How to Troubleshoot a Gas Pool Heater


As you might expect, there are dozens of ways that a heater can break down. Addressing all of them would turn this guide into a manual, so we will just address the common problems. If you use our pool heater troubleshooting checklist below, you will probably find the common problems and their causes. Below the checklist, we’ll go a bit deeper into troubleshooting and fixing some of the most common pool heater problems.

Common Gas Pool Heater ProblemsPotential Causes
No Hot WaterA) Heater is on standby
B) Heater is set to vacation mode
C) No power to the water heater
D) The heater isn’t capable of fulfilling the demand
E) The high-temperature limit switch is open
F) The upper-limit temperature sensor isn’t working
There Isn’t Enough Hot Water
The Water Is Slow to Heat
High Operating Costs
A) The air filter is dirty
B) The heating element isn’t working
C) The hot water usage exceeds the heater’s current mode or capabilities
D) There is a hot water leak in the faucet or the pipes
E) There is sediment or calcium scaling buildup in the tank
F) The temperature set-point is set too low
G) The water connections are reversed
H) Too much heat is being lost while traveling in pipes that are too long
Water Is Dripping from Temperature & Pressure Relief ValveA) There isn’t a thermal expansion unit, or the thermal expansion needs to be serviced
B) The water pressure is too high
C) The temperatures & pressure relief valve is not working
OtherA) The heat pump doesn’t run when it is set to efficiency mode
B) The water heater doesn’t start right away

Let’s focus on some of the most common problems

  • The heater will not heat/slow to heat
  • The heater cycles on and off
  • The pilot light will not light
  • The heater will not ignite

Voiding Your Warranty

Before you start to dig around the insides of your pool heater, you need to know that tinkering around the heater has the potential to void your warranty. Read the fine print of the terms and conditions to see if you need to obtain authorization from the manufacturer. When you are certain that you aren’t about to cause yourself any issues, we hope that we can help you troubleshoot your gas pool heater.

Common Gas Pool Heater Problems

Heater Will Not Heat or Is Slow to Heat to the Correct Temperature

Possible Causes

  • C1: The simplest possible cause might be that the temperature is simply set too low.
  • C2: The heater has a set timer that is too short, and it is shutting off before it reaches the correct temperature.
  • C3: The heater might be underpowered for the size of your pool.
  • C4: The gas supply might be insufficient for the heater. If the gas has been insufficient from the start, you should check gas-pipe sizing charts to see if the pipes are appropriately sized for your needs.
    • If your low-pressure issues developed over time, you should probably head over to our guide on troubleshooting low-pressure issues.
  • C5: The capabilities of your pool heater are insufficient. Pool heaters fight against the cold, but they do not always win that fight. If it’s wintertime, it’s possible that your heater is being overpowered. You will either need to upgrade your pool heater or wait till spring or summer comes around again
  • C6: The high limit switch might be faulty. When these switches go bad, they sometimes send the signal to turn off the heater before the correct temperature is reached
  • C7: Using a variable speed pump on too low of a setting. As we mention a few times in this guide, pool heaters require a minimum gallons per minute (GPM). If you have a variable speed pump that is set at too low of a setting, the pump may not be supplying the needed amount of water to the water heater. Try cranking up the GPM to see if that fixes your issue.

Heater Cycles On and Off

This is one of the most common, if not the most common, pool heater issue. It is generally caused by poor water circulation. Most gas pool heaters require a minimum of 40 Gallons Per Minute (GPM) in order to function. Some heaters will function with as little as 10 GPM, but that often leads to problems with either overheating or the heater cycling on and off. 

Time and again, you will find that knowing the optimal PSI of your pool pump is useful to know for pool maintenance. By optimal, we mean that the pump is working correctly and the filter is clean.

Damaged Heat Exchanger

Over time, the debris will clog the pool heater’s heat exchanger and pool chemicals will destroy it. Checking on it once a month should be plenty. If the heat keeps cycling on and off, it might be a good idea to start that routine by checking on it today.

The Thermostat Might Be Broken

It’s pretty self-explanatory. If the temperatures can’t be detected, there won’t be any reason to turn on the heater. 

If the Water Pressure Has Climbed

One of those reasons is that you can see the incremental changes to that pressure that let you know when it is time to clean the filter. If the pressure is 8 PSI or higher than the standard pressure, it might be time to clean, backwash, or change that filter. If the filter is too clogged, the water flow might be slowed down to the point of affecting the water heater’s ability to warm up the pool.

While you are cleaning the pool pump’s filter, you should also check the skimmer basket and the pump impeller. Whatever debris is in there is increasing the PSI, but is also dropping the water pressure. If there is a lot of debris, the low water pressure will likely shut off the water heater. If it doesn’t, the water heater will probably burn itself out.

If the Water Pressure is Too Low

Assuming that you have an appropriately sized pool pump, low pressure that has developed over time can often be solved by cleaning the following components:

  • Pool pump filter
  • Skimmer basket
  • Pump impeller’s eye

Water Levels are Too Low

Your water levels should be high enough to reach the midpoint of the skimmer. If they go higher or lower than that, your water pressure will be negatively affected.

Broken Water Pipe

Have you noticed that your water levels keep dropping faster than you expect? You might have a leak in the water pipe that returns water from the water pump. 

Air Leaks

If your pool pump has developed an air leak, it will lower your water pressure. An easy way to check for this is to look at the return jets. You should only see water returning to the pool. If you see air bubbles, your pool pump has almost certainly developed a leak. If the air leak is sufficiently large, your pool pump will lose its prime.

