Pool swim season in Yelm, WA
Swim season in Yelm without a heater, and with one
An unheated pool in Yelm never reaches 78°F in a normal year. It peaks around 67°F in Aug. The air allows up to 4 swimmable months with a heater (Jun to Sep).
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What each heater buys you in Yelm
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 0 | no month | $0 | $50 |
| Solar panels + cover | 2 | Jul to Aug | $0 | $283 |
| Heat pump + cover | 4 | Jun to Sep | $371 | $921 |
| Gas heater + cover | 4 | Jun to Sep | $862 | $1,287 |
For a 450 sq ft pool held at 80°F, at WA prices of 14.4¢/kWh electricity and $18/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Yelm
Heat pump + cover: 4 of 4 possible months for about $921 a year all-in, which works out near $230 for each month it adds.
A solar cover alone adds no swim months here, so if you want a longer season you need a heater (why); still use one at night to hold the heat you pay for. If you get quotes, ask for a heat pump sized to your pool's surface area; here it beats gas all-in (heat pump or gas).
Weather station: Olympia Airport, 12.5 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Yelm 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 | 46° | 40° | 1.1 | 37° | 43° | – | – |
| Feb | 49° | 41° | 2.0 | 39° | 45° | – | – |
| Mar | 54° | 44° | 2.9 | 42° | 48° | – | – |
| Apr | 59° | 48° | 4.2 | 47° | 53° | – | – |
| May | 66° | 55° | 5.2 | 54° | 60° | – | – |
| Jun | 71° | 59° | 5.7 | 60° | 66° | $131 | $296 |
| Jul | 78° | 64° | 6.4 | 65° | 71° | $76 | $184 |
| Aug | 78° | 64° | 5.4 | 67° | 73° | $62 | $151 |
| Sep | 72° | 59° | 4.0 | 63° | 69° | $102 | $231 |
| Oct | 60° | 50° | 2.4 | 54° | 60° | – | – |
| Nov | 51° | 43° | 1.3 | 45° | 51° | – | – |
| Dec | 45° | 39° | 0.9 | 39° | 45° | – | – |
Heating costs most in Jun (about $131 with a heat pump, $296 with gas). Sunshine peaks in Jul at 6.4 kWh/m²/day (NASA satellite data, 50.4 km grid cell). Highlighted months are swimmable with no heater.
How Yelm compares with nearby cities
Yelm ranks 76 of 91 Washington cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Lacey, WA | 10 mi | 0 | 4 | $372 |
| Tumwater, WA | 13 mi | 0 | 4 | $371 |
| Olympia, WA | 14 mi | 0 | 4 | $372 |
| Lakewood, WA | 16 mi | 0 | 2 | $125 |
| University Place, WA | 19 mi | 0 | 2 | $125 |
| Centralia, WA | 22 mi | 0 | 4 | $326 |