Pool Heater Not Igniting

Common Symptoms

• Pool heater won't ignite
• Ignition lockout error on display
• Heater clicks but no flame
• Pool heater error code (IF, IG, IL, or similar)
• Heater shuts off after ignition attempt

Most Likely Causes

1. 1

   Dirty or Faulty Flame Sensor / Thermocouple

   The flame sensor (also called a thermocouple or flame rod) detects whether the burner is lit by measuring a small millivolt signal from the flame. Carbon buildup on the sensor rod insulates it from the flame, reducing the signal below the minimum threshold and causing the control board to conclude the flame didn't light — triggering lockout. This is the single most common ignition failure cause. Cleaning the sensor rod with fine steel wool restores the surface and resolves the issue in minutes. If cleaning doesn't help, the sensor or its wiring harness may have failed and needs replacement ($20–$60).

2. 2

   Gas Supply Issue or Low Pressure

   Pool heaters require a minimum gas inlet pressure to sustain ignition: approximately 4 inches water column (WC) for LP/propane and 10 inches WC for natural gas. If gas pressure is below spec — due to a partially closed valve, undersized supply line, other high-demand appliances running simultaneously, or a regulator issue — the burner will light weakly or fail to stay lit, causing the heater to lock out. Always verify the main gas supply valve at the heater is fully open. Testing actual pressure requires a manometer and is typically performed by a licensed technician.

3. 3

   Cracked or Fouled Igniter Electrode

   The igniter electrode generates the spark that lights the gas at the burner. Carbon buildup on the electrode tip reduces spark intensity; a cracked ceramic insulator causes the spark to short to ground instead of jumping the gap to the burner. Either condition prevents reliable ignition. Inspect the electrode tip for heavy carbon deposits (clean with a soft brush) and look carefully at the ceramic body for hairline cracks. A cracked electrode must be replaced — it cannot be repaired.

4. 4

   Blocked Flue or Insufficient Combustion Air

   Pool heaters require a steady supply of fresh air for combustion and a clear flue path for exhaust. A blocked flue (bird nest, debris, collapsed vent pipe) creates back-pressure that prevents the burner from drawing air and can trip the air flow switch (AFS), causing ignition lockout. Insufficient combustion air — from installing the heater too close to a wall, enclosing it in a cabinet without vents, or a clogged air intake — produces the same symptom. Inspect the flue exit for obstruction and ensure the heater has minimum clearances per the installation manual.

5. 5

   High Limit Switch Tripped

   Pool heaters have one or more high-limit switches (sometimes called HLS or HL sensors) that shut down the heater if water or stack temperature exceeds a safe maximum — typically caused by low water flow, a dirty filter, a closed bypass valve, or an air-locked heat exchanger. Once tripped, the switch must be manually reset on many models. A heater that trips the high limit repeatedly has an underlying water flow or scale problem that must be addressed — resetting without fixing the root cause risks heat exchanger damage.

6. 6

   Ignition Control Board Failure

   The ignition control board (ICB) manages the entire ignition sequence: opening the gas valve, firing the igniter, confirming flame via the sensor, and running the blower. A failed ICB can cause no-spark conditions, failure to open the gas valve, or refusal to recognize a valid flame signal even when the sensor is clean. Board failures are confirmed by verifying all other components are functional and then testing board outputs with a multimeter. Replacement ICBs are $80–$200 depending on the heater model.

Quick DIY Checks

1. 1Read the error code on the display: most pool heaters (Pentair MasterTemp, Hayward H-Series, Raypak) display a 2–3 character lockout code. Common codes include IF (ignition failure), IG (igniter fault), AFS (air flow switch), PS (pressure switch), and HL or HH (high limit). Look up your specific code in the owner's manual or on the label inside the service panel — the code narrows diagnosis significantly. Photograph the display before resetting so you have a record of the fault.
2. 2Verify the gas supply valve is fully open: locate the manual gas shutoff valve on the supply line entering the heater. It should be parallel to the pipe for open and perpendicular for closed. If others in the household recently worked on gas lines or if the heater has been off for an extended period, someone may have closed this valve. Also check the main gas meter valve and confirm other gas appliances in the house are working normally — if nothing runs on gas, the issue is upstream of the heater.
3. 3Test gas pressure at the heater inlet (if symptoms suggest low pressure): attach a manometer to the inlet pressure tap on the gas valve (typically a 1/8-inch pipe plug on the valve body, labeled 'IN'). With the heater attempting to fire, record the inlet pressure. Minimum spec is approximately 4 inches WC for LP and 10 inches WC for natural gas — consult your heater manual for the exact minimum. If pressure is below spec, the cause is upstream (regulator, supply line sizing, or utility pressure). Do not attempt to adjust the gas regulator yourself — call a licensed pool technician or plumber.

