I know, I know. You came here for HVAC content, so why are we talking about pool heaters?

Since we live in Florida and don’t have that much of a heating season, we have to look for other forms of work during the winter months. Pool heaters fit the bill quite nicely since they’re actually quite similar to HVAC equipment. Plus, it’s Florida, so swimming pools are everywhere, especially in vacation rental homes. 

Like HVAC equipment, pool heaters typically come in heat pump and gas varieties (with a few electric heat ones here and there). We’ve already written about pool heat pumps, so we’re going to explore gas pool heaters this time around. We’ll cover their components and some things to keep in mind while installing or diagnosing pool heaters. 

Gas Pool Heater Components

Gas pool heaters have a user interface board, circuit board, and heat exchanger that all facilitate heat transfer from the heater to the pool water. That heat comes from combustion—the rapid oxidation of fuel. 

Gas pool heaters have components and safeties that manage combustion, like a gas valve that allows fuel to enter the appliance. Like gas furnaces, gas pool heaters typically use either natural gas or propane. Natural gas usually comes in from the gas line, and propane usually comes from a tank that may be partially buried underground. Propane tanks will need to be refueled from time to time, so be sure to inspect those to make sure they’re not running too low if a customer says their pool is too cold.

Ignition Components

Again, like gas furnaces, gas pool heaters have an ignition sequence and require igniters (either hot surface or spark) to work. The ignition module senses flame and sends power to the igniter and blower during the ignition sequence. Here are some other important components in the ignition sequence: 

• Hot surface igniters – use resistance-based heat to ignite the incoming natural gas, and they usually run on 120v power
• Spark igniters create an arc to ignite a pilot flame, which will stay on once the board detects ignition and opens the gas valve
• Flame rod (or flame sensor) – a flame rectifier that generates a DC microamp signal to the ignition module; if the module doesn’t receive this signal within three seconds of the gas valve opening, the ignition module will shut the gas valve (thus cutting off the fuel supply) and cycle the heater off
• Stack flue sensor – a thermistor in the flue that proves ignition by measuring the actual temperature rise against a set temperature rise

Safeties

Credit: Pentair MasterTemp™ Pool and Spa Heater Installation and User’s Guide (p. 41)

Gas pool heaters are combustion appliances, just like gas furnaces, and they have many of the same operational and safety risks. Forced-air models also need to have adequate air for ignition to occur. Safeties are in place to break the circuit and stop the pool heater if an unsafe operating condition occurs.

Here are the main safeties we’ll see in gas pool heaters:

• Air switches – check to make sure that there is adequate inducer airflow before ignition occurs in forced-air gas pool heaters
• High-limit switches – mounted on the heat exchanger and have a cut-out temperature to prevent the water from getting too hot and scalding people who use the pool
• Flame rollout switches – mounted just above the entry to the burner tray, and they melt whenever there’s a flame rollout to cut power to the heater

The heater won’t work once the flame rollout switch has opened, so customers need to call a professional to diagnose the heater, fix the problem, and replace the flame rollout switch before they can safely use their pool heater again. 

Bypass Methods

Water may need to bypass the heat exchanger in order to manage temperature in the pool and pressure in the heater. For that reason, gas pool heaters have the following components that allow water to bypass the heat exchanger:

• Manifold bypass – allows water to bypass the heat exchanger should the water pressure reach a potentially damaging level (8 in the diagram below)
• Thermal regulator – bypasses a small amount of water to keep the output water temperature at the desired level (15 in the diagram below)

Credit: Pentair MasterTemp™ Pool and Spa Heater 120/240 VAC Natural Gas/LP Gas Installation and User’s Guide (p. 34)

Common Issues

We’ve already alluded to one common issue that affects gas furnaces as well as gas pool heaters: flame rollout. The flame rollout switch will melt in a rollout condition, making it impossible for the pool heater to run without replacing that safety. When we come across a melted flame rollout switch, we don’t just want to replace it and go on our merry way. We need to identify the cause of the flame rollout to keep our customers safe. We should check the gas pressure and look for things like clogged burners—just as we would if we were diagnosing a furnace.

Water temperature sensor failure is another common problem. Since the probes are immersed in water, it’s possible for the water to get inside the probe and cause it to fail. Electrically diagnosing temp sensor failure is difficult, so the most accurate way to diagnose the problem is to replace the sensor with one that’s known to work and see if there is a difference. 

Another problem that can happen with LP or propane pool heaters is air in the fuel lines. That air can pass through the gas valve and cause the system to fail to finish the ignition sequence, which generates a fault code because failure to ignite indicates a possible explosion hazard. Propane heaters have a large outdoor tank, and the tank could simply be low on propane. If you notice the tank looks ~20% or less of the way full, notify the homeowner that they’ll need to refill it soon.

One of the main advantages that gas pool heaters have over pool heat pumps is that they can handle the heating loads a lot more quickly, especially in colder weather. While that’s pretty convenient for the customer, it also comes with a possible danger: scalding is far more likely. High-limit switches reduce the likelihood of that problem, but that just means we have to make sure we’re checking those high-limit switches whenever we service a gas pool heater.

Environmental Impacts on Equipment

Some issues are at least partially caused by the environment. Humidity, temperature, and wildlife can all affect gas pool heaters

Corrosion is quite common in gas pool heaters due to the proximity of chlorine and moisture. (In climates like Florida, the high temperatures throughout the year also make it worse.) Corrosion can lead to poor burner grounding, and the flame rectification circuit requires proper ground. For that reason, if the burner isn’t staying lit, it’s quite possible that the improper grounding makes it impossible to prove flame. Corrosion could very well be to blame. Inducer fans also often rust out and fall apart. In any case, if you come across corrosion, the best thing to do is to quote a replacement instead of a repair. On top of that, keeping gas pool heaters in this condition is unsafe, and we should NOT work on unsafe equipment that won’t last. 

The electrical circuit can also be damaged by animal activity. Rodents often break into pool heaters and nest in them. They are also notorious chewers and may chew the wiring. Insects can also get inside the equipment and accumulate on things like burners, which clog them and can cause flame rollout. 

In our climate, pool heat pumps are more common, and they usually handle the heating loads pretty well. They also don’t seem to corrode as often, work just like a normal heat pump, and don’t come with all the same risks as a combustion appliance. However, there’s still a lot of value in learning how to install and repair gas pool heaters. We still come across them, and they’re probably even more common in markets north of ours. 

Those of you who already work on gas furnaces are already pretty well equipped to work on gas pool heaters if you want some extra business. A lot of the same components and principles apply; you just have to manage water flow instead of airflow. (And yes, dirty filters are a common problem for pool heat pumps and gas pool heaters alike.) The manual will always be there to help you, and working on pool heaters is one of those things that gets easier as you work on them more often.