What Size Pool Heater Do I Need? Guide on How to Size A Pool Heater

What Size Pool Heater Do I Need? If you own a pool then it’s important that you purchase the right size because let’s be honest, pool heaters are not cheap and choosing the wrong size will definitely ruin your day. As with most things, education is the key and knowing all the facts will ensure you make the right decision when it comes to choose your pool heater.

Now, let’s have a look at what factors you need to consider when it comes to swimming pool sizes.

What Size Pool Heater Do I Need?

Owning a swimming pool means you and your family get to have a lot of fun together, and you can invite your friends and relatives on the weekends for pool parties. However, there are limitations on how long you can use your pool because of climate and day/ night cycles. Unless you are training to become a special forces operator, jumping into a pool filled with freezing cold water doesn’t sound like a lot of fun. And to make things complicated, different members in your family will have varying preferences when it comes to pool water temperatures. Some may feel that 75°F is a perfectly fine water temperature for the occasional early evening swim, whereas others will find it hard to step into an 80°F pool even with the sun shining right above them.

Heating the pool requires energy, but maintaining the heat is another problem with a different set of challenges. A pool is essentially a large water body sitting out in the open, and it loses both heat and mass over time to the atmospheric air through evaporation. Once your pool heater has brought the water temperature up to a comfortable level for swimming, it must also maintain the temperature caused by heat loss from the water surface.

How to Determine your Pool Volume

Finally, how quickly do you want your pool to be heated? If it is a simple design and you only plan on using it for a pool party on the weekends, then you’re probably fine with a heater that does the job in 48 or even 72 hours. But let’s throw something like a hot water spa or jacuzzi into the equation, and now all of a sudden you can’t afford to give your heater 3 days to get the water warm. Sizing your heater is a complicated matter, and more often than not you will be fine with a slightly oversized pool heater. In fact, a lot of pool professionals will recommend that you oversize your pool heater by a slight margin to compensate for things like unexpected weather changes, future pool upgrades (hot tub, spa, fountains), etc.

A smaller pool heater will consume less energy (gas, electric) to function, but it will also need to work longer. A larger heater will cost you more upfront, but it will get the job done in much less time and even though it consumes energy at a higher rate the overall energy expenditure isn’t all that different compared to a small heater if both of them are rated for the exact same efficiency.

In this article we are going to answer the question — “What size pool heater do I need?” And that’s not all, we shall also briefly overview the 3 major types of pool heaters based on fuel source : gas, electric, and solar. Each of you will have different requirements depending on where you live, the size and type of pool, etc. We shall provide the formulas needed to get a rough approximation of pool heater size, and back it up with a couple examples to further illustrate the calculation process. Without further ado, let’s dive in…

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Types of Pool Heaters

When you consult with a swimming pool professional, the first thing they are going to ask you is how quickly you want your pool heated. And then, they will examine the location of your pool as well as its surroundings. Are there any trees blocking direct sunlight from falling upon the pool? Do you have shrubs, fences, or cabanas around the pool to restrict wind speeds? And how large is the pool, how many gallons of water does it hold? The answers to these questions will help you determine the best type of pool heater for your needs.

Gas-powered pool heaters: If your pool is large, and you want it to heat up fast (within hours) there is nothing better than a propane or natural gas-powered pool heater. Most people get a 250,000 BTU gas heater if they can accommodate it within their pool heating budget. What makes a gas heater so unique is that it can actually extend your pool season, maybe even double or triple the number of months in a year that you are allowed to use your pool. For a city like Detroit, swimming season is usually the 2 month period consisting of July and August. But with a decently powerful gas heater unit, you can easily extend it on both ends — as early as June, and as late as September.

Natural Gas Pool Heater | Pentair 460736 MasterTemp

Unlike heat pumps, gas powered pool heaters can function in any climate or weather. They heat a pool really fast, do not depend on the temperature of ambient air, and are the only choice if you have a hot water spa or jacuzzi. And the upfront cost is lower compared to an electric heat pump, even though they end up costing much more in the long run because of how much gas they consume. It is not uncommon to get $800 or $1000 gas bills if you use one of these to heat up your pool.

Electric heat pumps: We like to think of electric heat pumps as a way to enhance your swimming experience, while spending less energy compared to a gas heater. A heat pump is typically used to maintain a steady pool temperature for extended periods of time, or in conjunction with a gas heater to compensate for surface heat loss after the gas heater has done its job of bringing the water to your desired temperature. It functions very much like an air conditioner, but in reverse cycle. Most heat pumps use refrigerant (typically R410A) which goes through phase changes while circulating between an evaporator and condenser.

