Last Updated: October 16, 2022

Sizing Your Pool Heater - What Size Do You Need?

Written By: Pool Care Guide

Sizing your pool heater can seem scary if you're unsure where to start, especially with the equipment essential to get the most out of your pool. 

Thankfully, it can be foolproof and stress-free, yet adequate to size your pool heater with a simple calculation and maybe a little help from an intuitive online tool. 

Step 1: Determine Your Pool's Surface Area

Why calculate the surface area instead of the total volume? The goal is to get a heater to keep all the water in your pool warm, right? 

While the total volume of the water is essential, pool sizes and shapes vary. The heat escapes through the pool's surface, so by calculating the surface area, you're covering both the volume and the surface, which is vital for retaining heat. 

This is a straightforward equation: length x width = total square feet (surface area). So, for a pool that's 45 feet long and 20 feet wide, it would look like this: 45 x 20 = 900 square feet. 

Step 2: Figure Out the BTU Size

BTUs, or British thermal units, measure the amount of heat from various energy sources, such as pool heaters. It's easy to compare energy output from different fuels such as electricity, natural gas, propane, etc. 

For pool heaters, BTUs measure the amount of heat your heater needs to increase the water temperature by one degree (Fahrenheit) per pound of water. 

Pool heaters vary by BTU size. To figure out the minimum size you need, you'll take your pool's surface area and divide it by three. This number (in hundred thousand) is the smallest size pool heater you'll want to consider. 

Extending the example equation above for a 45 x 20 pool (surface area 900 square feet), it would look like 900 / 3 = 300,000 BTUs. 

Consider What Type of Pool Heater You'll Use

Different types of pool heaters heat the water differently. So you'll want to consider your options and choose the one that works best for your lifestyle. 

Your best option is based on what factors are most important to you. For example, is being environmentally friendly vital to you? Or is it more crucial for the water to heat quickly?

Here are the types of pool heaters available: 

  • Gas. This is the most robust type available for residential purchase. They use propane to produce heat efficiently; therefore, they offer the largest sizes of pool heaters available as well. Their BTU output is higher than other types of heaters. 
  • Electric. These heaters use a combination of sunlight and ambient air to harness electricity and create heat. They're not as efficient as gas, although they cost less to run.
  • Solar. These heaters use heat from the sun to heat the pool water. They produce the lowest BTU output compared to other heaters but are also the most cost-effective. 

Your budget is something to consider when assessing different types of pool heaters as well. If budget is essential, looking at the cost savings from a gas heater to a solar heater is worth a bit of thought. 

Step 3: Adjust for Variables

It's not just a matter of surface area, though, if you want to be more precise. Some variables affect the heater size you'll want to consider.

Here are some additional factors you'll want to think about: 

  • Whether or not you'll be using a solar cover (especially at night)
  • Your ideal water temperature
  • The climate you live in
  • How often you use the pool
  • The coldest air temperature the heater will experience while maintaining the water temperature
  • Preferred time to heat

These factors give you a complete picture of what size heater you should purchase for your pool. Not all of them are included in the official equation, but they're essential considerations to keep in mind. 

This especially comes into play if your basic calculation from above yielded a number between two heater sizes. If you're unsure whether to size down to decrease the cost or size up to ensure you have enough heating power, consider some of these factors. 

For example, if you plan to use a solar cover, you might get away with the smaller BTU heater because the covers prevent much heat from escaping at night. 

Alternatively, if you spend time swimming in colder ambient air temperatures, your heater will need more power to keep the water heated, so you might be better off choosing the larger size. 

Assess Your Budget

When factoring in your budget, you'll want to consider the cost of installing the heater and its continual cost. 

This is important because some heaters cost more to install, but their running cost is much lower. Some are the opposite: they're cheaper to install but more costly down the road. 

There is a wide range of costs, so looking at your options ahead of time is a good idea. HomeAdvisor estimates the typical cost range to be between $1,767 - $4,023. 

Here are some costs to consider at a glance: 

  • Gas heater: $2,000 - $4,000 to install, and $200 - $850 per month to run
  • Electric heater: $2,000 - $5,000 to install, and $175 - $600 per month to run
  • Solar heater: $3,000 - $4,000 to install, and $0 - $10 per month to run

As you can see, there is quite an extensive range of costs here. You might want to consider a solar heater if you have a hefty upfront budget but need to be more frugal about continuing costs. 

On the other hand, if cost isn't as much of a factor moving forward, and you want your pool to heat more quickly, you'll likely benefit from choosing a gas heater. 

An electrical heater will run somewhere in the middle, which is also worth consideration. 

Get Sizing Help

Don't want to take a chance of choosing the wrong heater? Some helpful sizing tools are available to help run the numbers and make the best decision for your pool and budget, including this handy residential gas heater sizing tool from Raypak. 

This will take some guesswork out of the equation while adding crucial factors, such as your location. Your pool installer can likely help with the decision as well. 

It can help you figure out which size heater to purchase and provide a visual expectation of how much you can expect to pay to heat your pool.

Remember, sizing your pool heater doesn't have to cause undue stress. You can make an informed decision by using the simple equation above, using the heater sizing tool to take some additional factors into account, and possibly conferring with the installer for advice. 

Take a breath, grab a measuring tape, and punch a few numbers. It can be that simple.

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