Pool swim season in Gaithersburg, MD
When can you swim in an outdoor pool in Gaithersburg, MD?
An unheated pool in Gaithersburg reaches 78°F in Jul to Aug, about 2 months a year. The air allows up to 5 swimmable months with a heater (May to Sep).
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What each heater buys you in Gaithersburg
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 2 | Jul to 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 | $170 | $720 |
| Gas heater + cover | 5 | May to Sep | $263 | $688 |
For a 450 sq ft pool held at 80°F, at MD prices of 21.3¢/kWh electricity and $16/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Gaithersburg
Solar panels + cover: 4 of 5 possible months for about $283 a year all-in, which works out near $142 for each month it adds.
Before you pay for any heater, buy a solar cover: on its own it adds 1 month 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: Rockville 1 NE, 4.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Gaithersburg 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 | 42° | 35° | 2.0 | 36° | 42° | – | – |
| Feb | 45° | 37° | 2.8 | 35° | 41° | – | – |
| Mar | 54° | 45° | 3.8 | 42° | 48° | – | – |
| Apr | 66° | 56° | 4.9 | 52° | 58° | – | – |
| May | 74° | 65° | 5.4 | 63° | 69° | $141 | $214 |
| Jun | 82° | 73° | 6.1 | 72° | 78° | $24 | $41 |
| Jul | 87° | 78° | 6.0 | 79° | 85° | – | – |
| Aug | 85° | 76° | 5.3 | 79° | 85° | – | – |
| Sep | 78° | 69° | 4.3 | 74° | 80° | $5 | $8 |
| Oct | 66° | 57° | 3.1 | 63° | 69° | – | – |
| Nov | 56° | 47° | 2.3 | 51° | 57° | – | – |
| Dec | 46° | 39° | 1.7 | 41° | 47° | – | – |
Heating costs most in May (about $141 with a heat pump, $214 with gas). Sunshine peaks in Jun at 6.1 kWh/m²/day (NASA satellite data, 47.1 km grid cell). Highlighted months are swimmable with no heater.
How Gaithersburg compares with nearby cities
Gaithersburg ranks 10 of 23 Maryland cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Rockville, MD | 5 mi | 2 | 5 | $170 |
| Takoma Park, MD | 16 mi | 2 | 5 | $115 |
| Herndon, VA | 16 mi | 1 | 5 | $145 |
| College Park, MD | 17 mi | 2 | 5 | $173 |
| Vienna, VA | 18 mi | 0 | 5 | $211 |
| Falls Church, VA | 18 mi | 2 | 5 | $126 |