What Size Pool Heater Do I Need?
When I to talk to pool owners about heater sizing calculations I can usually see their eyes start to glaze over. Not that sizing a pool heater is super difficult, but there are some calculations to be done if you want to go by the book. Just as with pool pumps, more times than not, heater sizing is not calculated so much as installers use familiar sizing for certain applications. For example, if you have an attached hot tub on your pool, most installers will just tell you that you need a 400,000 BTU heater (the largest available in most residential applications). There was no calculation done, since the installer knows that with an attached hot tub that is circulated at pool temperature, when you want to use the spa you will need to wait for it to heat up. The higher the BTU output of the heater, the faster the heater will bring the water from pool temperature up to spa temperature. Nobody wants to have to turn on your spa two hours in advance just for it to warm up. With a giant 400,000 BTU heater, an average 8' x 8' spa could go from 80 degrees up to 104 degrees in about an hour. (more on this later in this article).
Similar to the above example, when a pool owner asks about what size pool heater they should get, most installers just pick their favorite heater that is between 200,000 BTU and 350,000 BTU knowing from experience that this will work on a pool approximately the size of the one in question. To a certain degree this works, but this article is intended to help you actually get comfortable with the calculations behind how all of this works (or doesn't). Nobody wins in a situation where you install a new heater only to find that it does not keep your pool warm enough, or it runs constantly just to be able to keep the water temperature where you want it.
Before we start with the calculation it is important to understand that it does not matter what kind of heater you are talking about, all types of pool heaters are measured in BTU's, which is a standardized unit of measurement (British Thermal Units) that indicates the amount of heat (energy) required to raise the temperature of precisely one pound of water by one degree Fahrenheit. The concept of BTU ratings applies to all forms of pool heaters:
Gas Pool Heaters - Natural gas pool heaters and propane pool heaters are the most powerful heating unit available for residential pools. These units have the highest BTU output of all heater types for pools, ranging from 100,000 BTU up to 500,000 BTU in some areas (400,000 BTU in most areas in Canada and the USA). For more information on these heaters you can also read these pool heater reviews which looks at the most popular brands and sizes currently available.
Electric Heat Pumps - Electric heat pumps use electricity, but not a heating element like a typical electrical powered water heater does. Instead an electric heat pump essentially operates like a reverse air conditioner, using ambient air temperature and sunlight to heat pool water. An electric heat pump is capable of producing much lower amounts of heat as compared to burning fossil fuels in a gas heater, but in the right environment can heat pool water more cost effectively than a gas heater can. For more information on electric heat pumps, how they work, and how to install them, you can read these pool heat pump reviews.
Pool Solar Heaters - Pool solar heaters are capable of producing the lowest BTU output of any type of pool heater, however solar heating also has the lowest per BTU cost since they utilize the heat energy from the sun to heat the water in your pool. Technology does exist for more efficient solar water heating than the thermal solar heating currently used to heat pool water with solar. The main limitation currently is that the cost of PV (photo voltaic) systems, as well as evacuated tube collectors limit the usability in residential pool applications. The technology is there for better, more efficient solar water heating, but the material and installation costs are simply too prohibitive for most installations. For more information about thermal solar heaters as well as how to install them on your pool you can read this article about pool solar heaters.
A note on electric heating element water heaters - Using a heating element to heat water, like you would find with an electric kettle, or a traditional hot tub heater, is the least efficient form of water heating. You can get by heating a small body of water with an electric element heater, like a hot tub, but it needs to be fairly well insulated on all sides or the costs rise quickly. For something as large as a swimming pool, even smaller ones, electric element heaters are not ideal. For example, take a look at this large 11 Kilowatt electric water heater. This heater would require up to 60 amps of electrical draw and can output a maximum of 37,000 BTU's. 60 amps is a huge electrical draw. Most older houses only have between 60 to 100 total electrical services, so a single unit drawing 60 amps is massive. With an output total of 37,000 BTU you would need to spend a LOT of money in order to heat anything larger than a hot tub with element style electric heating. Not just because of the low BTU output, but also due to the low efficiency of heating water via an electric element that results in much of the power you pay for being lost in the energy transfer.
