Sizing Your Pool Heater

Written by Michael Dean

swimming pool heater next to a pool

There are a lot of decisions pool owners and pool builders make during the heater buying process. Buyers must know how big or how small their heater needs to be. From my experience, this is the area where most pool owners have tons of questions.

The ole “bigger is better” statement. It is true that larger heaters heat your pool faster than smaller ones. A 400k BTU heater will heat a pool twice as fast as a 200k BTU heater and burn gas twice as fast. Heating up a pool versus a pool/spa combination requires less BTU power. Usually, if you have a pool and spa combination, I recommend purchasing 30% higher BTU, for example going from a 300k to a 400k. If you want to perform the calculations for sizing a pool heater, you can also do that.

Step 1: Calculate your Surface Area (Pool Length x Pool Width)

When you are determining which size pool heater to buy, the first step is calculating your pool’s surface area. A lot of pool owners assume that you use your pool’s gallon size to determine your heater size. In fact, there are some websites that DO use the number of gallons. If you have a pool with a large surface area, the heat has more space to escape and, subsequently, more water to heat. As a result, the heater must be large enough to compensate for the surface area and pool size.

To calculate the surface area of your pool, multiply the length times the width.

For example: If your pool is 15 x 30, then your surface area is 450 square feet.

Related: Pool volume calculator

Step 2: Divide the Pool’s Surface Area by 3

The pool’s surface area divided by three gives you the minimal BTU size recommended for that particular surface area. Continuing the example above with the 15 x 30 pool, after dividing by 3, you get 150. Therefore, the minimum size heater recommended for a 15 x 30 pool is 150,000 BTUs.

This is simply the recommended minimum size. I always go higher. Especially when it comes to gas heaters, you always want to oversize the unit. The job of a pool heater is to replenish the heat loss at the surface of your pool. Most heat loss happens overnight. Without a solar cover, there is no way to compensate for the loss. That’s when having a higher BTU size comes in handy.

Step 3: Consider Your Variables

Before deciding, consider all the variables that may affect the efficiency of your heater, such as owning a solar cover, climate, and how often you use the pool.

For a 15 x 30 pool without a solar cover, I might recommend the 250 k or 300 k BTU model, depending on the customer’s preferences. That way, it accounts for heat loss, evaporation, and unexpected windy conditions. It also accounts for pool owners who prefer to swim at night.

By adding a solar cover, you’re changing the game. Using a solar blanket in conjunction with your heater is the best tag team to provide the best heat in the shortest amount of time. With a solar blanket, you might not need a 300 K BTU heater. You might settle on the 200K or 250k heater. Sometimes, it’s cheaper to simply invest in a solar blanket than paying for a larger heater.

Residential Gas Heater Sizing Calculator

Raypak has a gas heater calculator that configures your city and state with your desired water temperature, average air temperature, and desired temperature rise. Raypak’s online calculator allows pool owners to input specific information about their pool and location and configure what size heater is best, not only for their pool but taking your physical location into consideration as well.

View the calculator here.

The calculator also can calculate the cost of an electric heat pump versus direct fire propane or natural gas heaters. This tool allows customers to compare energy costs by showing their region’s average temperatures and the cost to heat the water to your desired temperature, broken down for each month of the year. It’s a really useful tool I use to explain to pool owners the costs of heating their water and a realistic plan for which months they might want to close the pool, if at all.

Once you review the different options, you will understand that heat pumps are more efficient in milder climates where there is less temperature increase required. I expand on this in my how to heat a pool and how pool heaters work research. And propane is more expensive than natural gas. I will review solar and geothermal options in another article.

If you prefer to look at averages vs. using this calculator, I wrote a guide on average pool heater costs by type that should help. And if you’re experiencing trouble with your current heater, read my pool heater troubleshooting guide. And lastly, if you have an above ground pool, read my guide on how to heat an above ground swimming pool.

Hopes this helps; reach out to me with any questions!

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