We size heaters for South Florida pools every week, and the same three mistakes come up: buying by gallons instead of surface area, buying gas when a heat pump would cost a third as much to run, and undersizing to save a few hundred dollars up front. Here is how we actually do it.
Start with surface area, not gallons
A pool loses heat from its surface, mostly through evaporation, so the heater has to replace what the surface loses. Measure length times width in feet. A 15 by 30 pool is 450 square feet; a 20 by 40 is 800. Free-form pools: measure the longest and widest points and multiply by 0.85.
The BTU rule of thumb
For a heat pump in Florida, where you are maintaining temperature rather than heating from cold, plan on roughly 100 to 125 BTU per square foot of surface:
- Up to 400 sq ft (14 x 28): 80,000 to 100,000 BTU heat pump (Raypak R4450ti-E or R5450ti-E, AquaCal T75)
- 400 to 600 sq ft (16 x 32 to 18 x 36): 100,000 to 125,000 BTU (Raypak R5450ti-E or R6450ti-E, Pentair UltraTemp 120, AquaCal T100)
- 600 to 900 sq ft (20 x 40 and up): 125,000 to 140,000 BTU (Raypak R8450ti-E, AquaCal T135, or two units on a large pool)
- Pool and spa together: size for the pool, then make sure the unit can bring the spa up quickly; most owners pair a heat pump for the pool with a gas heater for the spa.
Gas heaters are rated higher because they are usually used for fast heating and spas: 200,000 BTU for a spa or a small pool, 266,000 for pools to about 500 square feet, 336,000 to 400,000 for larger pools and pool-spa combinations where you want a spa hot in 20 minutes.
Heat pump or gas?
Heat pump if you want to swim most of the year and keep the pool at a set temperature. In our climate a heat pump costs about one-quarter to one-third of gas to run, because it moves heat from the air instead of burning fuel. It heats slowly (one to two degrees an hour) and loses efficiency below about 50 degrees, which in Palm Beach County means a handful of mornings a year. The Raypak Digital ti-E series and AquaCal TropiCal are the units we install most.
Gas if you heat on demand (weekend swimmers, a spa you want hot tonight), if you heat a spa, or if you have natural gas at the house. A Raypak Digital 266A or a Pentair MasterTemp 400 will raise a spa 20 degrees in under half an hour. Propane works the same but costs more per BTU, so propane owners usually go heat pump for the pool and gas only for the spa.
Both is the premium answer for pool-and-spa combinations: heat pump holds the pool, gas blasts the spa. Automation switches between them.
Three things that change the answer
- Wind and shade. An open lot on the water or a screened enclosure changes heat loss by 20 to 30 percent either way. Go one size up in the open; a screen enclosure lets you stay at the lower end.
- A cover. A solar cover cuts evaporation loss by up to 70 percent and is the single cheapest way to make a smaller heater perform like a bigger one. See solar covers and reels.
- Electrical. A 100K to 140K BTU heat pump needs a dedicated 50 to 60 amp 230V circuit. If the panel is full, that is a cost to budget before the heater.
Do not undersize
An undersized heat pump runs all day, never reaches set point on cool weeks and wears out early. The difference between a 100K and a 125K unit is usually a few hundred dollars; the difference in running cost is nothing, because a correctly sized unit cycles off. When in doubt, go up one size.
Get a number for your pool
Send us the pool dimensions, whether you have a spa, your gas situation and a photo of the equipment pad, and we will size it and quote installation anywhere in Palm Beach, Martin and St. Lucie counties. Call 1-877-650-POOLS or use the contact form. Heaters and heat pumps over $300 ship free nationwide, or pick up free in Jupiter.
