Pool Heat Calculator – How to Estimate the Btu Rating and Heating Cost for Your Pool

This article will talk about the pool heat calculator for loads and losses, pool heater size estimate, and pool operational expense.

“You tell them – you tell them there’s a cost. Every decision we make in life, there’s always a cost.” Quoting Brad Meltzer, a writer, every decision that we make costs something.

So before deciding to get a swimming pool, it is essential for pool owners to calculate not just the initial expense but also the operational expenses-which includes pool heating.

To help you pool owners prepare for pool expenses, this article will talk about Pool Heat Calculator – How To Estimate The BTU Rating and Heating Cost for your pool. As well as estimate the pool heater size and pool operational expense. Let’s get started.

Pool Heat Calculator

Pool Heat Calculator – How to Calculate Heat Load

The first step in estimating heating expenses for a swimming pool is getting the heat load. Heat load refers to the heating required by the swimming pool. Usually, the unit for pool heat load in the specifications of a pool heater is BTU per hour.

Heat Load Factors

Many factors affect the heat load of pools, such as your location, pool size, pool volume, average air temperature, and desired water temperature. Although there are several others, those mentioned above are the major contributor to the pool heat load, and you can see their effect on the pool heat load formula.

Another deciding factor is the desired temperature or the temperature that you would like. But we will not talk about that since it’s subjective to pool owners. According to the U.S. Department of Energy, the ideal pool temperature is around 78 to 82 degrees Fahrenheit. With 78 degrees Fahrenheit as the most efficient pool temperature selection. [1]

Location and Average Air Temperature

Your pool location dramatically affects the pool heat load because it determines the climate-which affects the average ambient air temperature and pool season length. 

The climate of your location is heavily reliant on how far you are from the equator. To check the annually collected climate data, you can check out phone applications or websites like Accuweather. [2]

Pool Heat Location

When computing the pool heat load, the average ambient air temperature will serve as the base water temperature, which will be the unit value used to subtract from the desired pool temperature that you would like. The difference between these temperatures will determine the gradient that the heater needs to overcome in order to heat your pool efficiently.

Also, the climate affects the pool heater selection. For example, those who live in a colder climate may consider getting natural gas heaters rather than electric heat pumps. That is because the efficiency of heat pumps is reliant on the ambient air temperature. 

And for those of you who live in warmer conditions that want additional heating, installing an electric heat pump might be better. Or you can use a solar pool heater if you want a more eco-friendly pool heater.

The pool season length refers to the period in the year in which swimming is ideal. A warmer climate means a longer pool season. This means additional expense because it would mean a longer time for pool operation. 

Pool Size and Volume

Another factor in computing the heat load of a swimming pool is, of course, its size. The larger your pool is the more volume of water it holds-which will need more energy to heat your pool water.

Putting it in a technical perspective, this heat energy requirement is called the specific heat of the pool water. The specific heat of water [3] is a value that is defined by the amount of heat needed to raise the temperature of a certain mass of water by one degree of temperature. Of course, this value slightly changes when you add in chlorine and other chemicals, but it can be negligible since they only exist in small amounts and have very low specific heat.

In the English unit system, the value of the specific heat of water is 1 BTU / lbm – F. This means that you need 1 BTU for every pound mass of water before its temperature increases by 1 degree Fahrenheit.

With regards to the volume of the swimming pool, you can get it by simply computing the surface area of the water and multiplying it by its depth. Then we need to multiply this by its density to get the mass of the water in the pool.

Now that we’ve readied all the essential factors in heat load calculation, let’s dive into the formula that we will use.

Computing the Heat Load

In this estimation, we will be using the traditional way of computing for pool heat load. When I say traditional, it’s how the academes compute for heat loads. 

Suppliers and installers of pool heaters might use other empirical formulas or an automatic calculator to estimate the heat load. But you don’t need to worry since the answer that we will get using the formula below is close to the value that they will get.

For this discussion, we will only be using the common formula which is the mass multiplied by the specific heat of the water and multiplied by the temperature gradient.[4] Please see the formula below:

Heat Load = [mass of water x specific heat x (temperaturedesired – temperature)] / time

In order to properly use the formula and get a BTU per hour answer, we need to make sure that every unit used is in English system-pounds, gallons, Fahrenheit, BTU, and feet.

