Pool swim season in Johnston, IA
Johnston, IA: how long the pool season really is
An unheated pool in Johnston never reaches 78°F in a normal year. It peaks around 77°F in Jul. The air allows up to 5 swimmable months with a heater (May to Sep).
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Heater options for a Johnston pool, side by side
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 2 | Jul to Aug | $0 | $50 |
| Solar panels + cover | 4 | Jun to Sep | $0 | $283 |
| Heat pump + cover | 5 | May to Sep | $188 | $738 |
| Gas heater + cover | 5 | May to Sep | $272 | $697 |
For a 450 sq ft pool held at 80°F, at IA prices of 14.6¢/kWh electricity and $11/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Johnston
Solar panels + cover: 4 of 5 possible months for about $283 a year all-in, which works out near $71 for each month it adds.
Before you pay for any heater, buy a solar cover: on its own it adds 2 months here for about $50 a year (why a cover comes first). If you get quotes, ask for gas as well as a heat pump: once the purchase price is counted, gas comes out cheaper all-in here (heat pump or gas).
Weather station: Des Moines Wsfo-Johnston, 3.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Johnston month by month: air, sun and pool water
| Month | Air high | Air avg | Sun kWh/m²/day | Water, unheated | Water, covered | Heat pump $ | Gas $ |
|---|---|---|---|---|---|---|---|
| Jan | 30° | 22° | 2.0 | 23° | 29° | – | – |
| Feb | 35° | 26° | 3.0 | 24° | 30° | – | – |
| Mar | 49° | 39° | 3.8 | 33° | 39° | – | – |
| Apr | 62° | 51° | 4.8 | 47° | 53° | – | – |
| May | 72° | 62° | 5.6 | 59° | 65° | $132 | $185 |
| Jun | 81° | 72° | 6.3 | 70° | 76° | $31 | $50 |
| Jul | 85° | 75° | 6.4 | 77° | 83° | – | – |
| Aug | 83° | 73° | 5.5 | 77° | 83° | – | – |
| Sep | 76° | 66° | 4.6 | 71° | 77° | $25 | $37 |
| Oct | 63° | 53° | 3.1 | 59° | 65° | – | – |
| Nov | 48° | 39° | 2.2 | 45° | 51° | – | – |
| Dec | 35° | 27° | 1.7 | 32° | 38° | – | – |
Heating costs most in May (about $132 with a heat pump, $185 with gas). Sunshine peaks in Jul at 6.4 kWh/m²/day (NASA satellite data, 28.4 km grid cell). Highlighted months are swimmable with no heater.
How Johnston compares with nearby cities
Johnston ranks 15 of 41 Iowa cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Grimes, IA | 4 mi | 0 | 5 | $188 |
| Urbandale, IA | 5 mi | 0 | 5 | $188 |
| Clive, IA | 7 mi | 0 | 5 | $172 |
| Ankeny, IA | 7 mi | 0 | 5 | $207 |
| Des Moines, IA | 10 mi | 0 | 5 | $172 |
| West Des Moines, IA | 10 mi | 0 | 5 | $172 |