Pool Heater Won't Ignite: Common Causes

A gas pool or spa heater that clicks, glows, or shows an error but never lights is usually stopped by one of its safety controls, not by a single broken part. Modern heaters check water flow, water temperature, gas, and venting before they allow the burner to fire. This guide explains the common causes, which checks a homeowner can safely make, and when to call a licensed professional.

Safety first: If you smell gas, don't try to light or reset the heater. Don't operate electrical switches near it. Leave the area and call your gas utility or emergency services from a safe distance. Gas valves, burners, gas pressure, venting, and line-voltage wiring should only be serviced by a licensed, qualified technician. The checks below are limited to the things owners can safely look at.

Step 1: Start with the simple things

• Is the heater calling for heat? Make sure it's on, set to pool or spa mode, and set above the current water temperature. If an automation system or timer controls it, check that the heater is enabled there too.
• Is the pump running? Heaters won't fire without water flow. If the pump is on a timer or variable-speed schedule, it may be running too slowly for the heater.
• Is gas reaching the heater? Check that the gas shutoff valve to the heater is open. For propane, make sure the tank isn't low or empty. If other gas appliances also aren't working, the problem may be your gas supply.
• Is power on? Check the heater's breaker and any service switch next to it.
• What does the display say? Most heaters show an error code or a pattern of lights. Write it down and look it up in your owner's manual. It points straight to the circuit that's stopping ignition.

Step 2: Check water flow

Low water flow is one of the most common reasons a heater won't fire. The heater's water pressure switch (or flow switch) must sense enough flow before ignition can begin. Things that reduce flow include:

• A dirty filter. Clean or backwash it if the pressure is above its normal clean reading.
• Full skimmer or pump baskets
• A pump that has lost prime or is running at too low a speed
• Valves partly closed, or set to bypass the heater
• A pool cleaner or water feature diverting flow

If flow is good and the heater still reports a pressure or flow fault, the switch may be faulty or out of adjustment. Its setting depends on how high the heater sits relative to the pool, so a technician should test it.

Step 3: Understand the other safety controls

Gas heaters use several switches wired in a chain. If any one is open, the heater won't fire. Knowing what they do helps you talk to a technician and choose the right part:

• High-limit switches shut the heater down if the water inside gets too hot, usually because of low flow. A tripped or failed limit is often a sign of a flow problem.
• Fusible link or rollout switch opens if flame or excess heat escapes the combustion chamber. This can point to blocked venting, soot, down-drafting, or incorrect gas pressure. Treat a failed fusible link as a warning sign and have the cause found, not just the part replaced.
• Air pressure or vent switch (on fan-assisted heaters) confirms the blower is moving combustion air. A blocked vent or intake, a nest, or a failing blower can trip it.
• Thermostat or temperature sensor tells the control board the water temperature. A failed sensor can stop the heater or show an error.

Step 4: Ignition and flame problems

If the safety circuit is satisfied, the heater tries to light. Common faults at this stage include:

• Igniter: hot-surface igniters glow red before ignition; spark igniters click. A cracked igniter or one that never glows or sparks may need replacing.
• Flame sensor: if the burner lights briefly then shuts off, the flame sensor may be dirty or failing and unable to confirm the flame.
• Low gas pressure: undersized gas lines, a long run from the meter, an undersized propane regulator, or other appliances running at the same time can starve the heater. Measuring gas pressure takes a manometer and is a job for a professional.
• Gas valve or control board: if all safeties are closed and the igniter works but the burner never gets gas, the gas valve or control board may have failed.
• Air in the line: after new installation or gas work, it may take several tries to purge air from the line. Many heaters lock out after repeated failed attempts and need a reset per the manual.

Step 5: Look for outside causes

• Insects, spiders, or rodent nests in the burner area or vent are a common cause of ignition and rollout problems.
• Corrosion on wire terminals, often from heater placement near pool chemical storage, can interrupt safety circuits.
• Recent storms or power surges can damage control boards.

Store pool chemicals away from the heater; their fumes corrode heater parts.

Finding the right replacement part

Heater parts are very model-specific, and many differ between natural gas and propane versions. Before ordering, find the rating plate and note the brand, model, BTU size, fuel type, and serial number. Then browse Heater Parts, or shop by brand such as Raypak, Hayward, Pentair, or Jandy Zodiac. Have gas-train parts such as gas valves, burners, and orifices installed by a licensed technician.

If your heater is old and parts costs are adding up, compare new units in Pool & Spa Heaters and read Choosing the Right Heater.

Have your heater's model and error code? Use the chat on this page or reach us through our contact page, and we'll help you find the matching part.