When a pool heater stops working, the problem is usually one of five things: a flow restriction that trips the pressure switch, a dirty or faulty igniter, an error code pointing at a specific component, water chemistry that’s damaging the heat exchanger, or an electrical issue at the breaker or thermostat. Most of these are things you can diagnose yourself in 20 minutes before you spend $150 on a service call.
Start Here: Check Water Flow Before Anything Else
Low water flow is the single most common reason a pool heater refuses to fire. Every gas heater has a pressure switch that requires a minimum flow rate – usually 20 to 30 gallons per minute – before it allows ignition. If flow drops below that threshold, the heater simply will not start, and it won’t always throw an obvious error code to tell you why.
Work through this flow checklist first:
- Check your filter pressure gauge. If it reads more than 8-10 PSI above your clean baseline, the filter needs backwashing or cleaning.
- Empty the pump strainer basket and skimmer basket. A half-full basket can drop flow dramatically.
- Confirm all valves in the heater bypass loop are in the correct position. A partially closed valve is an easy thing to miss after a repair or winterizing job.
- Make sure the pump is actually running at full speed. On variable-speed pumps, check that the speed isn’t set too low for the heater’s minimum flow requirement.
If you’ve worked through all of that and the heater still won’t fire, move on down the list.
How Do You Read a Pool Heater Error Code?
Most heaters built in the last 15 years have a digital display that shows an error code when something faults. These codes are your fastest diagnostic tool, and they’re specific. Common ones include IF (ignition failure), HH or high-limit (heat exchanger overheated), PS (pressure switch not satisfied), and AFS (air flow sensor, common on Hayward models). Check your owner’s manual or search your heater’s model number plus the code – you’ll usually find the exact cause and fix within a few clicks.
If the display is blank entirely, that’s an electrical problem, not a component failure. Skip ahead to the electrical section below.
What to Check on a Gas Pool Heater That Won’t Ignite
If flow is fine and there’s no error code, or the code says ignition failure, work through the gas and ignition system:
- Confirm gas supply. Check that the gas valve at the heater is fully open, and verify the supply line isn’t also feeding something that might have interrupted pressure (like if other gas appliances in the house aren’t lighting either).
- Inspect the igniter and flame sensor. On most residential gas heaters, these are accessible behind the front panel. A cracked igniter or a flame sensor coated in oxidation will cause repeated ignition failures. Light cleaning with fine steel wool can sometimes revive a sensor, but a cracked igniter needs replacement.
- Check the vent and air intake. Debris, bird nests, or leaves blocking the combustion air intake will prevent ignition. This is especially common after a storm – if your heater worked fine and then suddenly stopped, a blocked flue is worth checking first, and it’s something the folks at The Heater Worked Until The Storm Rolled In cover in detail.
Igniter and gas valve repairs should be done by a qualified technician if you’re not confident working with gas appliances. This is the one area where “I’ll figure it out” can become a real safety issue.
Can Water Chemistry Cause a Pool Heater to Fail?
Yes, and this one surprises people. Water chemistry doesn’t usually cause an immediate fault, but it causes damage that shows up as reduced heating, strange noises, or eventually a cracked heat exchanger.
The two chemistry problems that hit heaters hardest are low pH and high calcium hardness. Low pH – below 7.2 – makes water corrosive. It slowly eats copper heat exchangers from the inside, and some heaters have sensors that detect acidic water and shut down to protect themselves. High calcium hardness – above 400 ppm – causes calcium scale to build up inside the heat exchanger, reducing efficiency and eventually blocking flow. Keep your pH between 7.4 and 7.6 and your calcium hardness between 200 and 400 ppm, and you’ll add years to that heat exchanger’s life. Keeping your overall water balance dialed in matters for equipment longevity – and it’s one reason AquaDoc formulates its balancing products to hold stable longer between dose-and-test cycles.
Electrical Checks: Breaker, Thermostat, and Wiring
If the display is dark and the heater does nothing when you call for heat, start with the obvious:
- Check the circuit breaker for the heater at your main panel. A tripped breaker is a two-second fix. If it trips again immediately after resetting, stop and call an electrician – something is drawing excess current.
- Check the thermostat or temperature setpoint on the heater itself. It sounds basic, but the set temperature being lower than the current water temperature will prevent the heater from running. Set it at least 4-5 degrees above the current pool temperature to confirm the call-for-heat signal is working.
- Look for a manual high-limit reset button. On many heaters this is a small red button inside the front panel. If the heater overheated – due to low flow or a previous fault – it locks out until manually reset. Press it once and attempt a restart.
- Inspect visible wiring for burn marks, loose connections, or rodent damage. Mice love warm equipment pads in the off-season.
Heat Pump Heaters: A Few Extra Checks
If you have a heat pump rather than a gas heater, most of the above still applies, but add these to your list. Heat pumps won’t operate efficiently (or at all) when air temperature drops below about 50 degrees Fahrenheit – that’s not a malfunction, it’s a design limitation. Also check that the fan on top of the unit is spinning freely, and that nothing is blocking airflow around the cabinet. A heat pump needs to pull a large volume of air to extract heat, so even a partial obstruction will hurt performance.
When to Call a Pool Heater Technician
Call a tech if: you smell gas anywhere near the heater, the circuit breaker keeps tripping, you see burn marks on wiring or components, or the heat exchanger is leaking water into the combustion chamber. Everything else on this list is within reach of a patient pool owner. But those four things are genuinely in “do not proceed” territory.
Frequently Asked Questions
Why does my pool heater keep shutting off?
The most common cause is low water flow triggering the pressure switch, which shuts the heater off as a safety measure. Check your filter pressure, pump basket, and any partially closed valves first.
Can bad water chemistry stop a pool heater from working?
Yes. Low pH (below 7.2) corrodes heat exchangers and can trigger scale sensors, while high calcium hardness (above 400 ppm) causes scale buildup that blocks heat transfer. Both conditions can cause a heater to fault or underperform.
What does a pool heater error code mean?
Error codes vary by brand, but common ones like IF (ignition failure), HH (high limit), or PS (pressure switch) point directly to the fault. Check your owner’s manual or search the model number plus the code for a quick answer.
How do I know if my pool heater igniter is bad?
If the heater attempts to start, you hear a clicking sound, but no flame establishes, the igniter or gas valve is the likely problem. This is one repair that typically requires a qualified technician.
Does a pool heater need a certain flow rate to turn on?
Yes. Most gas pool heaters require a minimum flow rate of around 20 to 30 gallons per minute to satisfy the pressure switch and fire safely. A clogged filter, closed bypass valve, or undersized pump can all drop flow below that threshold.
A heater that won’t run is frustrating, but it’s almost never a mystery. Work through flow first, then ignition, then chemistry, then electrical – in that order – and you’ll find the problem 90% of the time without waiting on a service truck. For more detail on specific heater fault scenarios, Poolwerx publishes practical equipment guides from their service teams that are worth bookmarking.