How to Properly Size a Replacement Pool or Spa Heater

When replacing a pool heater, is bigger better?

If you own a gas pool heater, chances are that you've either replaced it or will need to replace the heater at some point in the future. For most, a heater of the same size will be sufficient, so the search for the best size replacement pool heater will be simple. But, maybe your heater didn't heat up the pool water as fast as you'd like? For warm-water lovers, a good question to ask yourself may be: Could I and should I buy a larger pool heater?

Buy a larger replacement heater if:

  • You have an attached spa that you want to heat faster.
  • You want to have a greater temperature rise, for longer pool seasons.
  • You want to run the heater less, due to more rapid heat up.

How to match a pool heater to your pool size:

Multiply your length by the width to determine pool surface area, or for a round pool, the formula for surface area is Pi xR2 (where Pi is 3.14 and R2 is radius squared).

Other factors come into play that could affect the size of pool heater that would be best for your swimming pool.

  • Will you be covering the pool to prevent heat loss?
  • Do you have high winds in the area, without wind blocks?
  • Would you like to extend the pool season when the weather turns cool?
  • How many other competing appliances use the gas supply?

How to properly size a gas heater for a hot tub:

Multiply length x width x average depth x 7.5 to determine spa gallonage.

Besides the amount of time you can wait for your spa to heat up, other factors come into play that could affect the size of spa heater.

  • Will the spa be covered to prevent heat loss?
  • Is the spa located indoors or outside?
  • If outdoors, will it be operating year round?
  • Are there other competing appliances using the gas supply?

Do I need to increase the gas pressure or the gas pipeline size?

For natural gas heaters, if your gas pipe size is 1-1/4-inch in diameter, and the run is less than 100 feet from the meter to the heater, you'll probably be OK. Or, if you have a high pressure, 2-pound system, you probably will not need to increase gas pipe size or gas meter size. For propane gas pool heaters, you need to know the distance from the tank regulator to the gas valve on the heater. Pipe sizing for propane is a little more versatile since it can tolerate smaller pipes at greater distances, and usually (but not always) the propane tank is located near the pool heater.

The best thing to do if you are unsure is to call your gas supplier. They have records of your gas usage and may also know your heater size. The question to them is "Will my meter (or regulator for LP gas) be sufficient for a xxx BTU size pool heater? They can usually provide a quick answer over the phone for you. If a meter or regulator size needs to be increased, in my experience the homeowner is not charged for this, as the utility is happy to (presumably) sell more gas to its customers.

If you are still unsure about whether you could or should increase your pool or spa heater size, give our pool heater installation experts here a call. We've answered this question for hundreds of warm-water lovers and would love to speak to you about your gas pool heater size. Call us at 1-800-288-7946.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.