Sizing
How big the heater has to be
"What size pool heater do I need" is two different questions wearing one sentence, and they have different answers. One is about holding a temperature you already reached. The other is about getting there from cold in a set number of hours. Size on the wrong one and you either freeze in April or buy twice the equipment you needed.
Question one: hold the temperature
The maintain load is the heat loss at the worst hour of your season — night, no sun, cover in whatever state you actually keep it. For the default pool it is 110,594 BTU per hour, set by April conditions at 69.1 °F mean air.
This number is smaller than most people guess, and it is the number a heat pump has to satisfy. A heat pump that meets the maintain load will hold the pool indefinitely; it simply will not get you there quickly from a cold start.
Question two: get there from cold
Raising the whole body of water is a mass problem, not a surface problem. It takes one BTU to lift one pound of water one degree, and the default pool holds 159,815 pounds. Lifting it 10.9 degrees takes 1.74 million Btu regardless of how fast you do it. Divide that by the hours you are willing to wait, add the loss the pool is still running during those hours, and you get the heat-up load: 158,992 BTU per hour over 36 hours.
Halve the hours and you nearly double the required output. This is the entire argument for gas: a gas heater sized at 400,000 BTU per hour is not more efficient than a heat pump, it is just capable of dumping heat in fast enough to make a weekend possible. The gas page puts a dollar figure on that convenience.
Both sizing answers for your pool
The heat-up window is the one that changes the answer most. Try 12 hours against 72 and watch the required output move.
Maintain load 110,594 Btu/hworst hour in April
Heat-up load 158,992 Btu/hto lift 10.9 °F in 36 hours
Size the heater on 182,840 Btu/hthe larger of the two, plus a 15% margin
Water to warm 159,815 lb19,150 gallons
Energy in the heat-up alone 1.74 million Btuindependent of how fast you do it
Why the two answers diverge so far
The maintain load scales with surface area — it is a skin problem. The heat-up load scales with volume — it is a bulk problem. Make a pool deeper and the maintain load does not change at all while the heat-up load rises in direct proportion. Make it wider and both rise, but not at the same rate.
For the default pool, 512 square feet at 5 feet average depth, the heat-up load over 36 hours works out 1.4 times the maintain load. A deep diving pool at the same surface area moves that ratio further apart; a shallow play pool brings it closer together.
| Heat-up window | Required output | Compared to maintain |
|---|---|---|
| 12 hours | 255,787 Btu/h | 2.3× |
| 24 hours | 183,190 Btu/h | 1.7× |
| 36 hours | 158,992 Btu/h | 1.4× |
| 48 hours | 146,892 Btu/h | 1.3× |
| 72 hours | 134,793 Btu/h | 1.2× |
| 120 hours | 125,113 Btu/h | 1.1× |
The margin, and why it is a judgement
Everything above runs on monthly mean weather. Real weather has a worst week inside every month, and a monthly mean cannot tell you about it. Heat exchangers foul. Nobody has measured the wind in your particular yard. So the required output carries a margin over the computed peak.
15% is the default here, and it is a judgement, not a code requirement. There is no standard that specifies a pool-heater safety factor, and any page that tells you otherwise is inventing authority. Turn it to zero and the calculator will show you the raw computed peak.
Turning a size into a purchase
Gas heaters are sold by input rating, not output. A unit badged 400,000 BTU per hour delivers 328,000 BTU per hour of usable heat at the federal minimum thermal efficiency of 82%. Divide the required output above by the efficiency to get the input rating to shop for.
Heat pumps are sold by output at the AHRI rating condition, which is a warm summer afternoon. If your season includes cool months, the nameplate figure is not the capacity you get in those months. The heat-pump page derates it against air temperature so you can compare like with like.