11 Ways to Determine What Size Pool Heater You Need

As a pool supply company, we know a lot about pools. Because of our expansive knowledge of swimming pools, we get many questions about them, and the many accessories and other supplies pool owners require. There’s rarely a question we haven’t heard before, and there’s yet to be a pool-related question we can’t answer. But one of the most astute questions we get from new pool owners is this one: “What size pool heater do I need?”

Now, let’s take a deep dive into sizing a new swimming pool heater

What Size Pool Heater Do I Need?

Sizing and purchasing a pool heater will involve more than just measuring a pool’s dimensions. To make a well-informed decision on the size of the pool heater needed, it’s important to look at several factors.

Here are 11 ways to determine what size pool heater your pool needs: 

  1. 1. Pool Volume

This is the most accurate way to calculate the size of the pool heater needed, though other considerations may prove the best size may be smaller or bigger. Calculating average depth, length and width will give a pool owner a good idea of a swimming pool’s volume. This can then be translated into a British thermal unit (BTU), which we’ll go into more detail, along with a more in-depth calculation towards the end of the article.

2. Pool Area

Surface area is also important to determining pool heater size. While volume is important, shallower pools with larger areas will lose heat more quickly and more water due to greater evaporation. A rough calculation involves dividing the area of a pool by three to determine the minimum BTUs needed to heat a pool. This number is then multiplied by a thousand to establish the number of BTUs necessary for a pool heater. Thus, a swimming pool with an area of 450 square feet (about 41.8 square meters) should have a 150 thousand BTU heater at a minimum.  

3. Type of Pool Heater

How a pool heater heats also makes a difference when deciding on the size of the pool heater needed.

Pool owners should consider the following: 

  • Propane and natural gas pool heaters are the most powerful, offering the maximum BTU output and ranging from 100-500 thousand BTUs of output.
  • Electric pool heaters work like air conditioners, but in reverse, using sunlight and ambient air temperature to more cost-efficiently heat a pool.
  • Pool solar heaters produce the least BTUs but at the lowest cost per BTU; In contrast, heating water costs are drastically reduced; photovoltaic solar arrays require greater upfront costs and take longer to heat a swimming pool.

4. Adding a Cover

While not directly contributing to pool heating, adding a swimming pool cover that’s easy to close – particularly an automatic one – offers an added insulating layer when the pool’s not in use. This means a pool heater will have to do less work to heat the swimming pool, so should be considered when sizing any new heater.

5. Water Temperature

Establishing the preferred temperature most comfortable for swimmers is important in deciding pool heater size. Environmental aspects will also affect these calculations, but the ideal temperature should first be determined. Also, if a swimming pool has an attached hot tub, BTU requirements may be as high as 104?F (40?C).

  1. 6. Ambient Air Temperatures

Climate determines the ambient air temperature during swimming season, playing a major role in pool heater size. To calculate heater size, a pool owner needs to know the coolest ambient air temperature at which the swimming pool will be used. This will help determine how powerful a swimming pool heater needs to be.

Another consideration for swimming pool owners involves how long they’re willing to wait for their pools to attain an ideal temperature. Pools on their own don’t require speedy heating, but those with hot tubs will require more powerful heaters to bring the pool water to a comfortable temperature more quickly. Swimming pools with hot tubs attached will naturally want a more powerful pool heater, as it’s unlikely those wishing to use the hot tub will want to wait the extra time.

7. Temperature Differential

In addition to the ambient temperature, which is determined by climate, swimming pool owners should consider the maximum temperature differential it will likely be necessary to overcome during the swimming season. For example, the coolest nighttime temperatures would be 55?F (about 13?C). To raise the water temperature to an ideal of 80?F (about 27?C), these temperatures would have to be raised by 25 degrees Fahrenheit (about 14 degrees Celsius)

8. Time To Heat

Often pool heaters calculate raising temperatures over 24 hours. However, if a pool owner isn’t bothered by waiting a bit longer to conserve energy, it’s possible to use a smaller swimming pool heater. It will take longer for the water in a pool to reach its ideal temperature.

