How to Size a Pool Heater

General information, not legal, engineering or insurance advice.

A heater that's too small may never get your pool as warm as you want on a cool, windy day. One that's much bigger than you need costs more to buy and may need a bigger gas line or electrical circuit. Sizing is a job for your contractor, but it helps to understand what goes into it so you can compare quotes and ask good questions.

BTU basics

Pool heater capacity is usually stated in British thermal units (BTU) per hour. One BTU is the amount of heat needed to raise the temperature of one pound of water by 1°F.¹ A pool holds a lot of water, so heating it takes a lot of BTUs. Just as important, the pool keeps losing heat to the air while you heat it, so a heater has to replace that loss before it can warm the water.

What sets the size you need

• Pool surface area. Heat escapes mainly from the water's surface, so contractors size heaters by surface area (length × width for a rectangular pool), not just by gallons. Evaporation is by far the largest source of a pool's energy loss.²
• Desired temperature rise. The gap between the water temperature you want and the air temperatures during the months you plan to swim. Wanting to swim in the coldest weeks means a bigger rise and a bigger heater.
• Wind and exposure. Wind speeds up evaporation and heat loss. A screened enclosure, fence or landscaping that blocks wind can help; an open, breezy waterfront lot loses more heat.
• Cool nights and dry air. Cooler, drier air increases heat loss from the water's surface, so nights after a cold front are the hardest test.
• A cover. Pool covers can save 50% to 70% of pool heating costs², and a pool cover or blanket reduces heat loss and helps hold warm temperatures longer.³ If you'll use a cover, tell the contractor, since it changes how hard the heater has to work.
• How fast you want heat. Heating a pool only for weekends means warming it from cold quickly; keeping it warm all season means steadily replacing lost heat.
• Spa. If the heater also serves an attached spa, it has to bring a smaller volume of water to a higher temperature.

Heat pumps and air temperature

A heat pump moves heat from the outdoor air into the water.⁴ Air-source heat pumps with warm outside air are especially efficient², and they deliver less heat as the air cools. If you want to swim through cold snaps, ask the contractor to size the heat pump using the cooler air temperatures you expect, not mild-weather ratings, and ask whether a gas heater for quick boosts makes sense for how you use the pool.

Sizing solar pool heating

Solar is sized by collector area rather than BTU rating. The U.S. Department of Energy's consumer guide says:

• Solar pool heating requires a collector area of 50% to 100% of the pool's surface area. Location and other factors set the exact size.³
• For example, a 15-by-30-foot pool in Florida typically needs collectors equal to 100% of the pool's area, or 450 square feet of unglazed collectors, because many Florida owners swim year round. In northern California, where pools are used 6 to 8 months a year, systems are typically sized at 60% to 70%.³
• Adding collector area generally lengthens the swimming season and lets you use the pool in colder weather.³

Collectors also need an unshaded area facing south, southeast or southwest, such as a roof.³ If your roof space is limited, ask about ground or patio-cover mounting.

Ask to see the calculation

1. What surface area did you use, and what target water temperature and air temperatures?
2. Did you account for wind, shade and whether I'll use a cover?
3. What is the heater's output, and how long will it take to warm the pool on a cool day?
4. Will my pump, plumbing and electrical or gas supply handle this heater? Is any upgrade in the quote?
5. For solar: what collector area as a share of the pool's surface, which roof faces, and what shade did you find?

Sources

1. U.S. Energy Information Administration, “British thermal units (Btu)”, Energy Explained.
2. U.S. Department of Energy, Better Buildings, “Pool Efficiency” fact sheet.
3. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, “A Consumer’s Guide: Heat Your Water with the Sun” (DOE/GO-102003-1824, December 2003).
4. Florida Department of Community Affairs, Building A Safer Florida and University of Florida IFAS Extension, “Residential Swimming Pool Efficiency” fact sheet (May 2006).