How Long Does It Take To Heat A Pool?

small backyard cocktail pool with tropical green plants around it and a brown stained fence behind it

There are many times I’m sure you’ve wished you had a heater.  Sometimes the spring air heats up to the warmth of summer, but your pool water just isn’t quite warm enough.  Or maybe you’re enjoying a warm fall day, but the pool is already cold.  Whatever the case, maybe you’ve decided to buy a heater.  However, you’re probably wondering, how long does it take to heat a pool anyway?  Well, in this article we’ll go over some generalizations and tips for what you can expect when heating your pool.

What Are The Factors For How Long It Takes To Heat A Pool?

There are some variables to consider when determining how long does it take to heat a pool. The first is the size of your pool.  The gallons of the pool is important. Although, the most important thing to know is the square footage of the pool itself.  The majority of the heat loss in any pool is caused by evaporation through the surface of the pool.  For this reason, a heater will warm up a shallower pool slower than a deeper pool, assuming the volume is the same.

Additionally, the type and size of heater makes a huge difference when answering the question how long does it take to heat a pool. If you have a natural gas or propane heater, your pool will heat the fastest. When you are relying on an electric air source heat pump it’ll take longer, and have more variables to consider.  If you rely on a solar array, you’ll have even more time to wait.  

How Long Does It Take To Heat A Pool With Gas/Propane Heaters?

If you’ve shopped for natural gas or propane pool heaters, you know they are sold by the BTU, or British Thermal Unit.  This is a measure for how much heat energy can be produced by the heater. This is commonly and mistakenly confused with how much is transferred to the water.  For example, if you have a 400,000 BTU heater, the heater is capable of creating 400,000 BTU. However, you must also consider the energy efficiency rating to learn how much of that heat is actually transferring into the water.  

Commonly propane and natural gas heaters are rated around 86% efficiency.  This means a 400,000 BTU heater is actually 344,000 BTU of heat transfer to the water. This may sound like you’re losing a lot of heat. In reality it’s one of the most efficient ways to heat your pool.  There are higher efficiency heaters like the Jandy HI-E2 or the Pentair ETi-400. These are 95% and 96% efficient respectively.  Although, these heaters also have additional maintenance requirements. Therefore, they can cost more than three times as much.

How Long Does It Take To Heat A Pool With An Electric Heat Pump?

If you are using an electric heat pump, the calculations become extremely difficult. You’ll need to take into consideration the air temperature, humidity, existing water temperature, and so on.  In general, if you live in the northern half of the United States, an electric air source heat pump will be more of a frustration than a help. This is especially true when it comes to heating your pool if you are worried about time. Whenever you use an electric air source heat pump to heat the pool, you should never shut it off the entire swim season.  Set the heat pump to the desired pool water temperature, and let it do its job.  

If you try and use an electric air source heat pump as an ‘on-demand’ heater you will be sorely disappointed.  Expect your air source heat pump to take at least a few days. In fact, it may take over a week to get your pool up to temperature depending on the time of year.  There is however a reason you don’t want to turn them off. They can actually be used to maintain temperature quite well. However, heating the water up in the first place is not the heat pumps strong suit.  

How Long Does It Take To Heat A Pool With Solar Energy?

With solar, you will have to experiment.  This is due to the huge variation in geographic region, angle to the sun, time of year, and so on.  The variables with solar are additionally complex when you realize the sun isn’t consistent. It’s not always shining at the same intensity due to atmospheric conditions and cloud cover.  I’ve designed hundreds of pools for homeowners and many used solar energy.

Mostly, when my clients have solar heaters, they also have an additional heat source like natural gas to provide them with a guaranteed heat source.  By plumbing the solar into the system before the natural gas heater, you can reduce the amount of heating required, and reduce the amount of natural gas burned.

small pool showing how long does it take to heat a pool as being an hour or so with a gas heater

Pool Expert’s Advice For Heating Your Pool

When calculating how long does it take to heat a pool, you now can see there are quite a few variables to consider.  If you are using an electric air source heat pump, natural gas heater, or propane heater, it is best to buy the largest heater available. There is no energy efficiency benefit to buying a smaller heater.  A smaller heater actually can cause you to use more gas or electricity to provide the heat necessary.  When a heater has to run longer to transfer the same amount of BTU’s, the loss through evaporation and conduction through the pool walls will cause the heater to need to transfer more total BTU’s to achieve the goal temperature.  

How Long Does It Take To Heat A Pool Summary

There are several pool heat calculators you can use online. They will show you a ‘heat up time’ but these ignore the multiple variables beyond the heater BTU rating and the size of your pool.  If you ignore heater efficiency rating, air temperature, whether or not the pool has a cover on it, and so on, you’ll only get an idea of how long it’ll take in a virtual calculation perfect world. 

The best way to determine how long it’ll take is to plan a few days ahead if you have a gas heater and over a week ahead of time if you have an electric air source heat pump.  Get the pool to temperature and start tracking your own data for your specific pool.  If you are in the market for a heater for your above ground or in ground pool, consider buying the largest size you can afford.