Pool Heater Not Working: The Quick Troubleshooting Checklist

If your pool heater is not working, start with the four most common culprits before calling anyone: low water flow (usually a dirty filter or closed valve), a tripped breaker or gas supply issue, a thermostat set too low, or an error code the heater is already trying to tell you about. About 80 percent of heater calls get resolved by checking these basics first. Here is the full checklist to walk through in order.

Why Does a Pool Heater Stop Working in the First Place?

Pool heaters are actually pretty reliable machines, but they depend on everything around them working correctly. The heater needs adequate water flow, a clean return path, proper fuel or power supply, and balanced water chemistry. When any one of those conditions fails, most modern heaters are designed to shut themselves off rather than overheat or corrode. That safety behavior is good in the long run, but it means a single clogged filter basket can look like a “broken heater” from the outside.

Gas heaters (natural gas and propane) have the most components to check: ignition, gas pressure, venting, flow sensors, and high-limit switches. Electric resistance heaters are simpler but more sensitive to power supply issues. Heat pumps add refrigerant and fan mechanics to the mix. The checklist below covers all three, starting with the checks that apply to every type.

Step 1: Check Water Flow First

Low or restricted water flow is the single most common reason a pool heater shuts off or refuses to fire. Most heaters have a flow sensor or pressure switch that physically prevents ignition if the water is not moving fast enough through the heat exchanger.

1. Check your filter pressure gauge. If it is reading 8-10 psi above the clean baseline, your filter is dirty and restricting flow. Clean or backwash it before you do anything else.
2. Open all valves on the heater bypass. Many pools have a three-way valve that partially diverts water around the heater. Make sure flow is routing through the heater, not around it.
3. Clean all pump and skimmer baskets. A clogged pump basket can starve the heater of flow even with a clean filter.
4. Confirm the pump is running at full speed. Variable-speed pumps set to an economy speed often do not push enough GPM to satisfy the heater’s minimum flow requirement. Run the pump at full speed while heating.

Step 2: Check Power, Gas, and Fuel Supply

For electric heaters and heat pumps, go to your breaker panel and confirm the heater’s breaker has not tripped. A tripped breaker will sit in a middle position, not fully off and not fully on. Reset it once. If it trips again right away, stop – there is a wiring or equipment fault that needs a technician.

For gas heaters, confirm the gas supply valve on the line leading to the heater is fully open. If you have a propane tank, check the gauge – propane heaters are notorious for running empty mid-season and the symptom looks identical to a mechanical failure. Also check that no other large gas appliances tripped a safety shutoff on your home’s gas system.

While you are at the heater, look for any error codes or indicator lights on the display panel. Modern pool heaters – brands like Pentair, Hayward, and Raypak – all have digital readouts with fault codes. Write the code down and look it up in your owner’s manual; it will often point you directly to the problem.

Step 3: Check the Thermostat and Temperature Settings

This one sounds obvious, but it gets skipped constantly. The heater’s set point must be higher than the current water temperature by at least 4-5 degrees for the heater to engage. If your pool is 78°F and the thermostat is set to 78°F, the heater will do nothing – it thinks the job is done. Bump the set point up to 84°F and see if it fires within a few minutes.

Also check whether your automation system or app has overridden the heater’s local thermostat. Pool automation controllers (Pentair IntelliCenter, Hayward OmniLogic, etc.) can have their own temperature programs that conflict with what you set at the heater itself. If you have an automation system, manage the temperature there, not at the heater directly.

Step 4: Inspect the High-Limit Switch and Reset

Every pool heater has a high-limit safety switch that trips if the water inside the heater gets too hot – usually from a sudden loss of flow while the heater was running. When it trips, the heater locks out completely until the switch is manually reset. This switch is typically a small red button inside the front access panel of the heater.

To reset it: turn the heater off, wait 30 seconds, open the panel, press the red button firmly until you feel it click, close the panel, and power the heater back on. If it trips again within a few minutes, the root cause (almost always low flow) has not been fixed yet. Address the flow issue first, then reset.

Step 5: Look at Your Water Chemistry

Water chemistry will not usually cause a sudden heater failure, but it is a slow killer. Low pH (below 7.0) is acidic enough to corrode copper heat exchangers over a single season. High calcium hardness (above 400 ppm) scales up the heat exchanger and steadily reduces efficiency until the heater can no longer transfer heat. Keep pH between 7.4 and 7.6 and total alkalinity between 80 and 120 ppm. AquaDoc’s pool chemicals are formulated with this balance in mind, which is why some pool owners use them as their weekly maintenance line. If you have been fighting chemistry problems all season, that history may be connected to a heater that is suddenly underperforming.

If you are having broader equipment issues beyond the heater, the guide on how to repair pool equipment after a storm covers damage assessment that applies to most pool systems. And if you have been thinking about this situation as a reason to upgrade your setup, there is a solid overview of upgrading pool equipment for better efficiency worth reading.

Heat Pump Specific: Is the Fan Running and Is It Cold Outside?

Heat pumps have two failure modes that gas heaters do not. First, if the fan on top of the unit is not spinning, the heat pump cannot extract heat from the air and will shut down. A failed fan capacitor is a common and inexpensive repair – but it does require a technician. Second, heat pumps lose efficiency rapidly when air temperatures drop below 50°F, and most models will not operate at all below 45-50°F. If you are trying to heat your pool in early spring or late fall on a cold day, the heat pump may simply be outside its operating range.

Note: if you also own a hot tub and are dealing with a separate heater issue there, the process has some overlap – repairing a hot tub heater that’s not working covers the spa-specific steps. Pool and spa heaters share some failure modes but differ in flow rates and control logic.

For anything beyond what this checklist covers – cracked heat exchanger, failed ignition board, gas valve replacement, refrigerant issues on a heat pump – get a licensed pool equipment technician involved. Poolwerx and other professional pool service networks can often diagnose remotely before dispatching a tech, which saves time.

Frequently Asked Questions

Why is my pool heater turning on but not heating the water?

If the heater ignites but the water temperature never rises, the most likely causes are a cracked heat exchanger, a flow rate that is too low, or a thermostat set below the current water temperature. Check your filter pressure and pump speed first, then verify the thermostat is set at least 4-5 degrees above the current water temp.

Why does my pool heater keep shutting off?

A heater that cycles on and shuts off quickly is usually triggering a safety lockout due to low water flow, a dirty filter, or a tripped high-limit sensor. Clean your filter, confirm the pump is running at full speed, and look for any error codes on the heater’s display panel.

How do I reset my pool heater?

Most pool heaters have a reset button near the high-limit switch, typically a small red button inside the front panel. Turn the heater off, wait 30 seconds, press the reset button until it clicks, then power it back on. If it trips again immediately, the underlying cause – usually low flow – needs to be fixed first.

Can low water chemistry stop a pool heater from working?

Yes. Extremely low pH (below 7.0) corrodes copper heat exchangers and can cause the heater to fail or scale up internally. Keep pH between 7.4 and 7.6 and total alkalinity between 80 and 120 ppm to protect your heater long-term.

Should I call a technician or can I fix my pool heater myself?

Many common causes – tripped breakers, dirty filters, closed valves, thermostat settings – are owner-fixable with no tools required. Gas line repairs, heat exchanger replacements, and anything involving internal wiring should be left to a licensed technician.

The honest takeaway here: most pool heaters are not broken, they are protesting. They shut down because something upstream is wrong – flow, fuel, or water conditions – and they are doing exactly what they were designed to do. Run this checklist in order before spending money on a service call and you will solve it yourself more often than not.