Pool Heater Not Working: The Quick Troubleshooting Checklist

If your pool heater is not working, start with these five checks before calling a technician: confirm gas or power supply, verify water flow is strong, check the filter pressure, look at the thermostat and temperature settings, and read any error codes on the display. Most heater failures trace back to one of those five things, and the majority are fixable without a service call. Here is how to work through each one fast.

Why Pool Heaters Fail (The Short Version)

Pool heaters are not complicated machines, but they have a lot of safety switches built in. A pressure switch, a high-limit sensor, a flow sensor, a thermocouple – any one of these can shut the heater down the moment conditions drift outside the acceptable range. That is actually good news for you, because it means the heater is usually protecting itself from something fixable rather than suffering a catastrophic internal failure.

Gas heaters are the most common type in residential pools, so this checklist focuses there, but the flow and chemistry checks apply to heat pumps and electric resistance heaters too. If your heater suffered damage from a recent storm or flood event, this guide on repairing pool equipment after a storm covers the inspection steps specific to weather damage.

Step 1: Check Your Gas or Power Supply

For a gas heater, confirm the gas valve on the supply line is fully open – not halfway, fully open. If you have other gas appliances running fine, the supply line is probably not the issue. If the heater has a pilot light style ignition, check whether the pilot is lit. Most modern heaters use electronic ignition, so look for an igniter click when you try to start it. No click usually means a faulty igniter or control board, which is a technician call.

For a heat pump or electric heater, check the circuit breaker. A tripped breaker is one of the most commonly missed causes of a “broken” heater. Reset it once. If it trips again immediately, stop and call an electrician – there is an electrical fault that needs professional diagnosis.

Step 2: Is Water Actually Flowing Through the Heater?

Pool heaters will not fire without adequate water flow. Most residential gas heaters require a minimum of 25 to 40 GPM moving through the unit. The pressure switch inside the heater measures this, and if flow drops below the threshold, the heater shuts off as a safety measure. The result looks exactly like a broken heater, but it is actually a flow problem.

The first thing to check is your filter pressure gauge. If it reads 8 to 10 psi above the normal clean baseline, your filter is dirty and restricting flow. Backwash a sand or DE filter, or rinse a cartridge filter. Also confirm that the heater bypass valve, if you have one, is in the correct position – a partially open bypass sends water around the heater rather than through it, killing flow to the unit.

Step 3: Read the Error Code on the Display

Modern pool heaters display error codes when something trips a safety sensor. Do not skip this step. The code narrows the problem down from “something is wrong” to “the high-limit sensor is tripping” or “there is an ignition failure.” Common codes you will see on most brands:

• IF or IgnF: Ignition failure – the heater tried to light and failed. Often a gas supply issue, a dirty burner, or a bad igniter.
• HH or High Limit: Water temperature exceeded the safety limit. Check for a stuck valve or a closed return line trapping hot water near the sensor.
• PS or Pressure Switch: Insufficient water flow. Go back to Step 2.
• AFS or Air Flow Switch (heat pumps): The fan is not moving enough air. Check for debris blocking the unit.

Your heater’s manual will list all codes specific to your model. If you do not have the manual, the manufacturer’s website almost always has a PDF version you can pull up in two minutes.

Step 4: Check Your Thermostat and Timer Settings

This sounds too simple to bother with, but it catches more people than you would think. Confirm that the heater thermostat is set higher than the current water temperature. If the pool is already at 84 degrees and the thermostat is set to 82, the heater will not run – it thinks its job is done. Also check whether the heater is on a timer that is currently in an “off” window. Some automation systems have a separate heater enable setting that can get toggled off accidentally.

Step 5: Is Your Pool Chemistry Hurting the Heater?

Water chemistry problems will not usually prevent a heater from firing, but they can cause a heater to underperform and fail early. Low pH – below 7.2 – creates corrosive water that will eat through a copper or cupro-nickel heat exchanger over time. High calcium hardness above 400 ppm causes scale buildup inside the heat exchanger, which acts as insulation and dramatically reduces heating efficiency. If your heater seems to run constantly but barely warms the water, scale buildup is a strong suspect. Keep pH between 7.4 and 7.6 and calcium hardness between 200 and 400 ppm to protect the heat exchanger. AquaDoc makes a calcium and scale control product designed specifically for this kind of buildup in pool equipment, which is worth using if you run hard water or have had scale issues before.

When to Call a Technician

You can handle gas valve checks, filter cleaning, flow diagnostics, and settings adjustments yourself. Call a licensed technician for anything involving the gas line itself, igniter replacement, heat exchanger inspection, or a control board failure. A cracked heat exchanger is serious – it can allow pool water to contaminate the combustion chamber and is not a repair to attempt without experience. The Pool and Hot Tub Alliance maintains a directory of certified service technicians if you need help finding a qualified pro in your area.

If your heater is more than 10 to 12 years old and the repair cost is approaching half the price of a new unit, replacement often makes more financial sense. Newer heaters are significantly more efficient and include better diagnostic displays that make future troubleshooting much easier.

Frequently Asked Questions

Why is my pool heater running but not heating the water?

The most common causes are low water flow from a dirty filter or closed valve, an undersized heater struggling with cold air temperatures, or scale buildup inside the heat exchanger reducing its efficiency. Check your filter pressure gauge first, then verify all valves are fully open.

Why does my pool heater keep shutting off?

A heater that cycles off repeatedly is responding to a safety shutoff – usually low water flow, a high-limit sensor trip, or a pressure switch fault. Clean or backwash your filter, confirm strong flow through the unit, and check the display for an error code pointing to the specific trigger.

Can low pool pH damage a pool heater?

Yes. Water below pH 7.2 is corrosive and will degrade a copper or cupro-nickel heat exchanger over time. Keep pH between 7.4 and 7.6 consistently to protect the heater. If you need to lower your pH without overshooting, this guide on lowering pool pH safely walks through the process step by step.

What water flow rate does a pool heater need to operate?

Most residential pool heaters require a minimum flow rate of 25 to 40 GPM to stay running. A clogged filter, partially closed bypass valve, or undersized pump can drop flow below that threshold and trigger the pressure switch, shutting the heater down entirely.

Should I call a technician for pool heater problems?

You can diagnose and fix many issues yourself, including gas valve checks, filter cleaning, flow problems, and thermostat settings. Anything involving the gas supply line, igniter replacement, or heat exchanger repair should be handled by a licensed pool equipment technician.

A heater that is not working is almost always giving you a clue – through an error code, a pressure gauge reading, or a chemistry test. Work through this checklist in order and you will either fix it yourself in 20 minutes or walk into a service call knowing exactly what to tell the technician.