Pool Heater Not Working? 6 Common Causes and What to Do

Your pool heater fires up, runs for a few minutes, then shuts off before the water gets warm. Or worse - it won't ignite at all. Before you panic and call for a $150 service visit, run through these common causes. Most heater problems have simple explanations, and some you can fix yourself.

1. Low Water Flow (The #1 Cause)

Pool heaters have a flow switch or pressure sensor that prevents the heater from firing when water flow is insufficient. This is a safety feature - without adequate flow, the heat exchanger can overheat and crack (a $1,500+ repair).

Check these first:

• Is your pump running? Sounds obvious, but check.
• Is your filter dirty? A clogged filter restricts flow. Clean or backwash it.
• Are any valves partially closed? Someone may have turned a valve during maintenance.
• Is the pump basket or skimmer basket full of debris?

If your heater displays a "LO" or "FL" error code, low flow is almost certainly the problem. Fix the flow issue and the heater will fire normally.

2. Dirty or Failed Igniter

Gas heaters use a hot surface igniter (similar to a modern gas oven) to light the burner. Over time, these igniters crack, corrode, or get coated with spider webs and debris - spiders love the warm, sheltered space inside your heater.

What to do: Open the heater's front panel and visually inspect the igniter. If it's cracked, white/chalky, or visibly damaged, it needs replacement. This is a $40-80 part and a 20-minute job if you're handy. If you can see spider webs or debris around the burner tray, carefully vacuum or blow them out.

3. Gas Supply Issues

Your heater needs adequate gas pressure to fire and stay lit. Common gas-side problems:

• Other gas appliances running simultaneously - If your pool heater, home furnace, and gas dryer all fire at once, the supply line may not deliver enough pressure to all of them.
• Propane tank low - LP heaters will struggle or fail to ignite when the tank drops below 20%.
• Gas valve turned off - Check the manual shutoff valve on the gas line leading to the heater.
• Regulator failure - The gas regulator on your meter or propane tank can fail, reducing pressure to all appliances.

4. High Limit Switch Tripped

The high limit switch is a safety device that shuts the heater off if the heat exchanger temperature exceeds safe limits (usually around 135-150 degrees F). It trips when:

• Flow is restricted (see #1 above)
• The thermostat is set too high
• Scale buildup inside the heat exchanger is insulating the sensor
• The bypass valve is letting too much water skip the heater

Most heaters have a reset button for the high limit switch - consult your owner's manual for its location. If it trips repeatedly, you have an underlying flow or scale problem that needs addressing.

5. Corroded or Scaled Heat Exchanger

If your water chemistry has been off - particularly low pH (acidic) or high calcium hardness - your heat exchanger may have internal corrosion or heavy scale deposits. Symptoms include:

• Heater takes much longer than usual to heat the pool
• Heater cycles on and off frequently (short-cycling)
• Visible green corrosion on copper fittings
• High limit trips repeatedly

Prevention is key here: maintain pH between 7.4-7.6 and calcium hardness between 200-400 ppm. Once a heat exchanger is severely scaled or corroded, replacement is usually the only fix ($800-1,500 for the part).

6. Wind or Venting Issues

Outdoor heaters can struggle in high winds. If your heater fires but immediately shuts off on windy days, wind may be blowing out the pilot or disrupting the flame sensor. Solutions include installing a wind barrier or checking that your heater's venting (if applicable) is properly configured.

When to Call a Professional

If you've checked flow, gas supply, and the igniter and your heater still won't cooperate - or if you smell gas at any point - call a licensed pool heater technician. Gas appliance repair isn't something to guess at.

Need replacement parts? Pool Supply Express stocks igniters, thermostats, pressure switches, and other heater components for Pentair MasterTemp, Jandy JXi, and Raypak heaters. Free shipping on everything, and our team at 602-529-1901 can help you identify the right part for your model.