The BTU answer will then be divided by the BTU per hour rating of the pool size heater to get the heat pick-up time. We will show you that later in our sample pool heater selection.

How to Calculate Heat Loss

Heat loss in a swimming pool adds to the heating required. Thus, affecting the pool size heater needed. And that is why we must include heat loss in our computations.

Again, suppliers and installers of pool heaters might use other empirical formulas or an automatic calculator to estimate losses. And that’s fine since the answer we will get from this computation will be close to theirs.

Heat Loss Factors

There are four factors that affect heat loss in a swimming pool: evaporation, convection, and conduction. Although we will be discussing other heat loss factors, we will mostly focus on evaporation and convection since it’s the major heat loss factor (82%) in pools.[5][7]

Evaporation and Convection

Evaporation refers to the change of state of pool water from liquid to gas. It is due to the temperature difference between the pool water and the surrounding air. Approximately, evaporation and convection make up more than 82 percent of the total heat loss in a pool.

Pool Handle

Like we’ve previously said, evaporation is the largest factor in pool heat loss. And to give an idea of its effect, remember that it takes 1 BTU to heat a 1 pound mass of water by 1 degree Fahrenheit. But when 1 pound of pool water evaporates, it takes away 1,045 BTU of heat out of the swimming pool. [5]

The estimation formula to get the evaporative loss of heat is:

Heat Loss evaporation = heat loss factorsurface x pool surface area x (tempwater – tempair)

From the formula, the heat loss factor depends on the wind speed. For our value, we can try to extrapolate from the table provided by the U.S. Department of Energy [6] While the other values can be easily measured by using a thermometer and measuring tape.

To mitigate this evaporative effect, it is recommended to use a pool cover or a solar blanket. This will reduce the evaporation effect by 50 to 70 percent and generate savings in pool heater usage. [5]

There is another evaporative heat loss formula that is more specific, but we won’t be using it due to its complex. It involves a deep understanding of thermodynamics. But for the sake of information, we will be citing it below.

Heat Lossevaporation = latent heatevaporation x kinetic energy x partial pressure x surface areapool

Reradiation

Losses through reradiation happen when warm water in a pool and spa radiates heat to a cool sky. This mode of energy transfer accounts for around 17 percent of the total loss of heat in a swimming pool.[7] But we will not include this in our computation since this requires advanced mathematics for solving the radiation parameter that we will need. 

Conduction

Conduction is a mode of heat transfer when two surface areas in contact with each other. In the pool and spa setup, conduction happens at the surface area of the walls and floors. 

Although the surface area of the walls and floors may look large, losses through conductive means only make up 0.6% of the total loss of heat. This is due to the low heat conductance of the ground on which it’s installed. It is also the same for pools installed above ground. [7] So we can simply choose to not include this in our computation.

Computing the Heat Loss

In summary, we will only be using the evaporative heat loss formula in our computation. Recalling the formula:

Heat Loss Evaporation = heat loss factorsurface x pool surface area x (tempwater – tempair)

In order to properly use the formula and get a BTU per hour answer, we need to make sure that every unit used is in English system-pounds, gallons, Fahrenheit, BTU, and feet.

Again, suppliers and installers of pool heaters might use other empirical formulas or an automatic calculator to estimate losses. But that’s fine since the answer we will get from this computation will be close to theirs.

Example Estimations

Example 1 – Gas Heater

Given

What is the total pool heater size needed for a circular pool with a diameter of 20 feet and a depth of 6 feet? In 24 hours, we need the water to heat up from the current 62 degrees Fahrenheit to the desired pool temperature of 82 degrees Fahrenheit. The ambient temperature is 75 degrees Fahrenheit. 

What will be the pool heater size? What is the estimated monthly operating expense if I use a gas heater for 8 hours a day? 

Solution

The formula for the heat load of the pool:

Heat Load = [mass of water x specific heat x (temperaturedesired – current temperature)] / time

Substituting the given values to find the heat load:

= [[ℼ x (20 ft)2/ 4]x(6 feet)x(62.4 lb/ft3)x(1 BTU / lb-°F)x(82°F – 62°F)]/24hr

= 98,017.69 BTU per hour

Next is computing for the loss of heating. The estimation formula to get the evaporative loss of heat is:

Heating Loss evaporation = heat loss factorsurface x pool surface area x (tempwater – tempair)

Substituting the given values:

= (6 BTU / hour – ft2 – °F) x (ℼ x (20 ft)2/ 4) x (82°F – 75°F)

= 13,194.68 BTU per hour

Adding the two values, we get a total of 111,212.37 BTU per hour. To select a pool heater for this rating, we must get the nearest but greater than this value. 

