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Fix a heated pool that will not heat

We'll confirm the pump is circulating, rule out gas or power issues, then isolate the cause—pilot, thermostat, flow switch, or heat exchanger—or tell you when to call a pro.

Troubleshooting · Home appliances
20–60 min
What you'll need
  • Owner's manual (for error codes, flow switch location)
Step 1 of 9

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Steps

Goal: Confirm circulation, rule out filter and power/gas, then isolate the heating fault.

  • Check that the pool pump is running and water is flowing through the system.
  • Good: The pump is running and water is circulating. Proceed to Check circulation.
  • Bad: The pump is off or there is no flow—start the pump and check for blockages. See When to get help.

Check circulation

Goal: Rule out pump and filter issues before testing the heater.

  • Verify the pump is running and water is moving through the skimmer and returns. Check the filter pressure—if high, backwash or clean the filter.
  • Verify the circuit breaker (electric) or gas supply valve (gas) is on. Reset the breaker if tripped.
  • Good: Pump running, filter clean, and power or gas on. Proceed to Gas path or Electric path based on your heater type.
  • Bad: Pump will not run, filter severely clogged, or power/gas off—fix those first, or call a pro.

Gas path

Goal: Check the pilot and thermostat on a gas pool heater. If the pilot is out, call a technician.

  • Check the pilot light. If it is out, call a pool or HVAC technician—do not attempt to light it yourself. Gas pilot work is hazardous.
  • If the pilot will not stay lit, the thermocouple or gas valve has failed—call a pro. Do not work on gas valves yourself.
  • Set the thermostat 2–5°F above the current water temperature. Wait 15–30 minutes.
  • Good: The pilot is lit and the heater fires. The water warms.
  • Bad: Pilot won’t stay lit—call a technician. If you smell gas, evacuate and call 911 or your gas utility from outside.

Electric path

Goal: Check the thermostat and flow switch on an electric or heat pump pool heater.

  • Set the thermostat 2–5°F above the current water temperature. Wait 15–30 minutes.
  • Check the flow switch per your owner’s manual. If flow is adequate but the heater does not fire, the flow switch may be stuck or faulty.
  • Inspect the heat exchanger area for leaks. Leaks require a pro.
  • Good: The heater fires and the water warms.
  • Bad: Still no heat—check error codes or call a technician. Heat pump refrigerant issues require a pro.

When to get help

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside.

Call a pool or HVAC technician if:

  • The pilot is out or will not stay lit (thermocouple or gas valve).
  • The heat exchanger is leaking.
  • You have a heat pump and suspect refrigerant issues.
  • You are not comfortable with gas or electrical work.

Gas valve and refrigerant work require a licensed professional.

Verification

  • The pool pump is running and water is circulating.
  • The heater fires and the water temperature rises toward the thermostat setpoint.
  • No error codes, leaks, or gas smell.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm circulation Verify the pump is running and water is flowing.
  2. Filter and power/gas Check filter pressure, circuit breaker, and gas supply.
  3. Pilot and thermostat Pilot out—call a technician (gas); check thermostat setting.
  4. Flow switch and heat exchanger Test flow switch; inspect for leaks.
  5. Call a pro Gas smell—evacuate, call 911 or gas utility. Pilot won't stay lit, leaks, refrigerant—call a pool or HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Heater type (gas, electric, heat pump)
  • Whether the pump is running
  • Filter pressure
  • Pilot status (gas)
  • Thermostat setting vs water temp
  • Error codes (if any)
  • Steps already tried

Is the pool pump running and circulating water?

Pool heaters require water flow to operate. No flow means the heater will not fire.

Check that the pump is on and water is moving through the skimmer and returns. Good: pump running and flow visible—proceed to filter check. Bad: pump off or no flow—start the pump, check for blockages, then retest.

Yes No

You can change your answer later.

Is the filter clean and power/gas on?

Clogged filter or tripped breaker or gas off can prevent heat.

Check filter pressure—if high, backwash or clean. Verify circuit breaker (electric) or gas supply valve (gas) is on. Good: filter clean and power/gas on. Bad: filter severely clogged or power/gas off—fix those first.

Yes No

You can change your answer later.

Is the heater gas or electric/heat pump?

Gas heaters have a pilot; electric/heat pumps use a circuit and thermostat.

Check the heater type. Gas: look for pilot and gas line. Electric/heat pump: look for electrical connection and compressor. Good: you know the type. Bad: unsure—call a technician.

Gas Electric / heat pump

You can change your answer later.

Is the pilot lit?

Gas heaters need a lit pilot to fire the main burner.

Look for the pilot light. If out, call a technician—do not attempt to light it yourself. Pilot lit: check thermostat setting. Pilot won't stay lit: thermocouple or gas valve—call a pro. Smell gas: evacuate, call 911 or gas utility from outside.

Yes No

You can change your answer later.

Pilot out — call a pro

If the pilot is out, call a pool or HVAC technician—do not attempt to light it yourself. If the pilot will not stay lit after a technician has serviced it, the thermocouple or gas valve has failed—call a technician again. Do not work on gas valves yourself.

Done

Is the thermostat set above water temp?

The thermostat must be set higher than the current water temperature.

Check the thermostat setting. Set it 2–5°F above the current water temp. Wait 15–30 minutes. Good: heater fires. Bad: still no heat—check flow switch or error codes. Heat pump refrigerant issues require a pro.

Yes No

You can change your answer later.

Is the thermostat set above water temp?

The heater will not run if the setpoint is at or below water temp.

Set the thermostat 2–5°F above the current water temperature. Wait 15–30 minutes. Good: heater fires and water warms. Bad: still no heat—check flow switch, error codes, or call a pro.
Question

Is the thermostat set above water temp?

Continue

You can change your answer later.

Does the flow switch close when the pump runs?

The flow switch must detect flow for the heater to fire.

Check the flow switch per your owner's manual. If flow is adequate but the heater does not fire, the flow switch may be stuck or faulty. Inspect the heat exchanger for leaks—leaks require a pro. Good: flow switch works, no leaks. Bad: flow switch failed or heat exchanger leaking—call a technician.

Done

Call a technician

If you smell gas, evacuate immediately. Call 911 or your gas utility from outside. Call a pool or HVAC technician if: pump will not run; pilot won't stay lit; heat exchanger is leaking; heat pump has refrigerant issues; or you are not comfortable with gas or electrical work.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a pool heater run but not heat?
Common causes: pump not circulating (no flow), tripped circuit breaker or gas supply off, pilot light out (gas), faulty thermostat, blocked heat exchanger, or failed flow switch. Check circulation and power or gas first.
Can I fix a pool heater that will not heat myself?
Yes, for basic checks: confirm pump is running, check circuit breaker and gas supply. If the pilot is out (gas heaters), call a technician—do not attempt to light it yourself. Thermostat and flow switch checks are DIY if you are comfortable. Gas valve work, heat exchanger repairs, and heat pump refrigerant require a pro.
When should I call a technician for a pool heater that will not heat?
Call a pool or HVAC technician if you smell gas (evacuate and call 911 or gas utility from outside), if the pilot is out or will not stay lit, if the heat exchanger is leaking, or if you have a heat pump and suspect refrigerant issues. Gas valve and refrigerant work require a licensed pro.

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