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Swimming Pool Heaters: How to Pick a Replacement Unit

Posted by Blue Haven Pools & Spas on Dec 5, 2016 12:54:02 PM

What to look for when it’s time to buy a new gas heater for your aquatic retreat


A quality swimming pool heater can extend your family’s swimming season by several months, allowing you to enjoy the water into autumn in many parts of the country—even into winter in some warmer regions.

Controlling the water temperature means you never have to dive into an icy cold pool. If you have an inground spa with your pool, it means being able to quickly fire it up for hot, relaxing hydrotherapy after a long hard day.Today’s pool heaters are engineered to provide many years of reliable, efficient performance. But like any piece of equipment, it will eventually wear down and need replacing.

If the time has come to replace your pool heater, be informed before you buy: After all, you want the most efficient one possible for your pool and how you use it. Here’s what you need to know when selecting a new unit.

Three categories of heaters from which to choose

Pool heaters come in three basic types: gas, so­­lar, and electric heat pump. In some regions around the U.S., you will see solar or heat pumps capturing a meaningful share of the market, but nationwide by pure numbers, most pool owners opt for natural gas units. 

There are many reasons why. The first is that in most metropolitan areas across the country, natural gas is easily accessible and relatively affordable. And many homes are already piped for natural gas, making it fairly easy to plumb and install a gas heater.

Gas heaters also tend to last longer than their solar or heat pump counterparts. With proper installation and maintenance, today’s top-line gas units typically last seven to 10 years.

Now, depending on where you live in the U.S., a solar heater may be a smart option—especially if you live in an area with abundant sunshine or in a state that offers rebates for purchasing this green technology.

Solar systems do require a high upfront investment, and while they won’t heat your pool as quickly as a gas heater, this eco-friendly system will cost much less to operate and will save money over the long run—particularly if you want a heated pool for a good portion of the year.

Another alternative for heating backyard pools is an electric heat pump. Like an air conditioner in reverse, these electric-powered units draw heat from the ambient air.

They deliver heat more efficiently than gas heaters, and they work particularly well in warm-weather areas (except for certain desert climates).

However, these systems don’t heat the pool or spa as quickly as gas heaters, and they initially cost more to purchase. For those—like regular lap swimmers—who want a heated pool virtually year round for near daily use, a heat pump remains an excellent option.

Advantages of gas heaters

Gas pool heaters run on either natural gas or propane. They work by drawing water into a combustion chamber. The heat produced inside the chamber gets transferred to the water; then the warmed water returns to the pool.

Because gas heaters have the quickest heat transfer, they make a great choice if you need to boost pool temperature for short time periods or if your primary goal is to quickly warm an inground spa. And unlike heat pumps and solar systems, gas heaters can help boost water temperature regardless of the weather or available sunshine.

Most gas models carry a lower price tag than a heat pump or solar installation. Depending on the size and thermal capacity, gas pool heaters range from $1500 to $3500, including standard installation. (However if you need to install a gas-supply line, that will mean an additional cost.) 

Choosing the right gas heater for your pool and how you use it

If your current heater has served you well, the easy choice is to replace it with the same size and model. However, before going this route, do your homework. It can pay off big-time with real savings; equipment manufacturers are constantly making gas heaters more efficient and more affordable to operate.

If you installed your current heater more than three to five years ago, or if your pool heating bills seem higher than they should be, taking the time to investigate today’s models could save you serious money in the long run!

To select the right replacement heater for your pool, focus on three key elements: size, efficiency, and operating costs.

1. How to get the right size gas heater for your pool

The “size” of a gas pool heater is determined by the amount of heat it puts out per hour. This performance is referred to as BTU (British thermal unit) output. The measurement reflects the amount of work needed to raise the temperature of one pound of water by one degree Fahrenheit.

BTU ratings on gas pool heaters range from 75,000 for the smallest units to 500,000 BTU for the largest. Keep in mind that for convenience, these BTU numbers are often shortened by dropping the zeros; don’t be surprised to see numbers like “100” vs. “100,000”.

—BTU guidelines

As a general rule of thumb, pool professionals recommend the following BTU ratings based on pool size and surface area:

Heater Size                 Pool Size in Gallons               Pool Surface Area*

100 - 200 BTU            1,000 - 10,000 gals                  up to 300 sq. ft.

200 - 300 BTU            10,000 - 20,000 gals                up to 500 sq. ft.

300 - 400 BTU            20,000 - 40,000 gals                up to 800 sq. ft.

400-plus BTU             40,000 - 80,000 gals                up to 1200 sq. ft.

*For example, a rectangle pool measuring 15 feet wide by 30 feet long has a surface are of 450 square feet.

Determining the heater BTU rating that’s right for your pool involves others factors as well, starting with how and when you plan to use the heater.

—Heater usage

Gas heaters add one to two degrees of heat per hour to the average-size pool. So, if you plan to heat your water only for a few days at a time, you’ll need a large heater to achieve quick results.

If you prefer to maintain a constant temperature—at least for several weeks at a time or throughout the summer—a smaller size unit will do the trick. It takes longer to raise the water to your desired temperature, but a lower-BTU heater can still add a few degrees of warmth fairly quickly if needed.

—Weather conditions

The temperature outside can significantly impact how well your pool heater performs. Your choice of unit should take into account the following variables: humidity levels, the difference between pool temperature and average air temperatures, the amount of wind exposure around the pool surface, and temperatures on cool nights.

Pools exposed to frequent high winds, lower humidity levels, and chilly evening temperatures almost always need more power; to compensate for external forces that cool the water, these pools require heaters with higher BTUs to efficiently boost pool temperature.

