Despite environmentally motivated decarbonization efforts, gas-fired pool heaters remain the most popular choice for heating pools. Modern new gas-fired heater models are being manufactured with much higher efficiencies than older models. But depending on the climate and how often the pool is used, they may not be the most cost-effective option as compared to heat pumps and solar pool heaters.
Gas pool heaters use either propane or natural gas. As the pump circulates the pool's water, the water drawn from the pool passes through the filter and then to the heater. The gas burns in the heater's combustion chamber, generating heat that is transferred to the water as it is returned to the pool.
Ideal for quickly heating pools, gas pool heaters can be a good choice for pools that are only used periodically. Unlike solar pool heaters and heat pumps, gas pool heaters don’t rely on the ambient temperature, weather, or climate, and can maintain any desired temperature.
Selecting a Gas Pool Heater
When selecting a gas pool heater, the following factors should be considered:
• Costs Sizing a Gas Pool Heater Sizing a gas pool heater involves many factors and is really not a doit- yourself task; customers should understand that this job should be performed by a qualified pool professional. A heater is sized according to the surface area of the pool and the difference between the pool and the average air temperatures. Wind exposure, humidity levels, and cool night temperatures are other factors that affect the heating load for outdoor pools. Pools located in areas with higher average wind speeds on 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: • Determine the desired pool temperature.
• Determine the average temperature for the coldest month of pool use.
• Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed.
• Calculate the pool surface area in square feet.
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 1° to 1 ¼ °F temperature rise per hour and a 3 ½ mile-per-hour average wind speed on the pool surface. For a 1 ½ °F rise, multiply by 1.5. For a 2° F rise, multiply by 2.0.
In Northern California, for example, low winter temperatures average about 50 °F. For those wishing to swim in water of about 80 °F, the temperature rise would be 30 °F.
Next, calculate the pool’s surface area.
Multiply the surface area times the temperature rise times 12.
For example, if the temperature rise is 30 °F and the pool is 300 square feet, the required Btu’s needed are 108,000.
Determining Efficiency of a Gas Pool Heater
There’s a standard test to determine the energy efficiency of a gas heater based on their Btu 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. That means it wastes 20% of the fuel.
Gas pool heaters generally list their efficiency percentage on their nameplates. Manufacturers can also provide its efficiency percentage.
Modern gas pool heaters are rated from 89% to 95% efficiency. Compared to older gas heaters, serious financial savings can be realized by purchasing one of these newer energy efficient heaters. The accompanying Table shows how much customers 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.
This table provides a convincing argument for customers considering replacing an inefficient gas pool heater. In general use the following formula to determine customers’ annual cost savings with a higher efficiency gas pool heater model: Current Annual Cost x [1 – (Current Efficiency ÷ New Efficiency)] For example: If a customer currently has an old 55% efficient heater and is paying $1,000 a year, and is considering replacing the heater with a new, 84% thermally efficient heater.
$1,000 X [1 – (55 ÷ 84)] = $345 saved per year.
Estimating Gas Pool Heater Costs and Savings
The accompanying table from the U.S. Department of Energy estimates yearly gas pool heating operating costs by location, water temperature, with or without using a pool cover.
Table 1: Annual cost saving comparison with 95% efficient gas heater.
Table 2: Costs of Outdoor Pool Gas Heating by Location. Note: Figures based on a 1,000 square-foot, outdoor pool heated with an 80% efficient natural gas heater at $1.09 per therm,uncovered for 8 hours/day.