140K BTU Heat Pump | Hayward HP21404T HeatPro

An air conditioner takes the heat from air within your room and throws that heat into the air outside. A pool heat pump will extract the latent heat from ambient air surrounding it and transfer that energy into the pool water circulating through its heat exchanger. And unlike gas heaters, a heat pump doesn’t just convert energy from one type to another, it amplifies the heat energy before sending it into the pool water. There is a compressor within the heat pump that increases the pressure and temperature of hot refrigerant gas after it comes out of the evaporator. This way, a heat pump can achieve thermal efficiency values of 400 to 500 percent in optimal conditions (ambient air 80°F).

With an electric heat pump the Thermal COP can vary a lot depending on the ambient air temperature. On a sunny day in Florida, the COP can be close to 5 but during fall season in North Dakota the COP will drop close to 1. Which is why heat pumps are used to enhance your existing swimming pool season, not to expand it. Even the most efficient heat pumps will end up costing you 400 or 500 dollars a month if you use them frequently, because unlike gas heaters they can take days to heat up your pool by 10 or 15 degrees. While gas heaters go up to 400,000 BTUs in size, the most powerful electric heat pumps top out at around 140,000 BTUs.

Solar pool heaters: Solar energy has been around for quite a while, but it never got the mainstream popularity that we thought it would. And there are reasons for that- first of all, a solar pool heating system is extremely dependent on weather. It will work fine during the spring and summer season, but come fall or winter you will not get sufficient sunlight exposure for the solar hot water panels. In order to function properly, a solar pool heating system needs at least 8 to 10 hours of uninterrupted exposure to direct sunlight each day. While this is not an issue in Miami, you will never get that much sunlight exposure in Alaska. Not that you will find a whole lot of outdoor pools in Alaska to begin with, but just in case. Most pools in Alaska are probably indoors.

But if your house it located in a place where you get enough sunlight for at least 2 or 3 months a year to operate the heating panels, then you pretty much have a zero-cost, zero-maintenance pool heating solution. Well, the pool pump still draws electricity, but you get the point- the heating system itself doesn’t consume power like an electric heat pump or gas heater.

One of the things people do, is combine their regular gas powered pool heater with a solar heater. The gas powered heater runs for one day a week and shuts down, while the solar pool heater helps maintain the water temperature by compensating for heat loss. Another possibility is to use the gas heater exclusively for something like a hot water spa, while the rest of your pool is warmed by the solar heater. A solar heater in conjunction with a solar pool blanket, will help you cut down on pool heating costs tremendously. We shall discuss the benefits of a solar pool blanket and how it works in a later portion of this article, but first let us get into the steps for sizing your pool heater.

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How to Size A Pool Heater

There are two methods used by pool professionals when it comes to determining heater size —

Surface Area and Temperature Rise:

First you need to calculate the surface area of your pool in square feet. There are different formulas for various pool shapes-

Circular shaped pools: The area of a round pool is πr² where π is a constant with the value of 3.14 (approx.) and “r” stands for radius of the pool which is half of its diameter. With a circular pool that has a diameter of 10 feet, the radius is 5 feet which means we get a surface area of 3.14 x 5 feet x 5 feet = 78.5 square feet.

Oval shaped pools: This method applies to any elliptical pool shape. You take the smaller radius, let’s call it “A”. Now take the larger radius, let’s call it “B”. The area of an oval pool is- A x B x π or A x B x 3.14. Let’s consider an oval pool which is 20 feet long and 10 feet wide. The smaller radius will be 10/2 = 5 feet, and the larger radius will be 20/2 = 10 feet. Now we get a surface area of 5 x 10 x 3.14= 157 square feet.

Kidney shaped pools: First you take the two furthest away points on the pool and measure the distance between them. Next, you find the width on the smaller kidney bulge. And finally, you find the width on the larger kidney bulge. Now you can get the area of the pool with the following formula- (A + B) x L x 0.45, where A and B are the two separate widths of the smaller and larger bulges, and L is the length between the two furthest points on the pool (length of the kidney pool).

Rectangular pools: Multiply length by width to get area of a rectangular pool- for a 14’ x 8’ rectangular pool the area is 14 x 8 = 112 square feet.

Once you have measured the area of the pool surface in square feet, you can use the following formula to calculate the heater size in BTU/hour :

Heater size in BTU/hr. = Surface Area x Temperature difference x 12, where Surface Area is the area of the pool in square feet, temperature difference is the result of (desired pool temperature – current pool temperature), and 12 is our constant number based on the assumption that wind speed is around 3.5 miles per hour.