Pool Heater Sizing Considerations
Sizing and buying a pool heater is more complicated than just knowing how big your pool is. In order to make an informed decision about what size of pool heater you should get you will need to know the following information:
1) The volume of your pool
2) The temperature of the water you like to swim in
3) The coldest ambient air temperature you expect your heater to maintain water temperature in
In addition to these factors you also need to determine if the speed at which the water your water heats is important to you. For example, if you have only a swimming pool, but no hot tub, and it will remain at a constant temperature all the time, then the speed at which the water comes up to temperature is not very important. Conversely, if you have an attached hot tub that is at pool water temperature, then you will certainly want to consider how long it will take to raise the spa from pool temperature to hot tub temperature.
Pool Volume - The volume of a straight walled pool can be loosely calculated using the formula (in feet) of average length x average width x average depth x 7.5 which will give you the approximate volume of your pool in gallons. If you have a freeform shape pool (any pool without all straight walls) then a slightly more accurate calculation for volume is average length x average width x average depth x 7.
Water Temperature - In order to size your pool heater correctly for your needs you need to identify what temperature you prefer to swim at (in degrees Fahrenheit). This establishes what performance you expect your heater to maintain under different environmental conditions. If you have an attached hot tub, then your maximum water temperature for calculating BTU requirements would be as high as 104 degrees Fahrenheit.
Ambient Air Temperatures - A pool in a hot climate will have different environmental conditions to overcome than a pool in a colder climate area. For the purposes of calculating what size of pool heater you need, you need to know what the coldest ambient air temperature will be when you still expect your heater to be keeping your pool warm.
Once you are able to assign numbers to the above values you can begin to look at the nature of the heating equations for your pool. There are a few different ways that you can approach calculating your BTU heating requirements, such as whether you are looking to calculate your maintenance heat, or the amount of heat you need to raise the water from cold to warm. The answer as to which calculation you should use for your pool will depend entirely on your unique conditions such as whether you keep your pool one temperature all of the time, or whether you keep it cooler sometimes, and only heat it up periodically for swimming. The next section will provide examples for each of these potential conditions.
Pool Heating Calculations
In order to help all of this make sense to you it would be easiest to start giving real world examples that will help you to draw comparisons to your unique pool situation. For the following examples we will assume a 16'x32' rectangle pool with an average depth of 5' which would equal approximately 20,000 gallons of water (19,200 rounded up). Also for these examples we will assume that the pool owner likes to swim at a reasonable 80 degrees Fahrenheit.
BTU Constant = 1 lb of water heated by 1 degree F
Water Constant = 8.3 lbs of water per gallon
20,000 gallons x 8.3 lbs / gallon = 166,000 lbs of water that needs to be heated. Since it takes one BTU to heat one pound of water by a single degree, we now know that in order to raise this entire 20,000 gallon pool by one degree it will take 166,000 BTU's of heat energy. So, if you want to raise the water temperature by ten degrees, then you would need 1,660,000 BTU's of heat energy.
Temperature Differential - The next step of the heater sizing calculation is to use the ideal water swimming temperature as well as the coldest ambient air temperature for your pool to determine the temperature differential that your heater will need to overcome. For this example, let's assume that the coldest ambient air temperature at night for our pool will be right around 50 degrees Fahrenheit. This means that in a worst case scenario you will need to heat your pool water from 50 degrees up to 80 degrees.
Time To Heat - Next for our calculation we need to determine how long we want the heater to take to heat up the water from the coldest ambient temperature up to the ideal swimming temperature. Often a 24 hour window is used for this calculation, but in actuality this is a "made up" number in that you could choose a smaller sized heater and still be able to heat the water...it might just take longer to do.
So for this example we have a maximum temperature differential of 30 degrees Fahrenheit along with a 24 hour heating window that we expect the heater to work within. In order for 20,000 gallons of pool water to heat 30 degrees you would use the calculation:
20,000 x 8.3 x 30 = 4,980,000 BTU total.
