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What Size Pool Heater Do I Need: Finding the Right Fit for Your Needs

The sizing of the pool heater has to be done carefully. By having the right type of pool heater, you can extend the swimming session or even go to the pool in early spring. You may end up spending a lot of money and time when you don’t have the right pool heater size. It all depends upon the type of applications so that you can warm up the pool quickly. There are many factors to consider while buying a pool heater be it the size of the pool, heating preferences, or BTU requirements. To have the right size pool heater, you will have to perform basic calculations.

  • Gas Pool Heaters:Natural gas pool heaters along with propane pool heaters come with the highest BTU output. This makes them the most powerful heating unit for residential pools. It can have a BTU range of 100,000 to 500,000.
  • Electric Heat Pumps:As the name indicates, electric heat pumps use electricity. However, unlike a conventional electric water heater, it works like a reverse air conditioner. This type of pump uses ambient air temperature as well as sunlight for heating the pool water. In doing so, it can cost-effectively provide the right environment.
  • Pool Solar Heaters: If you are looking for the lowest BTU output, a pool solar heater serves as the right choice. Not only can it produce the lowest BTU, but it also has the lowest cost per BTU. This type of heater uses solar energy for heating the pool water. However, the photovoltaic systems of a pool solar heater can be expensive.

Pool Heater Sizing Considerations

1. Pool Volume

The pool volume is one of the major considerations for sizing the pool heater. By applying the right formula, you will be able to find out the pool volume of a straight-walled pool. In this formula, you will have to take the unit in feet. The formula is

Average Length X Average Width X Average Depth x 7.5 = Volume in Gallons

For calculating any pool that does not have straight walls, you can apply the following formula.

Average Length X Average Width X Average Depth x 7 = Volume in Gallons

2. Water Temperature

Another important factor to consider is water temperature. However, this completely depends upon individual preferences. You need to consider the temperature in Fahrenheit if you are looking to use the pool in different conditions. For a pool that has an attached hot tub, calculating the BTU requirement for maximum water temperature is 104 degrees Fahrenheit.

3. Ambient Air Temperatures

If your pool is in a warm climate, there will be many environmental conditions. It is important to know the coldest ambient air temperature so that the pool can stay warm.

Additionally, you will also have to consider other factors such as water heating speed and location.

Pool Heating Mathematics

BTU Constant: In the BTU constant, one pound of water is heated by one degree Fahrenheit. We can say that

BTU Constant = one pound of normal water heated by one degree Fahrenheit.

Water Constant: One BTU will heat 1 pound of normal water by 1 degree. The water constant is measured as 8.3 pounds of water per gallon.

If the pool capacity is 25,000 gallons, then it will be

Water constant = 25,000 x 8.3

Water constant = 207,500.

If you are looking to raise this temperature by 10 degrees, it will be 10 x 207,500 = 2,075,000.

Temperature Differential: Temperature differential is the coldest ambient air temperature and ideal swimming water temperature. For example, if the coldest ambient air temperature of the pool is around fifty degrees at night, you will have to increase it to up to eighty degrees Fahrenheit.

Time To Heat: We now come to the next important factor which is the time to heat. The time to heat is the speed at which the heater will raise the water temperature so that there will be an ideal swimming environment. For this calculation, a 24-hour time frame is used. However, one can easily choose the number according to their preference.

If we go by the above example of a pool with 25,000 gallons to reach it up to 30 degrees, you will have to apply the following formula.

25,000 x 8.3 x 30 / 24

6,225,000 / 24


According to the calculation, it will take 259,375 BTUs per hour to raise the temperature of a 25,000-gallon pool starting from 50 degrees to 80 degrees in 24 hours.

Consider When Sizing Your Pool Heater

We have seen that pool heaters will take 259,375 BTUs per hour for heating a pool of 25,000 gallons by 30 degrees in 24 hours. However, the real question is will a heater of 175,000 BTU can perform the same task. The answer is yes however it will take additional time for reaching the desired temperature. In this way, a solar-powered heater works by putting heat into a pool. If the BTU of the heater is reduced, it will not provide an accurate water temperature.

It has to be noted that pool heaters will not only heat the water but also overcome efficiency losses. In doing so, the pool would lose the heat quickly due to crosswinds and evaporation. It will also drop heat through the floor and walls. This is why you will have to get the pool heater size carefully and know how quickly the pool will lose heat. Even pool professionals perform calculations from the above for determining the BTU size to heat the pool. These professionals oversize the heater to 25% from 20%.

Heater Sizing For Pool & Spa

One of the main reasons for having a pool heater is to save time. We have said that professionals often oversize heaters to 25%. The main reason for this is to prevent environmental losses. Even a 175,000 BTU heater can perform the task but will take a longer time.

When there is a shared pool and spa combination, you will be able to have the largest possible BTU. All you have to do is to change the system to “spa mode” while using the spa. It will isolate the pool plumbing temporarily for increasing the spa water temperature by up to 104 degrees Fahrenheit.

We shall now calculate the time for raising the water temperature from 80 degrees to 104 degrees in a hot tub of 8 feet long, 8 feet wide, and 4 feet deep.

It will be 8 x 8 x 4 x 7.5 = 1920 gallons.

We shall now round it up to 2000 gallons and see how much time will it take to reach 104 degrees from 80 degrees.

As the hot tub has 8.3 pounds of water per gallon along with 16600 pounds of water.

It will require 16,600 x 24 = 398,400 BTUs for raising the temperature to 104 degrees.

Choosing your Heater – Which Type of Heater is Best?

By having the right type of pool heater, you can have a powerful option. However, you need to have stable temperature calculations and consider other factors like heat loss through evaporation. Compared to electrical heaters, natural gas and propane heaters are more powerful. You will require a temperature of over 50 degrees for electrical heaters for heat pumps. They are also less responsive. Solar heaters, on the other hand, are environmentally friendly but they don’t have the heating power of propane or gas.

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