Swimming Pool Blog

Gas Heater Sizing Guide

concrete skimmer replacement
by Mark Garcia August 27, 2013

Pool Heater Sizing Guidelines

gas pool heater sizing guideWhen you install a gas heater, you get reliable, consistent heat, with the largest temperature rise and fastest heat gain of all pool heater types.

Gas heaters for inground pools come in 4 or 5 sizes, all of which are the same height and depth, becoming wider as the heater size increases.

Manufacturers size their heaters according to their BTU output, from 100,000 to 400,000 BTU's. Model numbers reflect the BTU output, for instance Jandy makes the Legacy 125, 175, 250, 325 and 400 models (sizes), which equal their BTU output x1000.


Here are the factors to consider when sizing gas pool heaters for your pool.

Surface Area

If you have a large amount of surface area (pool length x width) the heat has more square footage to escape. It's also more water to heat. Quite simply, the larger the pool, the larger the heater should be to compensate for pool size and surface area.

Wind Speed

Wind - the largest heat thief around, a stiff breeze rolling across your pool surface can quickly accelerate heat loss. Covers and wind blocks such as walls, hedges and fences can reduce the wind pilferage to low amounts.

Some sizing guides have us look at average wind speed for the swimming pool, as a way to determine pool heater size. I'll bet that no one knows their average wind speed - but you probably know if your pool area is windy or usually calm.

Desired Temperature Rise

How hot do you want the pool? Larger heaters not only heat faster, but they can deliver a larger heat gain. Most pool heater sizing charts or sizing calculators will use this factor in their calculations.

If you want to run the pool during the 'shoulder seasons', those cooler months of spring and fall, then you may need more than a 15 degree temperature rise, which all heater sizes can accomplish.

spa heat up times with a gas heaterIf you have an attached 'outdoor spa', and you want to heat it up to 104 degrees, any pool heater size could do it. The smaller heaters can take an hour to heat up the spa, the largest (400 btu) can do it in under 20 minutes.

The range of temperature rise or heat gain that a gas heater can deliver, is from 15 degrees Fahrenheit, up to 40 degrees Fahrenheit.

Covered or Uncovered?

Covering the pool, with an automatic cover, solar blanket or liquid solar blanket, will retain 70% or more of the heat that is put into the pool. Cover manufacturers like to say "you wouldn't heat your house without a roof! Don't heat your pool without a cover!" 

Yes, covers are great, but they have what I like to call - the 'hassle factor'. Putting the on, taking them off - ugh, who needs it? Liquid solar blankets are very convenient, just pour in the liquid that forms an invisible shield on the surface of the water. And then there's the cost. How much of your heating cost savings are being eaten up - 20%?

Covering the pool can affect heater sizing; a pool that is covered can use a smaller heater size.

Key Takeaways

What have we learned today? 

  1. Gas heater model numbers indicate their size.
  2. Larger pools may need larger heaters.
  3. Wind is an lowdown heat thief.
  4. Knowing how many degrees of heat you want to add is helpful.
  5. Covering the pool can cut your heating bill in half.


Thanks for Reading!

Mark Garcia