Pool swim season in Herndon, VA
When can you swim in an outdoor pool in Herndon, VA?
An unheated pool in Herndon reaches 78°F in Aug, about 1 month a year. The air allows up to 5 swimmable months with a heater (May to Sep).
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Heater options for a Herndon pool, side by side
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 1 | Aug | $0 | $0 |
| Solar cover only | 3 | Jul to Sep | $0 | $50 |
| Solar panels + cover | 4 | Jun to Sep | $0 | $283 |
| Heat pump + cover | 5 | May to Sep | $145 | $695 |
| Gas heater + cover | 5 | May to Sep | $291 | $716 |
For a 450 sq ft pool held at 80°F, at VA prices of 16.9¢/kWh electricity and $17/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Herndon
Solar panels + cover: 4 of 5 possible months for about $283 a year all-in, which works out near $94 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 a heat pump sized to your pool's surface area; here it beats gas all-in (heat pump or gas).
Weather station: Washington Dulles International Airport, 4.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Herndon 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 | 43° | 34° | 2.2 | 35° | 41° | – | – |
| Feb | 46° | 36° | 3.0 | 35° | 41° | – | – |
| Mar | 55° | 44° | 4.0 | 41° | 47° | – | – |
| Apr | 67° | 55° | 5.0 | 52° | 58° | – | – |
| May | 75° | 64° | 5.6 | 62° | 68° | $118 | $232 |
| Jun | 83° | 73° | 6.2 | 72° | 78° | $23 | $50 |
| Jul | 88° | 77° | 6.0 | 78° | 84° | – | – |
| Aug | 86° | 76° | 5.4 | 79° | 85° | – | – |
| Sep | 79° | 69° | 4.4 | 74° | 80° | $4 | $9 |
| Oct | 68° | 57° | 3.3 | 63° | 69° | – | – |
| Nov | 57° | 46° | 2.5 | 51° | 57° | – | – |
| Dec | 47° | 38° | 1.9 | 41° | 47° | – | – |
Heating costs most in May (about $118 with a heat pump, $232 with gas). Sunshine peaks in Jun at 6.2 kWh/m²/day (NASA satellite data, 53.2 km grid cell). Highlighted months are swimmable with no heater.
How Herndon compares with nearby cities
Herndon ranks 23 of 39 Virginia cities for unheated swim months; Falls Church, Manassas Park, Rockville nearby get a longer season without a heater.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Vienna, VA | 9 mi | 0 | 5 | $211 |
| Fairfax, VA | 9 mi | 0 | 5 | $211 |
| Falls Church, VA | 13 mi | 2 | 5 | $126 |
| Leesburg, VA | 13 mi | 0 | 5 | $198 |
| Manassas Park, VA | 14 mi | 2 | 5 | $144 |
| Rockville, MD | 15 mi | 2 | 5 | $170 |