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How Long Does It Take To Heat a Pool?

Categories: How To Guides, Q&A Topics, Heaters
One of the biggest challenges of owning an in-ground pool is keeping it warm enough for swimming. Even living in a hot and arid environment doesn't mean the water will be acceptable for a comfortable swim! That's why having the right pool supplies is so important, like a pool heater.
But if you're trying to warm your pool from scratch, you may be wondering how long does it take to heat a pool? The short answer? It depends. There are multiple variables to consider, like the weather and your geographical location, the size of your pool, the size and quality of your pool heating system, if you use a solar cover, and more.
Today, we’ll share the proper formula so you can determine how long to heat a pool to your desired temperature for a comfortable swim.
Calculating How Long To Heat a Pool
The formula for calculating how long to heat your pool works like this:
Gallons of Water (GW) x Weight/Gallon (8.33 lbs) x Degrees Desired (i.e., 20 degrees) divided by the BTU output of your pool heating system. So, let's say you have a 20,000-gallon pool, and you want to heat it by 20 degrees with a heater that generates 336,000 BTUs of heat. The formula would look like this:
  • 20,000 x 8.33 (166,600 pounds of water)
  • 166,600 x 20 (3,332,000 BTUs Needed)
  • 3,332,000 / 336,000 = 9.91 hours
So, if you want to heat your pool from 60 degrees to 80 degrees, you can expect it to take upwards of 10 hours to reach the desired temperature. However, there are other variables at play, such as the ambient temperature, the amount of direct sunlight the pool gets, and your climate. Wind speed can also keep your pool cold, forcing the heater to work harder to generate the same results.
Costs of Heating a Pool
Most pool heaters use natural gas, which is calculated by "therms." A single therm represents 100,000 BTUs of heat per hour. So, using our 336,000 BTU heater from our example above, you'll be charged 3.36 therms per hour for about 10 hours. Then, you just calculate the number of therms based on what your gas company charges, such as $1.20 per therm.
The results would be 1.20 x 3.36 x 9.91, or $39.95 to heat your pool initially. As you may imagine, it costs a lot less to keep a pool warm than it does to heat it from a colder temperature. The exact cost will again depend on environmental variables, how many gallons are your pool, and your desired temperature.
Propane vs. Natural Gas
If your pool heater uses propane, you'll need a lot more gas to generate the same amount of heat. Propane is measured in cubic feet instead of therms, and one cubic foot equals 92,000 BTUs. So, we would need to divide 336,000 by 92,000, giving us 3.65 gallons of propane per hour. Then, you just have to multiply the amount of propane you're using by the cost per gallon. So, if you're paying $3 per gallon for 9.91 hours, you would have to spend $108.51 to heat your pool (3.65 x 3 x 9.91).
Factors That Affect How Long It Takes To Heat a Pool
We've mentioned some of these factors already, but here's a rundown of everything that can affect how much energy you need to heat your pool to your desired temperature:
  • Wind Speed - Higher wind speeds sap heat from the water faster, meaning you need more heat to compensate. For example, 11 mph winds can increase your energy usage by 25 percent. 
  • Elevation - Water boils at lower temperatures when you're far above sea level, so if your pool is in the mountains, you may need to use less energy (depending on the weather). 
  • Pool Depth - Shallow water heats faster than deep water because of the volume of liquid involved. 
  • Climate - If you live in a warm area, sunlight will heat your pool naturally, allowing you to use less energy. 
  • Surface Area - Larger pools may be easier or harder to heat, depending on how much sunlight hits the water and the amount of wind. 
  • Heating Method - Generally, electric pool heaters take longer because they don't heat up as quickly as gas models. 

Overall, there are so many variables that can change quickly, so it's best to use your calculation as a baseline, not a hard and fast rule.
Using a Solar Cover To Heat a Pool
Solar covers can make it much easier to heat a pool because the cover traps heat inside the water without evaporating. If you use a solar cover whenever you're not using the pool, you can cut down your energy costs drastically. Here are a few other benefits of solar covers:
  • Environmentally-Friendly & Affordable Heating Option 
  • Keeps Bugs and Debris Out 
  • Minimal Upkeep and Maintenance Required 
When using a solar cover, you want to ensure the bubble side is face down. Since there's more surface area touching the water, it heats the pool faster. If the water is too warm, flip the cover and draw heat away from the surface.
On average, you can expect about a 10 to 15-degree increase after roughly six hours of using the cover on a sunny day. Again, environmental factors can affect those results dramatically.
To help maintain your preferred pool temperature, we recommend covering the pool when not in use— even if you go days between uses. The longer the cover can stay on the surface, the easier it is to maintain a warmer temperature. Even at night, the cover can help trap heat so it doesn't evaporate and force your electric heater to work harder in the morning.
Pro Tip: When it’s time to store your pool cover, dry it out completely before putting it away for the season. You should also use bungee cords to keep the cover tight so it doesn't unravel when moving to and from its storage location. Finally, preserve your pool cover’s health by storing it in a cool, dry area.
Dive Into Efficient Pool Heating Solutions at Pool Supply Unlimited
Heating your pool is essential to a comfortable, relaxing swim—but you must also ensure your pool parts function correctly and efficiently. At Pool Supply Unlimited, we have expert knowledge and a wide selection of today’s best pool supplies for all your pool heating needs. Whether you need expert advice or are in the market for the latest and most efficient pool heater, we’ve got you covered.
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Pool Supply Unlimited is an e-commerce company that loves water. We love to pump it, filter it, heat it, chlorinate it, light it up and stare at it while we're barbequing a fat, juicy steak. There are real pool professionals working here. Our guys have seen thousands of pools in their careers, no exaggeration. We are based in San Antonio, TX and can land most products to your door (within the 48 contiguous states) in less than a week.
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