All of the swimming pool heaters at, from the small 100 (100,000 btu) unit to the largest 405's (405,000 btu's of output per hour) size will heat your pool. The smaller heaters are slower to heat, and will operate more often to keep up with the output of larger heaters. Larger heater sizes are a small bit more efficient than the smaller pool heaters. Use of a pool cover to retain heat and installing fences or other wind* obstructions can reduce demands on the pool heater and affect pool heater size.

Gas Pool Heater Sizing

If you are trying to heat a pool/spa combo, a 400 btu pool heater (the Big Boy) is usually used for fast (15 - 30 minutes) heat up of the spa. A smaller pool heater can also heat a spa, it will just take longer. Using a spa solar blanket cover can reduce the heat up time by a few minutes. Custom made insulated spa tops can keep most of the heat inside the spa, and have it ready for the moment you want to use it.

To size a pool heater, ask yourself a few questions about your anticipated usage patterns. If you will be heating intermittently, for example, turning on the heater on Friday, and turning it off on Monday, then a quick heat up is required. Gas heaters only add 1 - 2 degrees of heat per hour to the average pool. A larger heater is best for this usage pattern.

If you expect to be maintaining a temperature by setting the thermostat, and maybe adding a few degrees in the evening or on weekends, then a smaller size pool heater can be used.

Use the chart below as a guideline in sizing a heater to your pool size. Factor in the usage patterns and other considerations above and then consider going one size larger for good measure.

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

Pool Heat Pump Sizing

For pool heat pump sizing, as a general rule, plan on 50,000 BTU of pool heat pump for every 10,000 gallons of pool water. A pool cover can help immensely to retain the heat, especially used at night.

Pool heat pumps draw warmth from the outside air and convert it into warm pool water. They are electrical units, and in warm temperatures, they cost no more to run than a 100 watt light bulb. In cooler temperatures, it takes more energy to maintain the heat, but is still cheaper than gas pool heat. When outside air temps reach mid-50’s however, a pool heat pump will not be able to extract much heat from the outside air, and will eventually stop heating the water.

As compared to gas heaters, pool heat pumps are quiet, clean and much cheaper and safer to operate, but on the downside, they are slow to heat (5-10 degrees per day), and won’t operate at all during cold outside temperatures.

Pool heat pump sizing is also accomplished by their BTU output. Heat pumps are available in ranges from 50,000 to 150,000 BTU’s. For an inground pool choose one of the larger heat pump sizes. You can go smaller in size if you have a pool cover that will be used constantly, or if you are only heating during summer months. Larger pool heat pump sizes will heat faster, and maintain temperature more easily than smaller units.

Solar Pool Heater Sizing

Sizing a solar pool heater is less tricky than sizing a pool heat pump or gas heater. Every little bit helps, but if you want a 20 degree temperature rise, plan on installing enough solar panels that exceeds half the square footage of the pool. For example, if an inground pool has 600 sq. ft. of surface area, you will want to have at least 300 sq. ft. of solar panel surface area.

Other factors come into play as well when sizing a solar pool heater. How effective and productive your solar pool heater is depends on the amount of daily sunshine the panels receive. In addition, covering the pool or blocking the wind makes a big difference. Finally, a solar controller is highly recommended to automate the operation of the solar panels, for capturing and transferring the most heat possible.

*wind is the largest heat thief around your pool, with the possible exception of teenagers. To prevent other household members from cranking up the heat, (which doesn't heat the water any faster) you can disconnect a wire inside the heater – such as a pressure switch wire. Then the pool heater only works when you want it to!

Related Product Pages:

Gas Pool Heaters
Pool Heat Pumps
Solar Pool Heaters
Solar Pool Covers

Related Blog Posts:

Gas Heater Sizing Guide
Heat Pump Sizing Guide
Solar Pool Heater Sizing Guide