How pool heater sizing works
Heater sizing comes down to one piece of physics: it takes 1 BTU to raise 1 pound of water 1 degree Fahrenheit. Water weighs 8.34 pounds per gallon. Multiply gallons by 8.34 by the temperature rise you want, and you have the total energy the heater must deliver. Divide that by the hours you are willing to wait, add 25% for the heat the pool loses to air and evaporation while the heater runs, and you have the BTU/hr rating to buy.
Gas vs propane vs heat pump
Gas and propane heaters burn fuel and deliver heat fast in any weather, which makes them right for a pool fired up for the occasional weekend. Heat pumps move heat from the air instead of generating it, so they run 3 to 5x cheaper, but they heat slowly and lose efficiency once ambient air drops below 50F. For a pool kept warm continuously in a mild climate, a heat pump wins on operating cost. For on-demand heating or a cold climate, gas wins.
The pool cover multiplier
The single biggest lever on heating cost is not the heater, it is the cover. Most of a pool's heat loss is evaporation off the surface. A solar blanket or liquid cover cuts evaporation dramatically and reduces heating bills 50 to 70%. For any customer who heats their pool, the cover conversation is the easiest money-saving recommendation you can make, and a real add-on sale.
Pool heater FAQ
How big a pool heater do I need?
The standard formula: pool gallons multiplied by 8.34 (water weight per gallon) multiplied by the temperature rise you want gives the total BTU of energy needed. Divide by your target heat-up hours and add 25% for surface heat loss to get the BTU/hr heater rating. A typical 20,000 gallon pool raising 15F in 24 hours needs roughly a 150,000 to 200,000 BTU heater.
Gas heater or heat pump?
Gas heats fast and works in any weather, but costs 3 to 5x more per heat-up to run. Heat pumps are far cheaper to operate (COP around 5.0) but heat slowly and lose efficiency below 50F ambient air. Gas for on-demand weekend heating, heat pump for a pool kept warm continuously in a mild climate.
How much does it cost to heat a pool?
A full heat-up of a 20,000 gallon pool by 15F costs roughly $25 to $45 on natural gas, $55 to $90 on propane, and $8 to $18 on a heat pump. Holding temperature day to day adds up; a pool cover is the single biggest cost saver, cutting heating bills 50 to 70%.
Does a pool cover really matter?
Yes, more than the heater choice. Most pool heat loss is evaporation off the surface. A solar cover or liquid cover cuts evaporation dramatically and reduces heating cost 50 to 70%. Recommend one to every customer who heats their pool.
Why oversize the heater by 25%?
While the heater runs, the pool is also losing heat to the air, wind, and evaporation. The 25% oversizing covers that ongoing loss so the pool actually reaches target temperature in the time you planned. Skip it and heat-up takes far longer than expected.
How long should heat-up take?
Most operators size for a 24 to 48 hour heat-up from cold. A bigger heater heats faster but costs more upfront. For a pool kept warm year round, heat-up time barely matters; for a pool fired up for occasional use, a faster (bigger) heater is worth it.
Track equipment in Pooly
Log heater make, model, and install date per pool. Pooly flags aging equipment so you can quote the replacement before it fails.