What Size Pool Heater Do I Need? | Learn Everything About the Right Sizing for Optimal Heating

beautiful blue pool

“What size pool heater do I need?” may not sound like a necessary question to ask when purchasing the various essential pool equipment. But what’s worse than an ice cold pool? An ice cold pool from an ineffective heater.

There are many possible reasons why your pool heater is failing to adequately heat your pool. The main reason could be due to an incorrectly sized heater. To prevent finding yourself in cold water, you need to know what size pool heater will successfully heat your pool. You also need to learn how to heat an above ground pool.

Pool heater sizing is more than just the literal size of the heater. Instead, there are many factors that go into selecting the right size pool heater for your pool. A correctly-sized pool heater relates to its appropriate output of BTU to successfully raise the water temperature. Do you have an above ground pool? Is your pool in the sun or the shade? Do you live in a warm or cool climate? Do you have access to natural gas or propane? All of these factors need to be considered when selecting the right size pool heater for your pool.

What Size Pool Heater Do I Need?

First things first, pool heaters are measured using BTU, which stands for British Thermal Unit. A BTU is the amount of heat needed to raise water temperature by one degree Fahrenheit. But don’t let that scientific terminology scare you.This process doesn’t have to take you back to your high school algebra class, but you do need to do a little basic math in order to find the right fit. A little multiplication and you will find the exact BTU needed to heat your pool.

Additionally, there are other factors you can consider when choosing the right size pool heater. For example, how much sunlight exposure does your pool currently get? You can consider the amount of sunlight exposure the pool gets, your personal water temperature preference, and the climate in which you live. All these factors can attribute to selecting the right size pool heater. For example, a pool with excessive sunlight in a warm climate may not require the largest BTU heater.

A little preliminary information will help you select the right size heater for your pool. Make sure you know the size of your pool, the preference in which you swim, and your typical weather patterns. 

Pool Heater Sizing Considerations

To correctly size the heater of your pool, you will need to know the volume of water in your pool. Do you have a pool with all straight walls, or is your pool curved? Depending on your pool type determines the formula needed to calculate your total pool volume.

To find the volume of a straight-walled pool, use this formula (measured in feet):

  • Average length x average width x average depth x 7.5 = volume in gallons

Once you know how many gallons of water is within your pool, you will know the minimum required BTUs your heater will need to effectively heat your pool’s volume. But BTU amount isn’t the only factor to a successfully heated swimming pool.

Do you prefer a tepid swimming pool? Do you want your pool to feel like a bath? Water temperature preference is the second factor to finding the correct swimming pool heater for you. 

Last, what kind of climate will your swimming pool be in? Will the outside air temperature remain warm to help maintain the pool’s temperature, or will it get cool and reduce the temperature of the pool each day?

All of these factors need to be considered and known before selecting your swimming pool heater.

People also search for the best pool heater reviews when searching for the right heater size and type.

Pool Heating Calculations

As BTU is the method of heating water, what is the quantity of BTU required to heat a swimming pool with 20,000 gallons?

One BTU is responsible for heating one pound of water by one degree, so you have to know how many pounds of water are in 20,000 gallons. Then, it’s just simple math.

  • One gallon of water = 8.3 pounds

8.3 (pounds of water per gallon) x 20,000 (total number of gallons) = 166,000 total pounds of water to be heated

a thermometer floating  in pool water

Now, let’s talk about your preferred swimming temperature. A comfortable pool temperature is around 80 degrees, so we need to consider the dip in air temperature at night that will contribute to lowering the pool water temperature below the desired 80 degrees.

If the air temperature dips to 60 degrees, that is a 20 degree difference from the desired 80 degrees. (Remember that number.)

Next, you need to calculate how long it will take for you to reach the desired pool temperature of 80 degrees from 60 degrees within a 24-hour heating window.

For this calculation, you will use the following method, in order:

20,000 (total gallons) x 8.3 (pounds per gallon) x 20 (degree difference from 60 to 80) = 3,320,000 total BTU in 24 hours

3,320,000 ÷ 24 (hours in a day) = 138,333 BTU per hour to heat 20 degrees

This basic concept tells you the BTU needed to properly heat your pool to its desired temperature.

