Pool Heater Not Working: 7 Causes and How to Fix Each
Most pool heaters that won’t turn on fail one of four quick checks: a tripped circuit breaker, a thermostat setpoint below the current water temperature, a closed gas supply valve, or a flow switch that won’t trigger due to low water circulation. We recommend starting there because you’ll solve 80% of cases in under 10 minutes. For broader context on heater problems, our pool heater troubleshooting guide covers the full diagnostic landscape.
Video guide
Video: “Pentair pool heater turns on but does not heat” by Jack of All Trades
Is this guide right for you?
This guide is for you if:
- Your pool heater shows no display, no ignition attempt, or completely fails to start
- You want to work through the most common causes before calling a repair tech
This guide is NOT for you if:
- Your heater turns on but doesn’t heat the water, see heater runs but no heat instead
- Your heater shows a specific error code, see the Raypak, Pentair, or Jandy brand guides
- You smell gas near your heater, call your gas company immediately, do not troubleshoot
Quick answer: most likely cause
Most pool heater “no power” situations resolve in under 10 minutes. Check the circuit breaker and thermostat setpoint first before anything else. If those are fine, verify the gas supply valve is fully open and that the pool pump is running with clean enough filter pressure to trigger the flow switch.
Gas safety first
Before touching anything, note any error codes on the display. Many heaters show codes even after shutdown, and that code tells the technician (or you) exactly where the failure occurred. Write it down before you power-cycle the unit.
Safe to do yourself: checking the breaker, adjusting the thermostat, verifying valve position. We also consider flow switch verification (by cleaning the filter) a homeowner-level task.
Not safe to DIY: anything involving gas line connections, gas valve internals, or refrigerant. Verifying that a valve is open or closed is fine. Turning a fitting is not. Per NFPA 54 gas safety standards{:target=“_blank”}, gas line work requires a licensed professional.
Step-by-step: diagnose why your heater won’t turn on
Step 1: check the circuit breaker
Find the main electrical panel and look for a dedicated breaker labeled “heater” or “pool heater.” Pool heaters typically run on a 240V double-pole breaker.
A tripped breaker sits in the middle position between ON and OFF. To reset it, flip it fully to OFF first, then back to ON. Do not just push it from the middle to ON.
If the breaker trips immediately after reset: stop resetting it. A breaker that trips right away indicates an electrical fault in the heater’s wiring or components. At that point, call a licensed electrician. Repeated resets on a faulty circuit can cause fires.
Step 2: check the thermostat setpoint
This is the second most common cause of a “dead” heater, and the most embarrassing fix. If the water temperature has crept up during warm weather and the thermostat setpoint is now at or below the actual water temperature, the heater sees no demand and won’t fire.
Set the thermostat 5°F above the current water temperature to confirm the heater will attempt to fire. If it lights up and tries to run, you’ve found your issue.
Step 3: verify the gas supply valve is open
The gas shutoff valve at the heater should have its handle running parallel to the pipe when open, and perpendicular to the pipe when closed. If it’s at a 90-degree angle to the pipe, it’s closed.
If you recently had any gas work done at the house (water heater replacement, meter service, etc.), the technician may have closed the main gas valve and not reopened it all the way. Check the main shutoff too.
Checking valve position is safe. Do not attempt to adjust gas line connections or fittings. That work requires a licensed gas plumber.
Step 4: check the flow switch
Pool heaters have a flow switch that prevents the heater from firing when water circulation is insufficient. This protects the heat exchanger from overheating in a “dry fire” scenario. If the switch doesn’t detect enough flow, the heater will refuse to start.
The most common cause of a stuck flow switch is low water flow, which is almost always caused by a dirty filter. If your filter pressure gauge reads more than 8-10 PSI above your baseline clean reading, the filter needs cleaning before the heater will work.
Other causes of low flow: a closed bypass valve on the heater, a skimmer basket packed with debris, or a pump problem.
Step 5: reset ignition lockout
Pool heaters lock out automatically after three failed ignition attempts. This is a safety feature. The heater tries to light three times, fails, and shuts itself down to avoid dumping unburned gas.
Pool heaters lock out automatically after three failed ignition attempts. To reset the lockout, turn the heater’s power switch off, wait 30-60 seconds, and turn it back on. If the lockout repeats, the ignition system needs professional diagnosis.
