The sizing math
The rough formula: pool surface area (length × width in feet) × desired temperature rise (°F) × 12 = BTUs needed to raise the temperature in about 24 hours. A 16×32 pool that needs a 20°F boost works out to 16 × 32 × 20 × 12 ≈ 123,000 BTU per hour of continuous output — and because heaters do not run at perfect efficiency and Illinois nights pull heat back out, real-world sizing goes well above the formula minimum.
- Up to 15×30 pool: 250K-300K BTU minimum, 400K if you want fast weekend heat-ups
- 16×32 to 18×36 pool: 400K BTU is the standard recommendation
- 20×40 and larger, or pools with attached spas: 400K BTU, sometimes dual units
Why Illinois rewards oversizing
In a warm climate an undersized heater eventually gets there. In northern Illinois, May and September nights drop into the 40s and 50s, pulling several degrees out of an uncovered pool overnight. A larger heater recovers that loss in hours instead of days, which is the difference between a pool you actually use in the shoulder season and one you wait on.
Larger heaters also run fewer total hours for the same heat, so the operating-cost penalty of "too big" is far smaller than the usability penalty of too small. We install a lot of 400K BTU Pentair MasterTemp units for exactly this reason.
Gas heater vs heat pump in our climate
Heat pumps extract heat from the air, which makes them efficient in Florida and slow in Illinois shoulder seasons — their output drops sharply once air temperature falls below about 50°F, exactly when Chicagoland pools need heat most. Gas heaters deliver full output regardless of air temperature.
- Gas: fast heating, works in any weather, higher per-hour fuel cost — the default choice here
- Heat pump: cheap steady-state heat in July and August, weak in May and late September
- Hybrid strategy: gas heater plus a solar cover — the cover alone can cut heating costs 50-70%
Always pair the heater with a cover
Evaporation is the largest source of pool heat loss. A simple solar cover used overnight keeps most of the heat you paid for, shortens heater runtime, and often lets a smaller pool hold temperature through a cool week. If you heat without a cover in Illinois, you are heating the neighborhood.