Variables to Consider When Sizing Your Pool Heater

Using the above example, we know that a 20,000 gallon pool needs a pool heater of at least 138,333 BTU per hour to heat 20 degrees over a 24-hour period time and maintain that temperature. But this is just to heat the pool one time, so what about losses?

A surface loss can cause your pool temperature to drop below the desired temperature. This causes your heater to continue to maintain the desirable 80 degrees it worked so hard to achieve in the past 24 hours.

In this case, while a 138,333 BTU heater will successfully reach 80 degrees, it is also important to keep in mind that other factors can change your pool’s temperature. Typically, you will want to add between 20 – 25% BTU to account for additional factors. Therefore, a 138,333 BTU heater increased by 25% means the optimal BTU heater for your pool is 172,916. 

Another way to preserve your pool’s temperature without constantly running your heater is to invest in a solar blanket or solar rings.

The surface water is the warmest temperature. It can also escape quickly without proper preservation. 

solar rings in the pool

The placement of solar blankets or rings while the pool is not in use will help maintain your pool’s temperature and reduce the amount the temperature is lowered. This means your pool’s heater will not have to run as long to achieve the optimal temperature.

How to Size Electric Heat Pumps

The size of your electric heat pump, as well as the climate in which you live, is crucial for properly heating your pool, whether above ground or in-ground.

Electric heat pump sizes range from 75,000 to 125,000 BTU.

If you are attempting to heat your pool using an electric heat pump only, you will need to know the surface area of your pool in order to determine the correct BTU needed for optimal heating.

In addition, an electric heat pump may be best paired with an additional heating element in order to adequately heat your pool. For example, an above ground pool with an electric heat pump paired with a solar blanket or solar rings for preserving surface temperature will help extend the temperature of your pool, as well as the longevity of your electric pool pump.

Sizing Your Pool Heater

Should you have an above ground pool, in-ground pool, or a pool and spa combination will determine the size of heater you will need.

For example, a pool with attached spa will require a higher BTU heater, as the heater will be heating both the pool and the spa.

The highest BTU for residential use is 400,000. This BTU amount should be limited to pool/spa combinations, as these combinations will need the highest heating output possible.

Do you have an above ground pool in a warm climate? Your best pool heater may be a 125,000 BTU electric heat pump. Pair this electric heat pump with solar power rings or a cover and you have an optimal heating setup for your pool.

Do you have a large, in-ground pool with attached spa? You will want to choose the highest BTU output of 400,000.

Does your above ground pool have access to natural gas or propane and sits in a warm climate? You may want to choose a gas heat pump for minimal use to keep cost down.

Pool Heat Pump Sizing

To calculate the appropriate size of a pool heat pump, we will use a calculation similar to the calculation used to find the required BTU for a 20,000 gallon pool. The only difference is that we are using the surface area instead of pounds of gallons.

Keeping it simple, we will use a 16 x 32 pool.

To find the surface area of the pool, we will multiply 16 x 32. This gives us 512 square feet of surface.

Using the same 20 degree difference from 60 degrees to the desired 80 degrees, we use the following formula:

  • 512 square feet x 20 x 12 = 122,880 BTU
A swimming pool pump  heater

Electric heat pumps are sized between 75,000 and 125,000 BTU, so an electric heat pump would work for this size pool.

With that in mind, if your pool is larger than 512 square feet of surface, or the temperature difference is any greater than approximately 20 degrees, an electric heat pump would not be the heater you would want to choose.

Solar Pool Heater Sizing

One of the most environmentally practical methods for heating your pool is through solar energy. Solar pool heating is when solar panels connect to your pool’s heater and absorb the sun’s energy to power the heating element.

You will need to make sure you have adequate sun exposure to the solar pool heater for optimal results. After all, a solar pool heater with no sun will be ineffective.

For best results of your solar pool heater, the solar panels need optimal sun exposure of at least 4 – 6 hours of constant sunlight.

Do you have an above ground pool? A solar pool heater would be a good choice due to the amount of depth that an above ground pool lacks. 

For best results of your solar pool heater, the solar panels need optimal sun exposure of at least 4 – 6 hours of constant sunlight. 

Do you have an above ground pool?

A solar pool heater would be a good choice due to the amount of depth that an above ground pool lacks. 

solar panels by the pool

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