1. 4Clean the flame sensor / thermocouple: turn off the gas supply valve and allow the heater to cool completely. Locate the flame sensor rod — it is a metal rod (typically 1–3 inches long) mounted near the burner, connected by a wire to the control board. Remove the sensor by unscrewing its mounting bracket (usually one screw). Use fine steel wool (000 or 0000 grit) to lightly abrade the entire length of the sensor rod, removing any carbon or oxidation. Do not use sandpaper — it leaves abrasive particles. Reinstall the sensor, reconnect the wire, and test.
2. 5Inspect the igniter electrode: with the gas off and heater cool, locate the igniter electrode near the burner assembly — it looks like a spark plug with a ceramic body and a metal tip bent toward the burner. Inspect the ceramic body carefully for hairline cracks (use a flashlight and magnifier if needed). Examine the metal tip for heavy carbon deposits. If the tip has buildup, clean it gently with a soft brush — do not use steel wool on the electrode. If the ceramic is cracked, the electrode must be replaced before further testing. Check that the electrode gap (distance from tip to ground) matches the spec in your manual (typically 1/8 to 3/16 inch).
3. 6Check for blocked flue and combustion air: go to the flue exhaust outlet (the top or rear vent on the heater) and visually inspect for any obstruction — bird nests, leaves, spider webs, or collapsed vent pipe. Look inside the flue from the heater side if accessible. Also confirm the heater has the minimum clearances to walls and fences per the installation label (typically 6–12 inches on sides, more at the front). If the heater is in an enclosed cabinet or doghouse structure, ensure the enclosure has the minimum ventilation openings specified in the manual. A blocked or restricted flue prevents ignition and also creates a carbon monoxide risk.
4. 7Reset the high limit switch: locate the high-limit switch or switches on your heater — on Pentair MasterTemp models it's a red button on the top of the heater; on Hayward H-Series it may be inside the service compartment. Press the reset button firmly until you feel a click. After resetting, run the pool pump at full speed for 2–3 minutes to establish water flow through the heat exchanger before attempting to fire the heater. If the high limit trips again immediately, the underlying cause (low flow, dirty filter, closed bypass, scale in heat exchanger) must be resolved before the heater will run reliably.
5. 8Test ignition control board continuity: if all previous steps check out and the heater still won't ignite, the ignition control board may have failed. With the heater powered and in ignition attempt mode, use a multimeter to verify: (1) 24V or 120V (per your model) is present at the gas valve terminals during the ignition attempt — if not, the board is not commanding the valve to open; (2) the igniter electrode is receiving spark voltage — if the board fires the igniter you should see a spark at the electrode gap; (3) the flame sensor signal wire reads a millivolt signal (0.5–3.5 mV DC) when a test flame is held near the sensor. If the board is not sending or receiving these signals correctly and all other components test good, the board requires replacement.

Repair vs Replace

Recommended Tools & Parts

• Pool Heater Flame Sensor / Thermocouple

  Universal and OEM-specific flame sensor rods for pool heaters. The single most common ignition failure part. Clean first with fine steel wool — if cleaning doesn't resolve the lockout, replace. Confirm compatibility with your heater model number before ordering.

  $20–$60

  Buy on Amazon →

• Pool Heater Igniter Electrode

  Replacement spark igniter electrode for gas pool heaters. Required if ceramic body is cracked or electrode tip is heavily eroded. Available as OEM and aftermarket. Match to your heater model for correct gap and mounting.

  $30–$80

  Buy on Amazon →

• Pool Heater Ignition Control Board

  Replacement ignition control board (ICB) for pool heaters. Controls the full ignition sequence — gas valve, spark, and flame detection. Only replace after confirming all other components (sensor, electrode, gas supply) are functional. Model-specific — search your heater's model number.

  $80–$200

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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