Now, divide this BTU total by 24 (hours) to determine the BTU's / hour required to heat the water:
4,980,000 / 24 = 207,500 BTU's per hour
This means that it would take 207,500 BTU's per hour in order to raise this 20,000 gallon pool from 50 degrees up to 80 degrees in a 24 hour period of time. This is essentially the heater sizing equation that people use to determine the size of pool heater you need, but in actuality it is only one of the ways in which you can approach this question about what size of pool heater you need. For most pool owners you will only go from 50 to 80, or cold to warm, perhaps once per year. While this calculation seems unnecessary since you will not need to heat your water like this very often, this does help to establish a minimum pool heater size that you would need to raise your water to swimming temperature during the coldest parts of the year that you expect to be swimming.
Variables To Consider When Sizing Your Pool Heater
In the above example it was determined that we would need 207,500 BTU per hour in order to heat the 20,000 gallon example pool by 30 degrees over 24 hours. The reality is however that most pool owners will not need to heat their pool from 50 degrees to 80 degrees very often, plus we also arbitrarily assigned 24 hours as the period of time that the heater needed to accomplish this in. Does this mean a 175,000 BTU heater could not be used to heat the example pool? In fact, a 175,000 BTU heater could be used, and the only difference being that it would take longer than 24 hours to bring the pool from 50 to 80 degrees. The 175,000 BTU heater would take
28.45 hours to accomplish what the 207,500 BTU heater can do in only 24 hours. This is actually how a solar heater (or an electric heat pump) works to heat your pool. Unlike gas heaters, they have much lower BTU output potential, nowhere near to 200,000 BTU per hour. Still, these heaters work because they are adding heat to the pool, just much slower than a gas heater would.
Eventually, if you reduce the heater BTU's enough, the heater will no longer be able to maintain water temperature. This is due to the fact that a pool heater is doing more than just heating your water - it is also overcoming efficiency losses at the same time. First, your pool will lose heat very quickly from evaporation and cross winds. If you consider a pool in a calm area versus a pool in an area with a lot of winds it is easy to see how the pool will lose heat quickly from the winds blowing across the surface. In addition to losing heat though the surface area with the ambient air and wind, the pool will also lose heat through the walls and floor, though the vast majority is lost through the surface of the water. This is why it is so critically important to use a solar blanket on your pool at night to conserve the heated water that you are paying for. Proper use of a solar blanket on your pool can reduce your heat losses by as much as 90% - which, if you consider heating costs, is a huge sum of money to be leaving on the table.
Since your pool is constantly losing heat you need to consider this when sizing your pool heater. The actual calculations behind how fast your pool loses heat go beyond the scope of this article. Even if you hire a pool professional to come to your house and size your heater for you, they are not actually calculating wind speed and surface area in your backyard. They will perform the calculations above to determine the minimum BTU size to heat your pool from cold to warm in 24 hours, and then simply oversize the heater by 20% to 25% on average to accommodate for the losses that the heater needs to overcome. For the example pool that needed 207,500 BTU per hour to heat the pool, when you add another 20%, it becomes 248,400 BTU. So a reasonably safe bet for our example pool is a heater that is rated for around 250,000 BTU's.
Heater Sizing For Shared Pool & Spa Systems
Much like with pool pumps, pool owners tend to have a "bigger is better" mentality when it comes to sizing pool equipment. In the world of pool heaters there is really only one reason to buy the largest possible unit - and that is to save time. In the above example where we have been talking about a 20,000 gallon pool it was determined that a 207,500 BTU heater would be good, but we added 20% to the size to accommodate for environmental losses which pushed the size up to 250,000 BTU. We also addressed how a 175,000 BTU heater, as an example, could also work on this pool however it would take a little longer to heat the pool from cold to warm. In addition to this, the smaller heater would need to run more often per hour in order to maintain the heat than the larger size heater.
