Gas Pool Heater Sizing: How to Size A Pool Heater (Correctly)

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So you’re tired of swimming in cold water and have decided you want a heater for your pool. But how do you know which is the right one?

Correct pool heater sizing isn’t an overly complicated process, and we’ll walk you through how to determine what size heater will work best for your pool.

Important: This article is about sizing gas pool heaters, not electric heat pumps. If you’re looking for the latter, please read our article on sizing a pool heat pump.

Why Is Pool Heater Sizing Important?

We’ve all heard the saying “bigger is better”. When it comes to gas pool heaters, this is mostly true. However, it really depends on your pool size, and budget.

Generally speaking, the amount of BTUs required to heat pool water will be the same, no matter the size of the heater. But a larger one will work quicker than a smaller heater, especially if you have a large body of water you’re warming.

Note: BTU (British Thermal Unit) is a unit of measurement that measures thermal (heat) energy. Gas pool heaters are rated for BTU input/output and range from 100,000 – 400,000 BTUs. 

Current pool heaters are 85% to 90% efficient. This means if you have a heater with an input of 400,000 BTUs, you’ll be getting 340,000 BTUs of output based on 85% efficiency, though this drops over time due to normal wear and tear. Understanding BTU’s helps you determine the necessary heater size for the volume of water in your pool.

A smaller heater means you’ll have to run it longer, which means you’ll be paying more in operational costs. But once it reaches the desired temperature, it can easily maintain it.

When it comes to minimum size, the general rule is 1 BTU will raise 1lb. of water by 1 degree Fahrenheit. You can then determine the minimum BTU size of the heater you require when heating your pool by “X” amount of degrees.

How To Size A Pool Heater (Step-by-Step)

Sizing the right heater for your pool comes down to numbers and calculations.

Step 1. Determine Desired Temperature

The first thing you need to do is determine the desired temperature you want your pool water to be at.

A sweet spot for most heaters is between 82°F and 86°F, so for simplicity’s sake we’ll use 85°F.

Step 2. Calculate Temperature Shift

Next, figure out what the average air temperature is where you live. For this example we’ll use 73°F. Subtract this temperature from your desired temperature of 85°F. 

85°F – 73°F = 12°F

Next, divide the 12°F by 8, which is the number of hours required to fully heat the pool. 

12°F ÷ 8 (hours) = 1.5° per hour 

So, the heater has to raise the water temperature 1.5° per hour in order to heat the pool in 8 hours.

Step 3. Measure Surface Area

Not all pools are the same shape or size, so you’ll have to measure the surface area of the pool. 

For this example we’ll use a rectangular pool of 40 ft (length) x 20 ft (width). This gives us 800 square feet.

To calculate your pool’s surface area, choose from these options:

  • Rectangular Pool: Length x Width
  • Rectangular Pool with Rounded Ends: Length x Width x 0.8
  • Oval Pool: 1/2 Length x 1/2 Width x 3.14
  • Kidney-Shaped Pool: Length x Width x 0.75

Step 4. Consult Chart

Manufacturers will provide a chart to help you determine the right size heater for your swimming pool.

This chart will show you the amount of BTUs in proportion to your pool’s surface area.

Using our example of 800 square feet, to raise pool water 1.5° per hour we would need a heater that has over 400,000 BTUs.

IMPORTANT: This example is based on an 8-hour duration. If you were to heat slowly over a 24-hour time period, that 1.5° figure changes to 0.5° per hour. In this case, you’d only need a heater that has 155,000 BTUs to achieve this.

Sizing Considerations

Keep in mind a few things when deciding on heater size:

  • The convenience factor. Natural gas is pumped in via a gas line for an endless supply, whereas propane needs to be housed in an oversized tank. Refilling this tank can be a logistical nightmare as it weighs hundreds of pounds when full.
  • How fast the water is heated. Do you want to be able to warm the pool in only a few hours? Are you someone who wants to swim regularly in the cooler months? If this is you, a large BTU gas heater will heat fast, and you won’t need to run it for long stretches of time. Propane has an advantage over natural gas here as it burns hotter and heats faster.
  • The size of your natural gas line. The heater size will also require you to have the correct gas line size, both in diameter and length. For example, if you previously had gas pumping through a 20 foot long, ¾” pipe to a 150,000 BTU heater, an upgraded 400,000 BTU heater would require a larger 1¼” pipe to run gas the same 20 feet.

Other Considerations

There are some additional variables you should account for as well.

  • The type of gas you’re using. Both natural gas and propane will do the job but from a cost perspective, natural gas is the less expensive of the two and in some areas can be up to 80% cheaper. However, propane is energy rich, so you’ll use less of it to produce the same amount of heat when compared to natural gas.
  • The wind. Wind also plays a large role in the cooling of pool water. If your backyard has a lot of wind, you’ll need to run the heater more frequently and/or at higher temperatures.
  • The type of pool you have. If your pool is a standard chlorine pool, you can use any gas heater you want. However, those with saltwater pools should use a gas heater with a cupro-nickel heat exchanger. Cupro-nickel is resistant to the corrosive damage that salt and a fluctuating pH level can incur on a gas heater.
  • Where you live. Flow rate of fuel is balanced by the amount of oxygen in the air, which means living in a region over 2000 ft above sea level requires a high altitude heater, as there’s less oxygen available. Some models are rated for up to 5,000 feet, but you should always double check as an improper altitude rating can result in inefficient heating.
  • Environmental impact. Propane is slightly more eco-friendly than natural gas. While it does produce CO2, it produces fewer greenhouse gases. In its unused state, natural gas is considered a greenhouse gas (propane is not), so if you spring a leak, propane will have less of an environmental impact than natural gas.
  • Legal requirements. Some cities may require you to use a low emissions/low nox pool heater. Eco-friendly options (like the ones mentioned above) are designed to minimize the amount of nitrogen-oxide emissions the heater releases.

Size Really Does Matter

Contrary to what women say, size really does matter – at least, as far as swimming pool heaters go.

Correct pool heater sizing can be slightly tricky, but it mainly comes down to this: large heaters will heat quickly, whereas smaller heaters take more time to distribute heat throughout the pool. The choice is yours.

Categories: Pool Care, Pool Equipment