—Using the heater size formula

To get a rough idea of the right heater size for your pool, follow these steps:

  1. Decide on the temperature you want to keep the pool water. Let’s call this “DWT.”
  1. Determine the average water temperature, “AWT,” for the coldest month you use the pool. To arrive at this number, take the temperature of your pool water at the same time each day for a month; then calculate the average.
  1. Subtract the AWT from DWT.

This formula will tell you how many degrees you need a heater to raise the water temperature.                      

Next, calculate the surface area of your pool in square feet. To determine the required BTU/hour output, multiply the surface area by the needed degrees of temperature increase. Then multiply that number by 12.

Suppose your pool’s surface area equals 600 square feet. You want to keep the water temperature at 75 degrees, and your average water temperature during the coldest month (which you calculated in step 2 above) is 60 degrees.

75 – 60 = 15

15 x 600  = 9,000

12 x 9,000 = 108,000 BTU

In this example, you would need a heater with a BTU rating of 108,000 (or 108 after dropping the zeroes) to efficiently heat the pool.

If you don’t feel comfortable calculating the BTU rating, an experienced pool professional can perform a sizing analysis to determine the correct heater size for your pool.

Tip: Selecting the BTU rating that’s right for your particular pool will help your heater work more efficiently—and help minimize operating costs.

2. Evaluating heater efficiency

Heater efficiency determines how much it will cost to operate the unit on a monthly basis. The more efficient the unit, the lower your fuel costs!

Heater efficiency is defined as “the ratio of usable output (heat) to energy input (fuel).” No heating system is 100% efficient—some of the heat produced always gets lost in the transfer from the heater to the water. A pool heater’s efficiency indicates how much useful heat will be produced for the amount of gas used.

Suppose your pool heater produces $40 worth of useful heat for every $50 of fuel consumed. That means 20% of the fuel is lost, and the resulting heater efficiency rating is 80%. The higher the rating, the less you will spend on gas to fuel the heater.

Many of today’s gas heaters have efficiency ratings as high as 90 to 95 percent. If you have an old, inefficient unit, replacing it with today’s high-efficiency technology can save hundreds of dollars each year in fuel costs!

The following table illustrates annual savings (for every $1,000 in annual pool heating costs) by switching from a low-efficiency heater to one with a 95% efficiency rating. (Dollar costs will vary based on local gas costs.)

Current Heater          Cost w/95% Efficiency         Annual Savings

55%                               $580                                      $420

60%                               $630                                      $370

65%                               $685                                      $315

70%                               $735                                      $265

75%                               $790                                      $210

80%                               $840                                      $160


When heating a pool/spa combo, installing a 400,000 BTU pool heater will provide the quickest heat up of the spa.

Tip: High-efficiency models of gas heaters cost more to purchase, but they can save you hundreds of dollars each year with lower fuel costs.

3. Determining heater operating costs

The cost to operate a gas pool heater depends on several factors. Let’s take a look at what they are and how they make a difference.

—Minimum size (BTU’s)

Size can matter. While most pool builders may recommend a large-size unit on a new pool, if you are replacing an existing unit, it’s vital to get one with the correct minimum BTUs for your pool.

A unit a high BTU does use more energy, but will also heat your pool quicker—thereby decreasing power demand.  An under-sized heater will have to run more often; along with consuming more fuel, the longer operating times will decrease the equipment’s lifespan.

—Type of fuel

Gas heaters can burn natural gas or liquid propane. Due to its wider availability in some regions and its lower price, most pool owners opt for natural gas. However, in some regions of the U.S. and in some residential properties (often rural ones), natural gas is not available and propane is—sometimes at more than double the cost.

—Regular heater usage

How and when you use the heater has the biggest impact on operating costs. The longer you run the heater and the higher you want the water temperature, the more it will cost to operate.

One way to slash heating costs is to cover your pool and spa. Using an inexpensive solar blanket when the pool & spa is not in use helps retain heat. As a result, heat up time—and costs—can be slashed.

Tip: To minimize operating costs, avoid buying a heater with higher BTU rating than your pool really needs.

—The specific heater model and features

Altitude: If you live in an area above 2,000 feet, be sure to purchase a heater manufactured for this kind of high elevation.

Durability: Units made with a heat exchanger and tubing made of cupro-nickel—vs. copper—hold up better in pools with salt systems and fluctuating pH levels.

Ignition: Safer and cheaper to use, electronic ignition units light the burners with an intermittent spark. This technology means your heater will not need a pilot light burning all the time.

Dual thermostat: If you have a pool and a spa running off the same heater, get a dual thermostat. This way, you can fire up the only the spa without raising the pool temperature at the same time.

Forced air: Gas heaters with a forced draft system can eliminate outside weather variables, such as high winds, that can reduce heating performance.

LED control panel: This type of electronic display makes it easy to monitor you heater’s operating status and make adjustments with the touch of a button. Some even let you know when maintenance is needed.

Emission ratings: States like Texas and California mandate maximum NOx (nitrous oxides) emission levels for pool heaters. Even if you don’t live in one of these states, look for a “Low NOx” heater that meets these tough standards. This designation is a great indication that the heater is a highly efficient system that’s gentler on both the environment and your wallet. 

The takeaway

Picking a replacement for your pool’s heater should be done with care. Know how and when you plan to use the heater, figure out the right BTU rating for your size pool, and use the natural gas option when this fuel is easily and affordably available.

A unit that’s the right make, model, and size for your pool will provide years of efficient heating performance. With the best technology on your backyard resort, your family can make the most of warm, comfortable pool water.

Topics: Equipment/Technology, Buyer Tips, Maintenance, Heating


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