A pool heater that won’t work is almost always caused by one of five things: restricted water flow, a dirty filter, a tripped pressure or high-limit switch, an ignition fault, or water chemistry that’s out of range. Before you call a technician, work through this checklist in order. Most heater problems are owner-fixable in under an hour, and the ones that aren’t will at least be easier to diagnose when you can tell the tech exactly what you already ruled out.
Start Here: Is the Heater Actually Getting Flow?
Pool heaters require a minimum flow rate to operate – typically between 25 and 75 gallons per minute depending on the model. If flow drops below the threshold, the heater’s pressure switch or flow switch cuts power to the burner as a safety measure. The heater may appear to “turn on” (display lights up, fan runs) without ever firing the burner, which leads a lot of owners to assume an ignition problem when the real issue is flow.
Check your filter pressure gauge first. If it’s reading 8 to 10 PSI above its clean baseline, the filter is the problem – backwash your sand filter or clean your cartridge before troubleshooting anything else. Clogged skimmer or pump baskets also strangle flow. Pull them, rinse them out, and try the heater again. It takes about three minutes and fixes the problem more often than you’d expect. If the filter pressure looks normal but the heater still won’t fire, a stuck or partially closed bypass valve is worth checking next.
Is the Pump Actually Running at Full Speed?
Variable-speed pumps are great for energy savings, but if your pump is set to a low-speed schedule during the hours you want the heater to run, you may not have enough flow to satisfy the heater’s pressure switch. Set your pump to its highest speed, wait two minutes for pressure to stabilize, and try the heater again. This is one of the most commonly missed causes of heater problems on modern pools.
Also confirm the pump isn’t air-locking or cavitating. A pump that’s sucking air will show normal RPMs but deliver poor pressure. Signs include a gurgling sound from the pump housing and pressure gauge needle that flutters rather than holds steady. The fix is usually a loose lid O-ring on the pump basket or a small suction-side air leak.
Why Won’t My Pool Heater Ignite?
On gas heaters (natural gas or propane), an ignition fault is the second most common failure after flow issues. Start with the obvious: is the gas supply on? Check that the shutoff valve on the gas line to the heater is fully open – parallel to the pipe means open, perpendicular means closed. If you have propane, check the tank level. A tank that reads “20 percent” on the gauge can actually be empty if the gauge is stuck or inaccurate.
If gas supply is confirmed, the igniter itself may be dirty or failed. Most modern heaters use a hot surface igniter or a spark igniter. These wear out over time and can be fouled by insects, spider webs, or corrosion. Pull the access panel and look for an obvious obstruction. If the igniter looks intact but the heater still won’t light, that’s a job for a licensed technician – gas components aren’t something to probe without training.
One thing worth knowing: a storm with high winds or a sudden pressure drop can trip safety switches and cause heaters to lock out. If your heater was working fine before a weather event, check out The Heater Worked Until The Storm Rolled In for a closer look at what’s actually happening in those situations.
Check the High-Limit and Pressure Switches
Pool heaters have multiple safety switches designed to shut down the unit before it overheats or runs dry. The high-limit switch trips when the water temperature inside the heater exceeds a safe threshold – usually around 135 degrees Fahrenheit. This can happen if flow is restricted or if the heater fires while the pump is off. On many heaters, a tripped high-limit switch has to be manually reset by pressing a small button inside the access panel.
The pressure switch confirms that water is actually moving through the heater before allowing ignition. If this switch fails or its sensing port gets clogged with debris, the heater will behave as though there’s no flow even when the pump is running fine. Cleaning or replacing a pressure switch is a relatively low-cost fix, but you’ll need to confirm it’s the switch and not actual low flow before replacing it.
Does Your Water Chemistry Match the Heater’s Requirements?
This one gets overlooked constantly. Heaters – especially gas heaters with copper heat exchangers – are sensitive to water chemistry. Low pH (below 7.2) makes pool water corrosive, and that corrosion attacks the heat exchanger from the inside. You won’t notice it until you have a copper-colored stain in the pool or, eventually, a leak in the exchanger. Keep pH between 7.4 and 7.6.
High pH and high calcium hardness cause the opposite problem: calcium scale deposits build up inside the exchanger, narrowing the flow path until the heater either loses efficiency or shuts down on the pressure switch. If you’re dealing with repeated heater shutoffs and your chemistry has been running high for a season or two, your heater may be shutting off because it’s clogged with scale and gunk rather than any electrical fault. Target calcium hardness between 200 and 400 ppm. AquaDoc makes a calcium hardness increaser that pool owners use to dial in that range without overshooting.
Heat Pump vs. Gas: A Few Differences Worth Knowing
If you have a heat pump heater rather than a gas unit, the ignition checks above don’t apply – but the flow and chemistry checks still do. Heat pumps also have a minimum ambient air temperature requirement, typically 45 to 50 degrees Fahrenheit. Below that threshold, the unit won’t extract enough heat from the air to function and will shut down or display an error code. Running a heat pump in cold weather won’t damage it, but don’t assume a cold-morning shutdown is a malfunction – it may be doing exactly what it’s supposed to do.
Heat pumps are also sensitive to refrigerant issues. If the unit runs, the fan turns, water flows, ambient temperature is fine, but the water isn’t warming at all, low refrigerant is a possibility. That requires a certified HVAC technician to diagnose and fix – it’s not a DIY repair.
When to Stop Troubleshooting and Call a Pro
Stop the checklist and call a licensed technician if you smell gas near the heater, see error codes on the display that you can’t clear after resetting power, notice visible corrosion or green discoloration on copper components inside the unit, or if the heater continues to shut down after you’ve confirmed flow, chemistry, and switches are all fine. A good tech can run a combustion analysis and pressure test in under an hour and give you a clear answer.
If you’re dealing with an older heater that’s been through repeated issues, the honest conversation about repair versus replace is worth having before you invest in another service call. Sometimes the checklist reveals the heater is just done.
Frequently Asked Questions
Why does my pool heater turn on but not heat the water?
If the heater ignites but water temperature doesn’t rise, the most likely causes are a flow rate that’s too low, an undersized heater for your pool volume, or a faulty bypass valve diverting water around the heat exchanger. Check your filter pressure and confirm the pump is running at full speed before anything else.
Why does my pool heater keep shutting off?
Repeated shutdowns usually point to a high-limit switch tripping from restricted flow, a dirty or clogged filter, or low water pressure. A clogged heat exchanger from calcium scale is another common cause – it reduces flow enough to trigger the safety cutoff even when the pump and filter look fine.
Can high or low pH damage a pool heater?
Yes. Low pH (below 7.2) makes water corrosive and attacks copper heat exchangers from the inside. High pH (above 7.8) causes calcium scale to build up inside the exchanger, reducing efficiency and eventually blocking flow. Keep pH between 7.4 and 7.6 to protect the heater long-term.
Why won’t my pool heater ignite?
On gas heaters, ignition failures are usually a closed or faulty gas valve, a failed igniter, or an obstruction like a spider web in the burner assembly. Confirm gas is flowing to other appliances on the same line and that the shutoff valve on the heater’s supply line is fully open before assuming an igniter failure.
How do I know if my pool heater needs a service call?
Call a technician if you see error codes you can’t clear, smell gas near the unit, notice visible corrosion inside the heater cabinet, or if the heater continues to shut down after you’ve confirmed flow, chemistry, and switches are all within normal range. Those symptoms point to internal component failures that need professional diagnosis.