how long does it take to heat a pool?

How Long Does It Take to Heat a Pool?

No one likes swimming in a cold pool, especially when pool heaters exist. And if you own a heater, I’m sure you especially don’t like playing the waiting game. Once thought as a luxury item, swimming pool heaters are becoming a necessity for pool owners everywhere. Pool heaters add comfort to your swimming experience and can extend your swimming season for weeks.

One of the most frequently asked questions we hear from pool owners is, “How long does it take to heat a pool?” This is an important question because it helps pool owners determine what size pool heater they need to purchase for their pool and how long in advance they need to start their heater so that it is warm enough to swim. 

Overall, buying a heater that is too small for your pool is counterproductive.  Even if only for monetary purposes, it’s a good idea to know how long it will take to heat your pool.

click here to find your replacement pool heater parts

Are You Purchasing a Heater Or a Heat Pump?

Determining how long it takes to heat your pool depends on a couple of different things. Have you determined if you are purchasing a heater or a heat pump? Sure they both heat your pool, however, they do so in very different ways. Let’s dig a little deeper.

Heat pumps are unique pieces of equipment in the sense that they don’t create their own heat. Instead, it pulls heat from the ambient air and uses it to heat your pool water. If you live in a colder environment, you can see how this may present a problem. Heat pumps operate best when temperatures are above 50 degrees Fahrenheit. In temperatures below that, heat pumps cannot efficiently perform. As a result, pool owners need to run their heat pumps longer in order to heat their pool. That is why it is crucial to consider the average temperature in your area, especially during the cooler months, before purchasing your heat pump.

An undersized heat pump must run longer in order to heat the swimming pool. Longer running times mean a decrease in efficiency and higher operating costs. In short, it’s an all-around bad situation and purchases a gas heater instead.

Calculating Heat Run Time

Heat pumps and heaters, alike, are sized according to the British Thermal Units (BTU) per hour. One BTU raises one pound of water by 1 degree F. One gallon of water weighs 8.34 pounds. So, 8.34 BTUs raise one gallon of water by 1 degree F. 

FORMULA:  Determine the time required to increase water temperature by 1 degree F

# of gallons in pool  X  8.34 = ___________ lbs. of water

______ pounds of water  /  BTU Size = The hours it takes to raise the water temperature by 1 degree F


You have a 20,000-gallon pool and use a 125,000 BTU heater. Your water is currently 70 degrees F but you would like it to be a minimum of 80 degrees F. How long will it take before the pool water reaches 80 degrees F?

20,000 x 8.34= 166,800 lbs

166,800/125,000= 1.33 hrs to increase 1 degree

80-70= 10 degree difference

10 x 1.33= 13.34 hours of heating before the pool reaches 80 degrees F.

In most cases, calculations for a heat pump and heater are optimistic. For heat pumps, the efficiency varies depending on the air temperature and therefore the calculations cannot account for cooler temperatures. The listed BTU size is when the air is quite warm.

Gas heaters are more consistent, however, they are also less efficient. They are only about 80% efficient, which needs to be factored in. Also, you can always expect to lose heat overnight, increasing your overall heating time. After using this formula to calculate how long it takes to heat up your pool, it’s always a good idea to round up to account for heat loss and efficiency. The formula gives pool owners rough estimates, not exact times.


Pentair’s Heat Pump Calculator

Typically, I’m not a huge fan of online calculators because they’re usually not very user-friendly or accurate. However, the heat pump calculator on the Pentair website is user-friendly and provides everything you need when purchasing a heat pump. The Pentair calculator tool captures information about a pool’s location, size, and configuration to provide the right heat pump solution to fit a pool owner’s specific needs.

Here is a screenshot of a pool owner in Florida. The chart shows the average air temperature for each month, the running time, and the overall costs of consumption. Access Pentair’s Heat Pump Calculator here.


Other Resources

The best thing about heaters and heat pumps is that it allows pool owners to take full advantage of their swimming pool, even when the weather is not ideal. What isn’t ideal is having your party guests arrive at an ice-cold swimming pool. We always recommend planning ahead. It’s imperative to remember that even after you have completed your calculations and have a good idea about how long it takes to warm your pool, you are still unable to account for other variables such as the outside temperature, efficiency, and heat loss through evaporation. It’s always a good idea to round up when determining how long you should run your heater or heat pump. This gives you a little cushion room for unpredictable variables.