So, let’s take an example of a 40’ x 30’ rectangular inground pool in a Detroit home, month is September. Based on climate data (57°F lowest and 73°F highest), it is reasonable to assume an average starting temperature of 65°F for our pool water. What should our target temperature be? This can vary based on who is using the pool, and the purpose of the pool. For instance, the American Red Cross recommends a temperature between 78° and 82° for recreational and commercial pools. Kids and senior citizens prefer a water temperature higher than 80°F. For this example, we shall go with 80°F. Now let us use our formula- 40 x 30 x (80-65) x 12 = 216,000 BTUs per hour needed. Clearly an electric heat pump would not be sufficient for our application since those top out at 140,000 BTUs so you will need a gas powered pool heater. Something like the Hayward H250FDN Low-NO_x unit would be perfect.

Remember, our formula only works if the rate of temperature change is between under 1.25° per hour. If you want to get a 1.5°F rise in temperature per hour, multiply the result with 1.5 so for our example, 216,000 x 1.5 = 324000BTU/hr.

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Water Volume and Time

This is a very popular method for measuring pool heater size, it takes the volume of the pool in gallons and also factors in loss from surface through evaporation.

First, we need to know how many BTUs it takes to heat one gallon of water. One gallon of water weighs 8.34 pounds, and we know that a single BTU is the amount of heat energy required to raise the temperature of 1 pounds of water by 1 degree. So, to heat 8.34 pounds of water we need 8.34 BTUs, which is the amount of heat energy needed to raise the temperature of one gallon of water by 1 degree. Now, we need to calculate the number of gallons in our swimming pool which measures 40’ x 30’ x 6’. The volume is 7200 cubic feet, and when converted to gallons it is 7200 x 7.48 (one cubic foot is 7.48 US gallons) = 53,860 gallons.

We need 8.34 BTUs of heat to raise the temperature of one gallon by 1 degree. Taking our example of a swimming pool in Detroit during September, we need to go from a temperature of 65°F to 80°F which is a 15 degree rise. So, we need 15 x 8.34 = 125.1 BTUs per gallon of water for a 15 degree rise in temperature. Now accounting for the entire volume of our pool, the total heat energy required is 125.1 x 53860 = 6,737,886 BTUs. That’s a whole lot of heat energy, right? Clearly there is no company on Earth which will sell you a 7 million BTU/hr. pool heater. So, let’s do this over a 48- hour period which sounds reasonable- 6,737,886 BTU spread across 48 hours is 140,372 BTU per hour.

But we will also lose heat from the surface over a period of 24 hours due to evaporation.

To calculate water loss through evaporation, use the following formula :

K_s x ∆T x A, where K_s is “surface heat loss factor”, ∆T is the difference between ambient air temperature and water on the pool surface, A is the area of the pool. We began with an ambient of 65°F and our target temperature is 80°F which gives us a ∆T of 15°F. Value of K_s ranges between 4 and 7 for wind speeds between 2 to 5 mph, we shall consider it to be 5 for this example since our wind speed is pretty moderate. So, surface heat loss can be calculated as- H_s = (5 BTU/hr ft² °F) x (15°F) x (1200 ft²) = 90000 BTU per hour lost to evaporation.

With these new figures, our total BTU expenditure to heat and maintain the pool temperature at 80°F is (140,372 + 90,000) BTU/hr = 230,372 BTU/ hr.

If you have no shelters or fences around the pool, wind speeds will be higher resulting in more evaporation losses. You will have to oversize the heater by around 20 percent to compensate for the added load. For high altitude heating, add an extra 4 percent for every 1000 feet of elevation. Also note that you don’t need a 250,000 BTU heater to heat the pool used in this example, you can get away with lower BTU as long as it is above the 90,000 BTU/hr that is needed to maintain pool temperatures through surface heat loss. Only difference will be that it is going to take longer for heating the pool, in our example we needed 48 hours.

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The importance of solar blankets for your pool

Most of the heat loss from a pool happens in the form of evaporation, especially if the pool is outdoors and relative humidity of air is low. Loss through evaporation can happen both during the day, as well as the night, but it is maximized when pool water temperature is at its highest. And while there are other forms of heat loss from the pool surface such as radiation and convection, they only add up to around 20 or 30 percent of lost heat. With a solar pool blanket, you can prevent most of the heat loss that happens through evaporation and also suppress heat loss via convection. Not only are you retaining the pool heat for longer, but you also prevent loss of valuable pool chemicals through evaporation or convection.

Pool owners don’t realize how effective a solar cover really is when it comes to preserving pool water heat and lowering heater running costs. By spending just 50 to 200 bucks on a decent solar pool blanket, you can reduce the pool heater operating costs by up to 50 percent (even as high as 70 percent in some cases). It works by acting as a barrier between your pool surface and the wind, effectively reducing wind speeds down to zero underneath the blanket. It also keeps the relative humidity between the pool surface and solar blanket higher compared to the ambient air above the blanket. This higher relative humidity acts as a further deterrent for evaporation.