Love your pool, but hate having to wait for the temperature to warm up after a chilly morning? Do you want to go for a midnight swim, but the water has cooled off? Want to extend your swimming season to open it earlier in the spring and close it later in the fall? If you said yes to any of these questions, then you might want to consider installing a swimming pool heater so you can maintain your pool’s temperature at a comfortable level.

Swimming pool heaters can add 10-30 degrees F to the pool water using propane or natural gas, electric or solar, and can be underground or above-ground. With pilotless electronic ignition, state-of-the-art components, low emission, and energy efficiency, the technology has improved drastically.

Types of Swimming Pool Heaters

There are many makes and models of swimming pool heaters, with one difference being the type of energy usage, including gas, electric, and solar.

There are two types of electric pool heaters or heat pumps: water-sourced and air-sourced. Heat pumps don’t generate heat; they capture heat and move it, heating the pool water at a slow rate of 3-5 degrees per day, although they can maintain temperatures more efficiently.

Air-sourced pool heat pumps pull warmth from outside air and then operate as a reverse air conditioner: the outside air warms the refrigerant, compressing it to generate more heat, which it then transfers to your pool’s water. While air-sourced pool heat pumps operate at less than half the cost of gas pool heaters, they are limited to climates with outside temperatures of 50 degrees or more. Below that temperature and there is no heat to withdraw from the air.

Water-sourced (geothermal) heaters transfer heat from an underground body of water, operating without the influence of the temperature of the air. Water-based pool heat pumps pass water over an evaporator coil that contains Freon, a liquid refrigerant that heats and becomes gas. While the source water exits after cooling and goes back to its source, the Freon gas goes through a compressor, heating the gas even more. While the hot gas is going through the compressor, the pool water floods over the coil and is heated.

When considering natural gas or liquid propane heaters versus electric, one consideration is that the gas-run have different types of ignition (for example, millivolt or electronic). Millivolt pool heaters require a pilot light that must stay lit for the pool to sustain the heating process, where a pool heater that starts electronically does not use a pilot light.

Solar heaters use the sun and are highly efficient, cost much less than other types of traditional pool heaters, and can connect at a very low cost.

The Right Size Pool Heater

Smaller heaters are slower to heat and will have to work harder, shifting some of the savings from purchasing it to higher running costs. Keep in mind that the larger the pool heater, the faster it will warm your water, saving on heater running costs.

To determine the BTUs necessary to warm your pool water to the level you want, calculate the surface area of your pool. Typically, if you have a swimming pool with an attached spa, you’ll need to look at models over 400K BTU. Without a spa, look at the 250K or 325K sizes. Another consideration is whether your pool has a cover, or if you will only rarely use the heater. Installing wind blocks can reduce demands on the pool heater, and can affect the size of the heater you’ll need.

An additional consideration is whether the pool is an above ground pool heater or inground. Above ground can be in the 100K range for gas pool heaters. Above ground using heat pumps would be in the range of 50-75K BTU while inground would be in the 100-150K BTU range.

Sizing a pool heater is easier when you anticipate your usage patterns. If you only heat intermittently, like only on weekends, a quick heat up is necessary. Gas heaters add 1-2 degrees of heat per hour, so a more substantial heater would be better in this case. If you decide to maintain the temperature using a thermostat and only to increase the temperature in the evening, smaller heaters have to run more and are thus slightly less efficient.

Sizing a Gas Pool Heater 

As a guideline for sizing, factoring in usage patterns and other efficiency measures, here are some measurements to use for gas pool heaters (then go one more size up to be sure):

Size Sq. Ft. Surface Area Gallons
100-200 BTU up to 300 sq ft 1,000 – 10,000 gals
200-300 BTU up to 500 sq ft 10,000-20,000 gals
300-400 BTU up to 800 sq ft 20,000-40,000 gals
400 BTU up to 1200 sq ft. 40,000-80,000 gals

Sizing a Pool Heat Pump 

When sizing a pool heat pump, for every 10,000 gallons, plan on 50,000 BTU. Because pool heat pumps are electric, in warm temperatures they will cost less than a 100-watt bulb. While it takes more in cooler temperatures, it’s still cheaper than gas pool heaters to maintain the heat. After the temperature reaches down into the mid-50’s the pool heat pump will not work as well and will eventually stop heating the water.

While pool heat pumps are cleaner, quieter, and safer and cheaper to operate than gas pool heaters, they are slower to heat and will completely stop operating if the temperatures get too cold.

Sizing a Solar Pool Heater 

Sizing a solar pool heater is simple: install enough panels to exceed half the square footage of the pool. Then add in the amount of sunshine you’ll receive on a regular basis. If you have 4-8 hours of direct sunshine, the above calculation will be good. If not, adjust accordingly. Also, consider a solar controller to send water to the panels only when necessary during daylight hours.

Cost to Run a Swimming Pool Heater

The cost to run a swimming pool heater depends on many things. The biggest is the difference between the pool water and the air. Another is whether you use a cover or not, and whether it’s a solar cover or not). Add in the other variables like the surface area of the pool and the wind speeds, and it’s not easy to predict what your costs might be.

On average, the cost to run a gas pool heater for a medium to a large inground pool will be around $200 per month. While natural gas prices have dropped, propane costs are up, so this will vary depending on your location and the other factors mentioned above.

Heat pump pool heaters cost more initially, but over the long haul will have much lower operating costs because they are more efficient. They will also last longer than gas pool heaters when properly maintained.

Heat Your Pool More Efficiently

Here are a couple of tips for more efficient pool heating. First, make sure your guests know that turning the heater to maximum doesn’t heat the pool faster, and can cause your heater to overheat the pool, a costly and wasteful result.

If you only use your pool on the weekends, turn the temperature down 10-15 degrees on the days you don’t use it. Keeping a thermometer in the water will help you maintain consistency.

Set a timer to heat the pool up in the hours before use. Use a cover (preferably solar) to reduce heat loss. Wind is the biggest heat thief, so install wind blocks to reduce surface heat loss.


The biggest benefit of gas pool heaters is their quick heating time, in minutes or hours versus the time required with a heat pump. Gas heat is preferable when you use your pool sporadically, and want to heat your pool when needed. Gas pool heaters will perform well in any climate, heating your pool even in colder weather.

Heat pump pool heaters efficiently maintain the water temperature and can save up to 70% savings over a gas heater. However, at lower temperatures, the air-sourced heat pump version will not be able to heat the pool. If you live in a hot climate, an air-sourced heat pump is a good choice. If you have geothermal water available, a water-sourced heat pump is a very good choice.

In the end, you could install a gas or solar pool heater in combination with a heat pump. The combination will provide energy savings due to the heat pump’s efficiency, and the fast heat on demand from a gas pool heater will be there when you need it.

While we’ve advised on measurements and sizing, you should have a trained pool pro give you advice for your specific pool to determine the best swimming pool heater for your situation.

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