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How Do Pool Heaters Work? A Complete Explanation

Pool heaters need adequate water flow, gas, and electricity to function. They’re one of the most complex pieces of equipment for your swimming pool. Knowing how your pool heater works will help you prevent long-term damage. It’ll also allow you to adjust the settings and get the most out of your investment.

Pool heaters work by heating coils with ignited gas. The coils heat a condenser, which warms the water flowing through the heater. Swimming pool heaters have sensors that tell the thermostat when to increase the temperature, so there won’t be a flame when the water is hotter than the heater.

In this post, I’ll explain how a pool heater works, how often you should run it, and whether or not they’re worth it. I’ll also dive into the plumbing requirements for pool heaters and how you can add them to existing in-ground pools.

How Does a Pool Heater System Work?

A pool heater works by using gas or electricity to heat the coils and warm the water that passes through it. The pump needs to be on for the heater to warm the water. Gas heaters use electricity for the control board and display, but their main power source comes from a gas line from your house.

Keep these things in mind when using a pool heater:

  • Pool heaters can be used for spas, too.
  • The heater’s BTUs matter much more than their physical size (pool heaters often range from 150k BTU to 450K BTU).
  • The pump, filter, and booster pump need to go before the heater, whereas an optional salt cell needs to go after the heater in the plumbing.
  • All pool heaters have unique heater ranges, so it’s important to know what the make and model can do before installing it into your equipment pad.

Do You Run A Pool Heater All The Time?

You don’t run a pool heater all the time unless you want to keep the pool warm at night. Pool heaters maintain the temperature, but they won’t reduce it if the pool is hotter than the thermostat’s setting. The heater automatically ignites when the thermostat shows it needs to heat the water.

Here’s what you should know about running your pool heater all of the time:

  • Running a pool heater non-stop can cause long-term wear and tear. Pool heaters last a long time, but they can break apart if they’re used too often. You shouldn’t have to use your pool heater for more than a couple of hours per day. Spas and winter months typically require more heating, though.
  • Using your pool heater without turning it off will skyrocket your utility bill. Pool heaters use a lot of gas and electricity while they’re heating the pool. They stop using these energy sources when the pool is at the right temperature. However, I suggest turning off the heater at night since it’ll run non-stop.
  • Pool heaters should stay on and will automatically gauge when they need to ignite and heat the water. There’s a difference between leaving your heater on and running it all of the time. Activated pool heaters only heat when the water is below the desired temperature, while non-stop heaters always ignite and consume gas.
  • You can connect a pool heater to an automation system to turn it on and off with a remote. Automation systems work for Jandy, Hayward, Pentair, and Jacuzzi products. You can use them with pool pumps, salt generators, heaters, lights, etc. You’ll be able to adjust the temperature to ensure the heater doesn’t consume too much gas.

How Long Does A Pool Heater Take To Heat A Pool?

Pool heaters take between 1 to 48 hours to heat, depending on the pool’s size, the external temperature, the temperature setting, and the pump’s speed. Older heaters typically take much longer than new heaters because they don’t have the same ignition power. Furthermore, new heaters have much more efficient components for heat distribution.

Let’s take an in-depth look at what affects how long your heater takes to warm the pool:

  • Your pool’s gallon total is the most important factor. Big pools take much longer to heat than small pools. A deeper pool also takes longer because the warm water has to circulate to the lowest part of the water. You’ll have much better luck with a pool that has a uniform height.
  • The outside temperature can cool the water, making it harder to heat. Freezing temperatures make it much more difficult to heat a swimming pool. Keep a cover on the pool while you heat it to retain as much heat as possible. You can also use liquid solar covers if you don’t have a solar blanket.
  • The temperature difference will determine if it takes a couple of hours or more than a day. For example, if you set the heater to 80 degrees and it’s 30 degrees outside, it’ll take a couple of days. However, if you set the heater to 70 degrees and it’s 65 degrees outside, it’ll only take a couple of hours.
  • Faster pumps heat the water quicker, which is why I recommend getting a variable-speed pump. Turn the pump to its highest setting while you wait for the pool to heat, then lower it to its normal RPMs when you’re done. Variable-speed pumps save money and are much quieter than traditional single-speed and two-speed pumps.

Do Pool Heaters Use A Lot Of Electricity?

Gas pool heaters typically use between 500 to 1,000 watts per hour, whereas electric pool heaters use up to 5,000 watts per hour. Gas heaters often get most of their power from igniting the gas, which can cost up to $500/month. On the other hand, electric heaters can cost between $200 to $700/month.

Unfortunately, there’s no such thing as an energy-efficient pool heater. Solar heaters are getting better, but they’re not as effective as gas-powered or electric heaters. You can pair your heater with a solar cover to save electricity and gas, though. If your pool heater isn’t working, you likely need to upgrade the breaker to handle more voltage.

