The BTU Logic, Without the Mystery
A BTU is just a unit of heat, and heater sizing is just arithmetic: how much water are you heating, how many degrees are you raising it, and how fast do you want it done. Every degree you want to add to every gallon costs heat, and every hour the pool sits in cool air and wind, it gives some back.
The practical floor for heat pumps is about 50,000 BTU per 10,000 gallons of pool water. But the floor is not the recommendation. An undersized heater runs constantly, struggles on the cold snaps that are the whole reason you bought it, and wears out sooner. An oversized heater reaches temperature faster, cycles less, and costs only modestly more up front. In fifteen years of Treasure Coast winters, nobody has ever complained their heater was too big.
Surface area matters as much as gallons, because the surface is where a pool loses heat. A wide, shallow sun-shelf pool sheds more heat than a deep compact one holding the same water. That is why the table below is a starting range and a real sizing visit is the finished answer.
Sizing Ranges by Pool Size
| Pool size | Heat pump output | Notes |
|---|---|---|
| Up to 10,000 gal | 50K to 90K BTU | Small pools and pool-spa combos |
| 10,000 to 15,000 gal | 90K to 115K BTU | The most common Treasure Coast size |
| 15,000 to 20,000 gal | 115K to 130K BTU | Step up if unscreened or windy |
| 20,000 to 25,000 gal | 130K+ BTU | Large single units |
| Over 25,000 gal | 140K+ or twin units | Big pools, attached spas, fast recovery |
Then adjust for your yard. A screen enclosure knocks down wind and cuts heat loss; an open, breezy lot near the water does the opposite and earns a size bump. An attached spa you want hot on demand earns another. And a solar cover, the cheapest heating device ever invented, lets any heater in this table work less.
Heat Pump vs Gas in Florida’s Climate
This decision is easier here than almost anywhere in the country, because Florida’s mild winters are exactly what heat pumps are built for.
A heat pump does not make heat, it moves it. It pulls warmth out of the outside air and transfers it into the water, the same physics as your home AC running in reverse. That trick multiplies efficiency: for every unit of electricity it consumes, a heat pump moves several units of heat into the pool. The catch is that it needs warm-enough air to harvest, and its output falls as the air temperature falls. In Michigan that is a real problem. In Stuart, where winter afternoons sit in the 60s and 70s, it almost never is.
Gas heaters make their own heat by burning propane or natural gas, so they hit full output in any weather and raise temperature fast. That power costs money every single time: expect $300 to $500 or more in fuel for a typical heating month against $100 to $200 in electricity for a heat pump doing the same job. Gas earns its keep in one scenario we see weekly: the attached spa you decide to use at 7 pm and want at 103 degrees by 7:30.
The common Treasure Coast answer: a heat pump as the season-long workhorse, and gas only where instant spa heat is a priority. Both install at the equipment pad as part of a normal equipment upgrade.
| Heater type | Typical monthly cost, Florida winter | Best for | Heating speed |
|---|---|---|---|
| Electric heat pump | $100 to $200 | Consistent daily swimming | Slow, 1 to 2 degrees per hour |
| Natural gas or propane | $300 to $500 or more | Spas and on-demand use | Fast, 3 to 5 degrees per hour |
These estimates assume a solar cover on the pool at night, which can cut winter heat loss by up to half.
What Winter Run Times Look Like Here
Set expectations by season, because a heat pump is a marathon appliance, not a sprinter:
- October and November: the easy months. Water is still warm; the heat pump tops up a few degrees on cool mornings and barely registers on the electric bill.
- December through February: the working months. After a real cold front the heater may run most of the day for a day or two to recover, then settle into maintenance runs. Holding a steady temperature all season costs far less than letting the pool go cold and reheating it from the 60s for each visit, because raising a typical pool even a few degrees takes many hours of run time.
- March: the victory lap. The sun does most of the work again.
With a heat pump, winter operating cost lands around $100 to $200 per month for a typical screened Treasure Coast pool held in the mid 80s, and less in the shoulder months. Two habits keep it there: run the heater during daylight hours when the air is warmest and the machine is most efficient, and cover or screen the pool to hold what you paid for overnight. Pair the heater with a variable-speed pump and the whole pad sips power; Florida’s efficiency rules already push every replacement pump that direction, as our pump rule explainer covers.
Why We Install AquaCal
CoastLife is an AquaCal dealer, and that is a deliberate choice, not a logo on a truck. AquaCal builds heat pumps a few hours from here for exactly this climate and this salt air, and as a dealer we size, install, warranty-register, and service the same machines with our own crew. If an AquaCal unit ever needs attention, the company that installed it answers the phone at (772) 266-4355, Mon to Fri, 7:30 to 4:30. The full line, and what fits which pool, lives on our pool heat pumps page.
Heater Sizing Questions
What size heat pump do I need for a 15,000 gallon pool?
Start around 110K BTU and size up, not down, if the pool is unscreened, windy, has big surface area, or includes a spa. Around 50,000 BTU per 10,000 gallons is the floor, and the next size up is usually the right buy.
Do pool heat pumps work in Florida winters?
Yes, they are ideal here. Heat pumps harvest warmth from the air, and Treasure Coast winter air stays warm enough for efficient operation nearly all season. Output only sags during the coldest snaps, which is what right-sizing is for.
Is a gas or electric heat pump heater better in Florida?
For keeping a pool warm all season, the heat pump wins on operating cost by a wide margin. Gas wins for speed: instant high heat for spas and spur-of-the-moment weekend heating. Many pools with attached spas run one of each.
How long does it take to heat a pool?
A correctly sized heat pump typically raises pool temperature about 1 to 2 degrees per hour depending on size, air temperature, and wind. Recovering from a cold front takes a day or so; holding temperature after that takes far less.
Can I add a heater to an existing pool?
Almost always. A heat pump needs pad space, a dedicated electrical circuit, and a plumbing tie-in; gas needs a fuel line. We confirm the electrical panel capacity during the quote and handle the install as one equipment upgrade visit.