Pool Heater Not Working: The Quick Troubleshooting Checklist

If your pool heater isn't working, start with these four things before calling a tech: check that the circuit breaker hasn't tripped, confirm your pump is running and flow is strong, verify your filter isn't clogged, and make sure the thermostat is set above the current water temperature. Most heater failures trace back to one of these, and you can check all four in about ten minutes.

Why Pool Heaters Stop Working (The Short Version)

Pool heaters - gas, electric heat pump, or solar - are built with safety lockouts. The moment something falls outside normal operating parameters, the unit shuts down to protect itself. That sounds frustrating when you just want a warm swim, but it means most "broken" heaters aren't actually broken. They're reacting to a problem elsewhere in the system. Your job is to find that upstream problem before assuming the heater itself is the culprit.

A note before you start: if your heater is displaying an error code, write it down. Every manufacturer publishes fault code lists, and that code tells you exactly what the unit is unhappy about. A quick search for your model number plus the code will usually point you straight to the cause.

Step-by-Step Troubleshooting Checklist

Work through these in order. Each step rules out a possible cause before you move to the next one.

1. Check the circuit breaker and power supply. Go to your electrical panel and look for a tripped breaker. Even a partial trip (breaker looks "on" but is sitting between positions) can cut power. Flip it fully off, then back on. For gas heaters, also confirm the gas supply valve is fully open and that the pilot light or igniter is getting gas.
2. Confirm the pump is running and water is flowing. A heater needs adequate water flow to fire and stay on. No flow, no heat. Look at your pump basket - is it full of water? Is the pump making normal running noise? A pump that's air-locked or cavitating won't push enough flow to satisfy the heater's pressure switch.
3. Check your filter pressure. A dirty filter is one of the most common reasons heaters shut down. If your filter pressure is reading 8-10 psi above its clean baseline, flow is restricted. Clean or backwash your filter before proceeding.
4. Inspect the skimmer basket and pump basket. Leaves, debris, or a waterlogged floating chlorinator stuck in the skimmer can choke your flow. Pull both baskets, empty them, and reinstall before retesting the heater.
5. Verify the thermostat setting. This sounds obvious, but set your thermostat at least 4-5 degrees above the current measured water temperature. Some heaters won't engage if the differential is too small. Use a thermometer to get the actual water temp - the heater's display can read incorrectly if the sensor is faulty.
6. Check the bypass valve position (if your system has one). Many heater installations include a bypass valve that routes water around the heater for summer use. If it's partially closed or accidentally diverted, the heater gets little to no flow.
7. Look for error codes or indicator lights. Gas heaters and heat pumps both have control boards that log faults. Check your owner's manual for the code list. Common codes point to ignition failure, high-limit switch trips, or low-pressure faults - all of which have specific fixes.
8. Inspect the heat exchanger and burner area (gas heaters). Insects, especially wasps and spiders, love to nest in burner tubes and gas orifices during the off-season. A blocked burner tube causes ignition failure or weak heat output. If you see a nest or debris in the burner compartment, clear it with compressed air or a soft brush - with the gas off.

Gas Heater vs. Heat Pump: Different Problems to Expect

Gas heaters most often fail because of ignition issues (dirty igniter, faulty thermocouple, blocked burner) or flow-related shutdowns. If you hear clicking but no flame, the igniter is seeing a problem - either no gas reaching it or the igniter itself has failed. A gas pool tech can diagnose this in one visit.

Heat pump heaters have a different failure mode. They pull heat from outside air, which means they rely on the refrigerant cycle staying healthy. If your heat pump is running - fans spinning, compressor humming - but water isn't warming up, a refrigerant issue or a failing compressor is the likely cause. That's a certified HVAC/refrigeration job, not a DIY fix. Also remember that heat pumps stop working efficiently below about 50 degrees Fahrenheit - if the outside air is cold, that's not a malfunction, it's physics. If you're wondering whether your heater is properly sized for your pool's volume and your local climate in the first place, this guide on what size pool heater you need is worth a look before assuming the unit is defective.

Chemistry Problems That Damage Pool Heaters

Bad water chemistry doesn't just hurt your swimmers - it actively destroys your heater. Low pH (below 7.2) makes pool water acidic enough to corrode a copper heat exchanger in a single season. High calcium hardness above 400 ppm causes scale buildup inside the heat exchanger, which reduces flow and insulates the metal so it can't transfer heat efficiently. Keep pH between 7.4 and 7.6 and calcium hardness between 200 and 400 ppm to protect the equipment. If you suspect scale buildup is restricting your flow, a professional acid wash of the heat exchanger may be needed. AquaDoc's pool chemical line is one option pool owners use to keep these numbers dialed in through the season without the guesswork.

When to Call a Professional

Some repairs are straightforward enough for a confident DIYer. Others aren't worth the risk. Call a licensed pool heater tech or HVAC professional if you're dealing with any of the following: a gas smell near the heater (turn off the gas first), a heat pump with a refrigerant issue, a cracked or leaking heat exchanger, persistent ignition failure after you've confirmed gas supply and cleared the burner, or any repair involving the gas valve or electrical components inside the unit.

For parts you can replace yourself - pressure switches, temperature sensors, thermostats, bypass valves - browse the Pool Heater and Chiller Parts section to find components by type. Having the part number from your owner's manual makes matching much faster.

Preventing This Next Time

Most heater failures are preventable with basic seasonal maintenance. At the start of each heating season: clean the filter before firing the heater for the first time, clear any debris from around the unit and inside the burner compartment, check that all valves are in the correct position, and test your water chemistry before running the heater for extended periods. A few minutes of prep prevents hours of troubleshooting later. And if you want to cut your energy bill while you're at it, reading up on when to turn off your pool heater to save energy is a practical next step once you've got it running again.

Poolwerx and other pool service companies consistently report that the majority of heater service calls trace back to flow issues - dirty filters, blocked baskets, and closed valves - rather than actual heater failures. Check the simple stuff first every single time.

Frequently Asked Questions

Why is my pool heater running but not heating the water?

If the heater fires but water temperature isn't rising, check for low flow first. A dirty filter or closed bypass valve can starve the heat exchanger. Also confirm your thermostat is set at least 4-5 degrees above the current water temperature.

Why does my pool heater keep shutting off?

Most heaters shut off as a safety response to low water flow, a clogged filter, or a faulty pressure switch. Clean your filter, confirm pump flow is strong, and check the pressure switch for debris or corrosion.

How do I reset my pool heater?

Most gas and heat pump heaters have a reset button on the control panel or inside the unit near the burner assembly. Press and hold it for 3-5 seconds, then restart normally. If it trips again within minutes, there is an underlying issue that needs diagnosis.

Can dirty pool water damage a pool heater?

Yes. Water with low pH (below 7.2) is corrosive and will eat through a copper heat exchanger over time. High calcium or scale buildup can also restrict flow and reduce efficiency. Keep pH between 7.4 and 7.6 to protect your heater.

At what temperature is a heat pump pool heater ineffective?

Most heat pump heaters stop working efficiently when outside air temperature drops below 50 degrees Fahrenheit. Below 45 degrees, many units will not run at all. A gas heater is a better choice for cold-weather or shoulder-season heating.