Let’s say, the nearest but greater gas heater rating is 150,000 BTU / hour. Now, let’s try to compute the operational expense. 

In the U.S., 1 therm or 100,000 BTU costs around 1 dollar.[8] So if we are operating pool gas heaters at 8 hours per day, then our monthly operational expense is around 360 dollars every month for a natural gas heater.

Example 2 – Heat Pump

Given

What pool heater size should I consider getting if I have 10,000 gallons of water to heat in my pool. In 24 hours, I want a pool temperature of 78 degrees Fahrenheit from 58 degrees. The ambient temperature around the pool is 70 degrees Fahrenheit. The square pool has a depth of 5 feet.

Solution

What will be the pool heater size? What is the operating expense if I use an electric heat pump?

The formula for the heating load:

Heating Load = [mass of water x specific heat x (temperaturedesired – temperature)] / time

Substituting the given values to the heating load formula:

= (10,000 gallons) x (8.345 pounds / gallons) x (1 BTU / lb-°F) x (78°F-58°F) / 24 hr

= 69,541.67 BTU / hr

Substituting the given values to the heating loss formula:

= (6 BTU / hr – ft2 – °F) [(10,000 gallons) / (7.481 gallons / feet3) / (5 feet)] x (78°F – 70°F)

= 12,832.51 BTU / hr

Adding the two values, we get a total of 82,374.18 BTU/hr. To select a pool heater for this rating, we must get the nearest but greater than this capacity. 

Let’s say, the nearest but greater heat pump rating is 90,000 BTU/hr and it has an efficiency or coefficient of performance (COP) of 5.0. Now, let’s try to compute the operational cost.

If the heat pump unit has a COP of 5.0 then its input power is a fifth of its rated power. Dividing 90,000 by 5, we will get 18,000 BTU/hr or approximately 5kW. If we are using the heat pump for 8 hours a day and a kWhr costs 8.5 cents, then we will end up with a monthly operational cost of $102. [9]

In summary, estimating the BTU rating and operational expenses for a pool heater involves several factors such as pool location, pool size, and pool heater losses.

You can also use other formulas for estimation aside from what we have used. The results that you will yield will be close to each other. But if you are in doubt of your estimations, you can always call a swimming pool professional for consultation.

“Count the cost before you make your choice.” – Quoting Lailah Akita, a Ghanaian writer, it is wise to count the costs first before deciding.

Now that you know how to estimate the costs of operating your pool heater, this will prevent financial problems due to a sudden large pool heater bill.

Do your family and friends love to swim? Are you planning to add a pool to your home? Check this article, A Detailed Guide to Investing in the Best Solar Pool Heater for Your Home, surely it will help you!

And after that, I recommend you read about Solar Pool Covers. It will guide you to select the well suited for your Pool. Amazing fact: To cut heating costs, using pool covers is one of the most effective ways.

  • [1] https://www.energy.gov/energysaver/managing-swimming-pool-temperature-energy-efficiency
  • [2] https://www.accuweather.com/
  • [3] https://www.usgs.gov/special-topic/water-science-school/science/heat-capacity-and-water?qt-science_center_objects=0#
  • [4] https://www.khanacademy.org/science/physics/thermodynamics/specific-heat-and-heat-transfer/v/specific-heat-and-latent-leat-of-fusion-and-vaporization-2
  • [5] https://www.energy.gov/energysaver/swimming-pool-covers
  • [6] https://www.energy.gov/sites/prod/files/2014/05/f16/steam19_vessels.pdf
  • [7] http://www.fsec.ucf.edu/en/publications/html/FSEC-IN-23-83/in-23-83-2.pdf
  • [8] https://www.energy.gov/energysaver/gas-swimming-pool-heaters
  • [9] https://www.energy.gov/energysaver/heat-pump-swimming-pool-heaters

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