9. Wind & Evaporation

The water surface of a pool loses heat quickly due to evaporation from the wind. On hot, windy days, pool water will lose heat especially quickly. A more powerful pool heater may be necessary for pool owners who live in windy areas. Wind is also a reason many swimming pool owners consider a solar cover.

10. Adding Solar

Investing in a solar cover is sometimes cheaper than purchasing a larger heater. Solar covers combined with swimming pool heaters will significantly lessen the size heater you need and shorten the time it takes to heat a pool. For example, instead of a 300 thousand BTU heater, a 250 thousand, or even 200 thousand, BTU pool heater may suffice when used with a solar cover. Using a solar cover on swimming pools overnight will also keep the water from cooling, potentially reducing heat loss by up to 90 percent.

11. Choosing Between Efficiency & Power

Swimming pool owners must choose between a more powerful pool heater and greater efficiency. While natural gas and propane pool heaters are more efficient these days, other types of heaters offer better efficiency. Electric pool heaters provide better efficiency, whereas solar pool heaters are the most environmentally-friendly alternative, though they’re also the least powerful. Choosing the most efficient option may lessen the money spent on energy expenses, but it will also likely shorten the swimming season by a few weeks.

Doing the Calculations: BTU & Swimming Pool Volume

While it’s certainly possible for people to guess what size pool is needed, it’s usually a good idea to work out more exact figures. Before doing these calculations, however, it may be advantageous for some pool owners to brush up on their high school geometry and mathematics. To simplify, however, we’ll use some more basic measurements.

An analysis has been done on pool size in the United States, revealing that they’re on average 5.6 feet (about 1.71 meters) deep, 15.7 feet wide (about 4.79 meters), and 30.1 feet long (about 9.17 meters). As these are rather difficult numbers with which to work, however, let’s just say we’re looking at a swimming pool that’s 30 feet long (about 9.14meters), 15 feet (about 4.57 meters) wide, and an average of 5 feet deep (about 1.52 meters).

This translates to 16,875 gallons in volume (about 63,.879 liters) and an area of 450 square feet (about 41.8 square meters). A British thermal unit (BTU) is used to measure the amount of heat necessary to heat a pound (.45 kg) of water by one degree Fahrenheit (.56 degree Celsius) from its greatest density. BTUs can thus be used to determine the energy needed to heat a body of water, so it’s particularly useful for determining the size of the pool heater you need.  

Water weighs about 8.3 lbs. (about 3.76 kg) per gallon (about 3.79 liters). Therefore, 16,875 gallons (about 63,.879 liters) times 8.3 lbs. (about 3.76 kg) per gallon = 140,062.5 lbs. This means that a pool owner would need just over 140 thousand BTUs to raise the pool temperature by a single degree Fahrenheit (.56 degree Celsius) and 10 degrees Fahrenheit (5.6 degrees Celsius) would require just over 1.4 million BTUs.

Now, let’s apply this to a real-world situation. Say that a swimming pool owner wants a pool heated to 80?F (about 27?C) from 60?F (about 15.6?C). To determine the time it would take to raise the temperature over a 24-hour period by 20 degrees Fahrenheit (11.2 degrees Celsius), that number should then be divided by 24. Thus, 140 thousand BTUs divided by 24 would take just over 5833 BTUs to raise the temperature one degree Fahrenheit (.56 degree Celsius) over 24 hours.

This would require over 116,660 BTUs to raise the temperature of this pool by 20 degrees Fahrenheit (5.6 degrees Celsius) over a day. However, adding about 20 percent to this figure is usually recommended to cope with evaporation and heat loss, which makes it 139,992 BTUs. So, in this case, the pool owner needs a pool heater that generates about 140 thousand BTUs of heat energy.

Halogen Supply has been a trusted distributor of pool and spa supplies for decades. If you’re asking, “What size pool heater do I need?” or have other swimming pool questions, contact the experts at Halogen Supply today.