If it locks out again immediately after reset, the problem is in the ignition system, not the controls. See our guide on pool heater ignition failure for the full diagnosis of igniters, thermopiles, and gas valves.
Step 6: check for error codes
If the display shows any code at all, write it down before power cycling. Here are the most common codes to watch for:
Pentair MasterTemp / Max-E-Therm:
- PS (pressure switch) = flow issue, not enough water through heater
- HLS (high limit switch) = flow problem or overheating
- E01 = stack flue sensor failure
Raypak:
- E1 / HI LIMIT = high limit sensor tripped
- IGN FAIL / IF = ignition failure at burner
- E3 / INLET SENSOR = inlet temperature sensor failure
Jandy LXi / Hi-E2:
- E04 = high limit tripped
- E06 = ignition lockout after 3 failed attempts
For deeper code lookups, see Pentair heater diagnostics documentation{:target=“_blank”} and Raypak heater support and error code guide{:target=“_blank”}.
Step 7: inspect the high-limit switch
If the heater attempted to run and shut off rather than failing to start at all, the high-limit switch may have tripped as a safety response. The high-limit switch is a thermal sensor that shuts the heater off if water temperature inside the heat exchanger gets too high, which usually indicates low flow.
Some heaters have a manual reset button on the high-limit switch. Check your owner’s manual for the location. If the button is accessible, press it to reset.
If the high-limit switch trips again after reset without any adjustment to water flow, professional diagnosis is needed. Repeated high-limit trips on a heater with apparently good flow can indicate a failing heat exchanger.
When to call a pool heater technician
Call a pro immediately if:
- You smell gas at any point
- The circuit breaker trips again immediately after reset
- Error codes persist after the 30-60 second power cycle reset
- The high-limit switch trips repeatedly after reset
Safe to attempt yourself: all 7 steps above.
Service call cost typically runs $75-$150 plus parts, and the diagnostic fee is usually credited toward the repair bill if you proceed with the same technician.
For heater repair vs replacement cost: if your heater is 8+ years old and the repair quote exceeds $500, get a replacement quote before committing. Heat exchangers alone run $500-$800 in parts. At that price point on an older unit, replacement math often wins.
FAQ
Why did my pool heater suddenly stop working?
The most common cause of a sudden pool heater shutdown is a tripped circuit breaker, a flow switch triggered by a dirty filter, or an ignition lockout after three failed start attempts. In warm weather, a thermostat setpoint that falls below the rising water temperature can also cause the heater to appear dead when it’s actually working normally. Start with the breaker and thermostat before assuming component failure.
How do I reset my pool heater?
To reset a pool heater, turn the power switch or disconnect off and wait 30-60 seconds, then restore power. This clears ignition lockout states and resets most error conditions. If the heater still won’t start after a full reset, note any error codes on the display and work through the 7-step checklist above. If the error code returns immediately, the underlying problem has not been resolved.
How do I know if my pool heater’s flow switch is bad?
If your heater refuses to fire but the breaker is fine, the thermostat is set correctly, and the gas valve is open, the flow switch is the next suspect. To test it: clean your filter and verify the pump is running at full speed, then attempt to start the heater. If it fires after improving flow, the flow switch was triggering correctly due to inadequate flow, not because it failed. A truly failed flow switch (stuck open = heater never fires, stuck closed = heater fires without flow and overheats) requires replacement by a technician.
Can a dirty pool filter cause my heater to stop working?
Yes. A dirty pool filter is a common cause of heater shutoff: high filter pressure reduces water flow through the heater, tripping the flow switch or high-limit safety. If your filter pressure gauge is reading 8-10 PSI above its clean baseline, clean the filter cartridge or backwash the sand/DE filter before troubleshooting the heater itself.
How much does it cost to repair a pool heater that won’t turn on?
Repair costs depend heavily on the cause. A tripped breaker or thermostat adjustment costs nothing. An igniter replacement runs $50-$150 in parts (relatively easy repair). A circuit board replacement is $150-$300. A heat exchanger failure costs $500-$800 or more in parts alone. A service call for diagnostics typically costs $75-$150, usually credited toward the repair. On heaters older than 8 years, compare the repair quote against replacement before proceeding.