Heaters and heat pumps are big expenses. As a result, pool owners are very careful and strategic about the use and operation of their heat pumps or heaters. However, it’s not uncommon for pool owners to run into issues concerning their heaters. To better assist pool owners with their heaters, here is a list of other informative blogs related to heaters.

Inground Pool Equipment Selection Part 3: Heaters

Sizing Your Pool Heater

Pool Heater Answer Guide

How Much Does It Cost to Run My Pool Heater?

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24 thoughts on “How Long Does It Take to Heat a Pool?

  1. Reading a lot of these articles, I can’t figure out why my pool installer wants to use a natural gas heater? To me, that makes no sense in Las Vegas. Any time I would be swimming, it’s going to be over 75 degrees, so a heat pump seems to make the most sense to me and would cost the least money long-term, especially because we’ll have a robust solar array so I think I can power the entire house and the pool from solar power because it’s a large rooftop. I’m just concerned that he’s building the system for “normal person” use, and I’m not a normal person. I work from home, I intend to swim daily from mid-March through October, so it doesn’t do me any good to say, “Ah well, leave the pool unheated all week, then I’ll crank it up for the weekend!” No, I’m just as likely to use it on a random Tuesday afternoon than I am on Saturday or Sunday, maybe more likely during the week even because I wouldn’t be going anywhere. At the same time, I don’t want a system that costs me $20/day to heat, that’s just ludicrous. I also suspect some people like much colder pools than I do. If it’s 105 or 110 outside, sure, I’m fine with 82 degrees, but normally speaking I want the pool 85 degrees if I’m going to enjoy being in it. I don’t swim laps, just float around, so if it’s cold that’s a miserable experience.

  2. Have you heard of this issue: the temp reading on the heat pump will read something like 65 when I first turn it on and immediately begin to climb. Within a few minutes and as soon as the compressor turns on, the temperature reading immediately begins to drop by a degree every 2 or 3 seconds or two, making it look like it’s cooling the pool. Is this normal? Thanks a lot

    1. Yes it would. Heat pump loses performance the colder it gets outside obviously. I install heat pump systems for a living (HVAC) and they heat homes by themselves down to outside temps of about 35-40 degrees. Once it gets that cold the home needs more btu’s than a heat pump will produce. So if you are drawing warmer air at your heat pump yes it will help. But it will obviously cost more to run your patio heater. I just bought a house in Arizona and got my heat pump pool heater installed the other day.

  3. My pool deep end is about 8 feet deep. Would that make a substantial difference in the heating time?
    i am having my pool replastered by a local company that claim it would not.
    thanks for your input

  4. AC system heat recovery works pretty well for a 40 gallon water tank but not so much for a 15,000 gallon pool

  5. The correct term I should have used is Heat Recovery. It is a unit that connects to your house air conditioner unit and heats your pool.

  6. Has anyone used a Heat Exchanger connected to their home air conditioner to heat your pool? We are considering going this route. Once you have the initial investment of the heat exchanger; it is free to heat your pool because you are using the hot air from your air conditioner to heat the water.
    Just wanted to know if anyone has experience with this method.

      1. I would like to know who installed it for you cuz I have one I just purchased a couple of weeks ago from hotspot energy recovery and can’t find anyone to do it any suggestions please

        1. Friend did, I installed the piping, was interesting, will heat the pool but it’s a pain to install

    1. I purchased one and I can’t find anyone to put it in I bought it from hotspot energy I live in Tampa do you know anyone that could put it in

    2. I researched this but went away from it as the air conditioner is likely not running substantially when you would most want to heat the pool early or late in the season.

      1. If you have an old house, maybe? Or constantly leave your windows open? The second that it hits 70 degrees, my house needs AC badly, and interior temperatures will reach the upper 70s. It retains the heat generated inside and the exterior temperature doesn’t allow much heat exchange. I am running AC pretty much as soon as it’s 65 or so outside. Then when it’s consistently above 75 the AC is on all day long.

      2. Exactly! I stopped looking into it as well when I realized the AC has to be running and my house would be an igloo. Plus the biggest AC for a home is typically a 5 ton/60,000 btus and you need twice that size at a minimum.

    3. no this is not a good idea i have little one and i tried to do that and my air conditioner blew up after 3 months

    4. Yes that’s an awesome system. I was going engineer one myself and then I saw they have that system already available. My only issue with that system is your AC has to be running for it to work. I won’t be using my AC when it gets cold out.

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