Most of the time, you can find the source of this issue at one of three locations: the drain plugs, the lid, or the inlet fitting. You often fix the leak by swapping out any worn-out o-rings. 

If everything seems good at the pump, the leak is probably located somewhere before the pump. Check the diverter valves and any fittings or unions.

The Pilot Light Will Not Light

If you can’t get your pilot light to actually light, there are several easy-to-check potential causes to this problem.

Thermocouple Has Gone Bad

The thermocouple is a creative solution to prevent unburned gas from being released into the air. When the thermocouple is warmed by the pilot light, it sends an electrical signal that keeps the gas line open. This supplies gas to the pilot light. When the pilot light is shut off, the thermocouple cools, stops sending its signal, and closes the gas lines.

Over time, a thermocouple will develop a sheathe of carbon which hinders its ability to send its signal to the gas line. When this happens, the pilot light does not have enough gas to remain lit. 

You should use a multimeter to test its signal. If it isn’t working, you will need to either clean or replace the thermocouple. It’s cheap and easy to swap.

Loose Connections

Sometimes parts, such as the thermocouples or coils haven’t gone bad, they just need tightening. Feel around the connections and see if anything stands out. 

Insufficient Gas Pressure

Assuming that the gas is set to “on,” there are a couple of other common reasons for low or nonexistent gas pressure; either there is poor air supply or bad venting.

If the pilot light has never worked, there’s a chance that the pipelines might be too small to supply the gas. 

Flooded Vents

If there is enough gas pressure, it might be a good idea to check the vents for flooding. If your heater is positioned poorly, water pouring from the roof or water from the sprinkler systems can be the source of your problem. It’s uncommon but common enough to be worth checking

Clogged Pilot Light

Along the same vein of thought, insects and rust have been known to clog up the hole for the pilot light.

Heater Will Not Ignite

There are a few obvious yet easy-to-miss points that can keep the heater from turning on and heating the pool.

  • Check the heater switch. Is it set to “on?”
  • Check the gas supply valve. Is it set to “on?”
  • Check the thermostat settings. Is the current water temperature higher than the current setting?
  • Check the thermostat contacts. Some models, especially the older ones, have a tendency to get dirty. Whenever an older thermostat system sits unused for an extended period of time, dirt has a tendency to collect on those contacts. For this reason, you should turn the thermostat knobs back and forth a few times. This can clean off some of the accumulated dirt and return the heater to an operational state.

Broken Pressure Sensor

As we mentioned earlier, heater pumps protect themselves by sensing the water flow. If it senses that there is a sufficient GPM to keep itself operational, it will heat up the water when appropriate. If that sensor is broken, the water heater will likely never work. If you want to test your pressure sensor, you will need to do the following:

  • Step 1: Check the filter. If the filter is dirty, it can slow the flow to the point of tripping the sensor.
  • Step 2: Turn on the pool pump. 
  • Step 3: Use a multimeter to test if power is getting to the pressure switch.
  • Step 4: If the multimeter isn’t detecting power, you probably need to replace the sensor.

Too Many Water Features

Your water pump might be overburdened by the needs of your pool and water features. Let’s say that you have a 15,000-gallon pool. Now let’s assume that it requires around a 20 GPM output from your pool pump. 

If your pool pump has an output of 40 GPM that is being sapped by a water feature that requires 30 GPM, it will only leave you 10 GPM for the pool. That might be the reason that the water heater isn’t working.

The Heater is Leaking

Corroded and Weather-Worn Parts

Pool chemicals and fluctuating temperatures wear down components. If given enough time, all of the pool components will become corroded. It’s time to take a look inside and see if anything catches your eye. Replace anything rusted and worn to the point of malfunctioning.

Loose Connections

The leak might be as simple to fix as tightening a few nuts and bolts and replacing some worn-down washers.

Heater Leaks When Burner is Lit

There are a few potential reasons for this type of a leak


The apparent leak might not be a problem at all. When temperatures are a bit colder, the cold water mixing with the heat of the burner causes condensation. 

Bad O-ring in the Winterizing Plug

When the colder temperatures hit, owners can use the winterizing plug to drain water from the areas. Draining the water prevents damage from the ice when those cold temperatures drop to freezing temperatures. When the temperatures warm up again, many find that the winterizing plugs o-ring.

  • Condensation, heating cold water
  • Missing or damaged internal bypass in the front header
  • Excessive water flowing through the heater from an oversized water pump
  • Check heat exchanger for any buildup of soot and make sure internal bypass is working
  • For high-speed pumps, installing an external bypass might be needed to reduce water flow coming from the heater

Rust in the Pool

As we mention a few times, pool chemicals can be harsh on the pool equipment. The resulting corrosion produces and releases rust in the swimming pool. This becomes the most noticeable when the heater is running. If you have a cast iron heater, you are more likely to encounter this particular problem.

Top is Black and/or the Exhaust is Dark

Inefficient heaters can blow out dark exhaust and blacken their containers in a similar manner to an inefficient truck that blows its unburned black exhaust out its tailpipe. This is very likely going to be an airflow problem. Test for poor air supply and issues with venting.

If you live in an area with frequent strong winds, your heater might have issues due to a strong downdraft that is throwing off the airflow. If this is the case, you might need to install a high wind stack to obtain sufficient airflow.

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