The one specific condition where you want the largest possible BTU heater is when you have a shared pool and spa combination system. These common shared systems have a pool and spa both operated at pool temperature. When you want to use the spa, you simply change the system to "spa mode" which isolates the hot tub from the pool plumbing temporarily and then increases the spa water up to 104 degrees. It is not practical to need hours to heat your hot tub up when you want to use it. Most designers of these systems aim for between 30 minutes to one hour of heating time to bring the spa water up to temperature. It is these systems that benefit the most from having huge 300,000 and 400,000 BTU heaters.
Example Hot Tub: 8' length x 8' width x 4' depth x 7.5 = 1920 gallons
So, rounding up to 2000 gallons we need to determine how long it will take a heater to raise the water from 80 degrees to 104 degrees. Since there is 8.3 lbs of water per gallon, we know that this hot tub has 16,600 pounds of water to heat. We also know that the water needs to increase 24 degrees from 80 to 104.
16,600 x 24 = 398,400 BTU's total to raise the spa from 80 to 104.
So for this example we can see that it will take approximately one hour for a 400,000 BTU heater to raise a 2000 gallon hot tub from 80 to 104 degrees. One hour is barely adequate already, depending on how much time you consider acceptable to heat your spa, and the original 207,500 BTU heater that we calculated for our 20,000 gallon pool would take two full hours to heat a hot tub from pool temperature. This is why a pool without a hot tub can get away with a smaller heater, simply allowing longer for the water to heat. However with a shared pool and spa combination, the conventional wisdom is that you want the spa to heat up quickly, and two hours or more will simply be too long to be attractive to most pool and spa owners.
How To Size Electric Heat Pumps
Solar heaters and electric heat pumps do not work in the same way that natural gas and propane pool heaters do. With gas heaters, you simply feed money into the machine and it will make your pool warmer. You could have a million square miles of thermal solar heaters for your pool, but in the dead of night they will provide a total of zero heat for your pool. In this sense, when you size a solar heater or an electric heat pump for a pool you need to consider the maintenance BTU's required in order to hold your temperature constant. The 24 hour heating time that we use for a gas heater simply does not apply to heat pumps or solar heaters since there are none that can output enough heat to meet this timeline. This is why heat pumps and solar heaters do not work on every pool and every climate.
In order to calculate the amount of heat energy needed to maintain your pool temperature we need to know the surface area of the pool, the desired water temperature for your pool, as well as the ambient air temperature around the pool. Using the same example pool that we have been using all along in this article we can calculate the following:
16 x 32 = 512sf of surface area
80 degrees Fahrenheit desired water temperature
50 degrees ambient air temperature
These numbers are then multiplied by 12 to reveal the BTU size needed for the heat pump:
512 sf pool area x 30 degrees temperature difference x 12 = 184,320 BTU
In this example it is determined that a 185,000 BTU heat pump would be needed to meet the heating requirements of the pool. What is interesting about this is that electric heat pumps are usually only sized between 75,000 to 125,000 BTU's. In short, for this pool and this example, an electric heat pump would NOT meet the heating requirements of the pool. The 50 degree ambient air temperature is simply too cold, and the temperature differential too great, for the heat pump to overcome. If you were to calculate the same example, only this time imagine that the pool is located in a warmer climate and 70 degrees is the ambient air temperature, not 50 degrees, the numbers will work out better:
512 sf pool area x 10 degrees temperature difference x 12 = 61,440 BTU
In addition to the temperature variation of 30 degrees being too great, in a real world setting you would also find that 50 degrees is simply too cold for most electric heat pumps to operate. This is why electric heat pumps are great for applications where it does not get too cold outside, and you get quite a bit of ambient heat and sun. Areas where it gets colder in the winter, and a pool heater is often used to try to start the swimming season early, or end late, a heat pump may not be the best (or even a viable) option.
The type of heater you choose and the amount of heat that you need will depend on your specific swimming pool requirements as well as the climate where you are located. If you follow the calculations and advice on this page you will have everything you need to determine the size of gas heater of electric heat pump your pool needs. If you are still not sure, and would like some assistance choosing the best heater for your pool, please email Steve directly at SwimmingPoolSteve@gmail.com for assistance.
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