How to Perfectly Sizing Your Pool Heater?

Pool heaters are the pool equipment you want to have if you plan to open your pool earlier in the spring or if you live in areas that experience cold summer. However, these units are large investments. Before you can run buying a pool heater, it’d be best to spend some time sizing the best one for your swimming pool.

Not sizing it well could cost you more in terms of purchase and bills, or you might spend more time waiting for the swimming pool to heat up. That’s why we prepared this pool heater calculator for you.

How To Calculate the Perfect Heater for your Swimming Pool

1) Determine Your Pool Size

The powerfulness of your pool heater is directly affected by the size of your swimming pool. The bigger the swimming pool, the more powerful the heater must be. That means it’d be best to determine the amount of water your swimming pool holds before moving forward.

Start by calculating the surface area of your pool and multiply it with the pool depth. If you’re not a math guy, then you can use an online pool calculator.

How to Find Your Pool’s Surface Area

Square or Rectangular Pool: Measure the length of your pool and its width in feet. Multiply the two to get the surface area.

L (length) x W (width) = Surface Area

Irregular Shaped Pool: divide the pool into regular shapes and find their lengths and widths. Multiply and add them to get the total surface area. For example, if you have an L-shaped pool, break it into two rectangles, multiply the length and width of each triangle, and then add the values.

Surface Area of Triangle A + Surface Area of Triangle B

Circular Shaped Pools: take the diameter (distance across the pool through the centre) and divide it by two to get the radius. Square it to get radius squared and multiply it by pi (3.14) to get the surface area.

Radius x Radius = r2
r2 x π = surface area

Oval shaped pool: multiply the length (longest diameter) by width (shortest diameter) to get the surface area.

Longest diameter x shortest diameter = surface area

Kidney shaped pool: for this type of swimming pool, you need to add the shortest and longest widths, then multiply your answer with the length.

(Width A + Width B) x Length = Surface Area

How to Find Your Pool’s Volume

If your swimming pool has a uniform depth, then your work becomes easier. All you need is to take your pool’s surface area and multiply it by the depth.

Surface Area x Depth = volume

If your swimming pool has shallow and deep sides, take the two depths, add them and divide by two to find the average. You can now multiply the average depth with the surface area.

(Depth A + Depth B)/2 = Average Depth
Surface Area x Average Depth = volume

Convert Pool Volume to Gallons

Take your pool volume in square feet and multiply it by 7.48 to get the volume in gallons.

Volume x 7.48 = Gallons

2) What Are Your Pool Heating Preferences

You can run a pool heater in two ways:

  • Heat your pool from cold, or
  • Maintain constant warm temperature

And as you well know, you still have to start from somewhere. The question is, how often will you be heating your swimming pool?

If you are using the pool heater on-demand to heat the water on those chilly days, you’ll need a more powerful unit that can warm the swimming pool on a need-to-heat basis.

But if you need a unit that keeps your swimming pool warm day in and day out, then a smaller pool heater would be excellent.

3) Determine the Required BTUs

Get ready to dig into the pool heater calculator math, starting with the heating power a pool heater output measured in BTU (British Thermal Units).

a) The BTUs You to Raise the Temperature

Suppose you need to raise one pound of pool water by one degree Fahrenheit; you need one BTU from the pool heater. And since 8.33 pounds of water are one gallon of water, it means you need:

8.33 BTU to heat one gallon of pool water by one degree

In an example: if your swimming pool is 15,000 gallons, you need a pool heater that can deliver:

15,000 gallons x 8.33 = 124,950 BTU

Divide the number by 24 hours to know the BTUs required to rise by one degree Fahrenheit in a day:

124,950 BTU ÷ 24 = 5,206.25 BTU

Assuming you want to raise your pool temperature by 90 degrees, subtract this from the current pool water temperature. Let’s say your current temperature is 60 degrees Fahrenheit;

90 – 60 = 30 degrees

The next step is to multiply the BTU/hour needed to raise the water temperature by a degree by the difference in the temperature you got above. From our example;

5,206.25 BTU/hour x 30 degrees = 156,187.5 BTU

If you need something to heat your pool faster, you need a unit to deliver more than the above BTUs. It’d also be best to BTU by 20% to account for the natural heat loss through evaporation and other environmental factors.

b) The BTU You Need For Maintaining Temperature

Before you can maintain the pool temperature, you must warm it in the first place. For that, you need 10 BTU/hour per square foot (your pool’s surface area) to raise a degree Fahrenheit.

It means, if your swimming pool has a 500-square-feet surface area and you want to raise the temperature from 60 degrees Fahrenheit to 80 degrees Fahrenheit and maintain it there, you would need:

500 x (80-60) x 10 = 100,000 BTUs

What’s the wind factor in the pool heater calculator?

If your swimming pool is in an exposed area, you will want to increase the heater BTU capacity by 1.25 for 5-mph wind velocity or two for 10-mph wind velocity.

In an example: suppose your swimming pool is 1500 sq. ft. in surface area, and your desired temperature is 80 degrees Fahrenheit, the lowest pool temperature is 50 degrees Fahrenheit, and the wind velocity is 5-mph, you’d need a pool heater capable of delivering:

1500 x (80-50) x 10 x 1.25 = 562,500 BTUs

If you would like to dig deeper into the engineering math on sizing a pool heater, read more here.

What If You Have A Solar Cover?

Installing a solar cover such as a solar blanket can help reduce the amount of energy and time your pool heater needs to warm your swimming pool. It cuts down the BTUs required to heat or maintain the pool water temperature by 50-80%.

Pool Heater Calculator Vs Pool Heater Types

The calculations above will help you size the perfect pool heater for your swimming pool, but they will input very little when it comes to the best pool heater type.

Gas or Propane pool Heater: With gas and propane pool heaters being the most powerful pool heating units you can use for residential use, they have BTU output ranging from 100,000 to 500,000. The pool heater calculator is more applicable in sizing a gas or propane pool heater. Getting the wrong size could cost you a fortune in purchase and fuel.

Electric Pool Heat Pump uses electricity, not for heating an element like a standard electric water heater, but for capturing the ambient air temperature and heating the pool water. It operates as a reverse air condition by transferring warmth from the sunlight and the ambient air temperature to the pool water. It’s not capable of heating your swimming pool like a gas heater can do, but it can effectively reduce pool heating costs if you live in the perfect environment. It’s something you will need for a small-size swimming pool.

Solar Pool Heater: a solar pool heater produces the lowest BTU output compared to the other two types. But that’s understandable considering it uses the sun’s warmth to warm the swimming pool, which can be beneficial and cost-saving.

Pool Heater Calculator – Perfect Way to Size a Pool Heater

Note that the above calculations are estimates to help you size your pool heater. It’d be best to have a 20% efficiency coefficient to add to the amount of BTUs you need from a pool heater. Also, calculating the heater output isn’t science- you might want to consider heat loss either naturally through evaporation or swimming activity.

About the author

Sharif Miah

Hi! I'm Sharif, the founder of Globo Pool® and I have been working in the pool & hot tub industry for the last few years. I love to share my experiences with people & hope you are enjoying my information and lessons!

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