline) by installing a 95% efficiency gas pool heater.

Table 1. Annual Savings by Gas Pool Heater Efficiency*

55% $1000 $422
60% $915 $337
65% $845 $267
70% $785 $207
75% $732 $154
80% $687 $109

*For every $1,000 in annual pool heating costs relative to a 55% gas pool heater (baseline)

Estimating Gas Pool Heater Costs and Savings

Table 2 below estimates annual costs of a using a gas pool heater for an outdoor swimming pool by location, by water temperature, and with or without using a pool cover.

Table 2. Costs of Outdoor Pool Gas Heating by Location*

78° 80° 82°
MIAMI 1/1–12/31 $2910 $3880 $4905
W/COVER 1/1–12/31 $567 $796 $1090
PHOENIX 3/1–10/31 $1886 $2420 $3019
W/ COVER 3/1–10/31 $131 $229 $349
DALLAS 4/1–10/31 $2060 $2616 $3346
W/ COVER 4/1–10/31 $251 $382 $556
ATLANTA 4/1–10/31 $2321 $3063 $3924
W/ COVER 4/1–10/31 $436 $578 $807
LOS ANGELES 5/1–10/31 $2540 $3237 $3957
W/ COVER 5/1–10/31 $229 $414 $643
KANSAS CITY 5/1–10/31 $1954 $2552 $3248
W/ COVER 5/1–10/31 $392 $567 $741
NEW YORK 5/1–9/30 $1973 $2594 $3248
W/ COVER 5/1–9/30 $283 $403 $545
CHICAGO 5/1–9/30 $2208 $2823 $3455
W/ COVER 5/1–9/30 $294 $403 $523
DENVER 5/1–8/31 $2394 $2889 $3404
W/ COVER 5/1–8/31 $168 $229 $331
BOSTON 5/1–8/31 $2333 $2856 $3412
W/ COVER 5/1–8/31 $316 $447 $628
MINNEAPOLIS 6/1–9/30 $1814 $2420 $2965
W/ COVER 6/1–9/30 $262 $338 $523
SAN FRANCISCO 6/1–8/31 $2126 $2529 $2954
W/ COVER 6/1–8/31 $262 $436 $643
SEATTLE 6/1–8/31 $2078 $2431 $2801
W/ COVER 6/1–8/31 $414 $578 $752

*Figures based on a 1,000 square-foot, outdoor pool heated with an 80% efficient natural gas heater at $1.09 per therm and uncovered for 8 hours per day.

If you're replacing a gas pool heater, you can use the following formula to determine your annual cost savings with a higher efficiency gas pool heater model:

Current Annual Cost x [1 – (Current Efficiency ÷ New Efficiency)]

Installation and Maintenance

Proper installation and maintenance of your gas pool heater can optimize its efficiency. It's best to have a qualified pool professional install the heater and even perform complicated maintenance or repair tasks.

Read your owner's manual for a maintenance schedule and/or recommendations. You'll probably need to tune up your pool heater annually. Also, scaling in the burner or heat exchanger may decrease efficiency over a period of time.

With proper installation and maintenance, gas pool heaters typically last five or more years.