Are Pool Heaters Worth It?

Pool heaters are worth it because they warm the water and extend the swimming season by at least three months. Swimming pools often take a couple of months to get warm on their own, especially if you don’t have heaters, covers, etc. Deep pools take much longer than shallow pools, making heaters even more worth the investment.

So, should you get a pool heater? Ask yourself these questions:

  1. How often do you swim in the pool? If you love swimming, why not enjoy your preferred temperature? You can set the heater to its maintenance mode to prevent the water from getting colder than 70 degrees Fahrenheit, making it usable throughout the year (especially if it’s an indoor pool).
  2. Do you have solar covers or other heating methods? Solar covers help you save money on your gas and electricity bill by using the heater less. You can heat the pool to 75 degrees and watch as the solar cover maintains it without needing more gas or electricity. It could even hike the temperature by a few degrees.
  3. Does your pool get cold enough to freeze the equipment? Running your pump prevents freezing, but you can heat the water to keep the PVC pipes warm during the winter. Freezing temperatures can crack the plumbing and ruin the equipment, so a heater can be a helpful preventative tool.
  4. Do you want a spa someday? You can plumb the heater into an in-ground spa, then use diverter valves to choose if you want to heat the spa, pool, or both. All heaters can be used for in-ground pool and spa combos. Attach the valve to an automation system for automatic heating switches.
  5. Do you enjoy swimming during the off-season? Most people swim in their pools between May and August, but adding a heater can let you swim comfortably between April and October. Those in moderate climates can swim throughout the year since the temperature likely won’t get below 65 degrees Fahrenheit.

What Do You Need for a Pool Heater?

You need plumbing, electrical wires, and a gas line for a pool heater. Make sure there’s enough space on the equipment pad to handle a full-sized heater. Check which heater size you need, then ensure you can place it before the salt system if you have one. Don’t forget to hire a professional to run a gas line to the heater.

Below, I’ll explain each heater requirement in further detail.

  • Proper plumbing: Pool heaters use 1.5” or 2” plumbing, so you need to know which size the heater needs before adding it to the equipment pad. I suggest avoiding adapters since they’ll limit the water flow and reduce the pressure. This issue puts more pressure on the unions instead of the heating element, which can cause leaks.
  • Electrical wires: Make sure there’s enough room in the breaker box for a new heater. All pool heaters use three wires (black, red, and green in most cases). The green wire grounds the heater, while the red and black wires provide the power source. Make sure you know if the heater requires 110v or 220v.
  • Gas lines: Many pool services avoid installing gas lines, but you can hire an expert to extend your home’s gas line to the heater. If you already have one, all you have to do is use pipe dope and threaded galvanized pipe to supply gas to the heater. Tighten the couplings and elbows with a pipe wrench to prevent leaks.
  • Adequate space: Pool heaters typically have the biggest footprint on the equipment pad. They can range from a 3’ x 3’ footprint to a 4’ x 4’ footprint. There needs to be enough room to accommodate the heater and ensure there are about 18 inches on all sides for ventilation and airflow.

Can You Add a Pool Heater to an Existing In-Ground Pool?

You can add a pool heater to an existing in-ground pool. All you need to do is run a gas line, attach the necessary wires to the breaker box, and plumb it into the PVC pipes. Pool heaters can work with corrugated plastic hoses. Most of them can’t work with copper pipes, either. You can also add a heater to an above-ground pool if it uses PVC pipes.

If you’re looking for a new pool heater, consider our list of the best gas pool heaters on the market. There’s something for everyone in any budget range.

The main thing I’d suggest to anyone getting a pool heater for their existing pool is to make sure they get the right size. Pool heaters come in a wide BTU range. Below, I’ll add a table to help you determine which heater size you need.

Pool Gallon TotalBTU MinimumMax BTU (Don’t Exceed)
10,000 gallons50,000 BTU100,000 BTU
15,000 gallons85,000 BTU125,000 BTU
20,000 gallons110,000 BTU250,000 BTU
25,000 gallons125,000 BTU400,000 BTU

Quick Reminder: Pool companies list BTUs as ‘XXXK,’ so a 450,000 BTU heater will be called a 450K BTU heater.

How Hot Can a Pool Heater Make the Water?

A gas pool heater can make the water 40 degrees (Fahrenheit) hotter, whereas most electric pool heaters can make the water about 10 to 15 degrees hotter. Gas heaters work much quicker because the ignition process is instant rather than slowly increasing like an electric heater. Furthermore, the flame can get much hotter than the electric sensor.

Pool heaters can usually make the pool about 80 degrees Fahrenheit before they experience trouble. However, they can heat a spa up to 105 degrees Fahrenheit. It’s much easier for a pool heater to heat a spa because it’s more shallow and has fewer gallons than a swimming pool. Pool heaters work best during the hottest part of the day when the pool has direct sunlight.