Pool Heater Not Working?

When your pool heater fails to fire up, something in the system needs attention — and in most cases, the fix is simpler than you’d expect. This troubleshooting guide covers every common cause of pool heater failure, from basic settings issues to component failures, covering gas heaters, electric heaters, and heat pumps.

Quick Diagnostic Checklist (Start Here)

Before diving into individual components, run through this checklist. Many pool heater service calls are resolved in minutes by checking one of the following:

• Thermostat set higher than current water temperature?
• Pool pump running and water flowing through the heater?
• Filter clean and flow rate adequate?
• Circuit breaker not tripped?
• Gas valve open? (gas heaters)
• Pilot light lit? (older gas heaters with standing pilot)
• No error codes on the display?
• All plumbing valves fully open?

If you checked everything above and the pool heater still won’t work, move to the specific issue sections below.

1. Dirty or Clogged Filter (Most Common Cause)

A dirty pool filter is responsible for up to 70% of all pool heater service calls. Pool heaters require a minimum water flow rate to operate. When a clogged filter restricts flow below that threshold, the heater’s internal flow switch or pressure switch detects insufficient flow and shuts the unit down as a safety measure.

• Check the filter pressure gauge — pressure 8 to 10 PSI above clean baseline indicates it needs cleaning
• Clean or backwash your filter
• Clear the pump basket and skimmer basket of all debris
• Open all plumbing valves fully
• Restart the heater

2. Thermostat Set Too Low or Malfunctioning

The heater’s thermostat must be set higher than the current pool water temperature for the heating cycle to initiate. This is one of the most frequently overlooked causes of a pool heater not working.

• Set the thermostat at least 2 to 3 degrees F above current water temperature
• If setting is correct but heater won’t start, the thermostat itself may be faulty
• Use a separate thermometer to verify actual water temperature vs. displayed temperature
• A malfunctioning thermostat in a gas heater requires professional replacement

3. Low Water Flow

Beyond a dirty filter, other factors reduce water flow to levels too low for the heater to operate:

• Variable speed pump running at too low an RPM for the heater’s minimum flow requirement
• Partially closed bypass or return valves
• Air lock in the plumbing
• Undersized pump for the pool volume

Heat pumps are especially sensitive to water flow issues. If your heat pump cycles on and off repeatedly, low flow is the most likely cause. Check your heater’s specifications for minimum GPM required, and increase pump speed on variable speed units accordingly.

4. Gas Supply Issues (Gas Heaters)

• Gas valve turned off — often happens after winterization or routine maintenance
• Low propane tank level — check and refill if applicable
• Gas pressure too low — other appliances drawing from the same line simultaneously
• Supply line undersized for the heater’s BTU demand

Important: If you smell gas at any point, shut off the gas supply immediately and call your gas utility or a licensed technician. Do not attempt to restart the heater.

5. Ignition or Pilot Light Failure

Standing Pilot Issues

• Pilot light out — relight following the manufacturer’s procedure
• Pilot won’t stay lit — the thermocouple is worn out and needs replacement
• Clogged pilot orifice — carefully clean with compressed air

Electronic Ignition Issues

• Ignitor not clicking — check for power supply and continuity
• Ignitor clicking but no flame — gas valve not opening, or ignitor element needs replacement
• HSI (hot surface ignitor) failure — element glows but gas does not ignite; requires professional replacement

Do not attempt repeated manual restarts on a gas heater that isn’t lighting. Doing so floods the combustion chamber with unburned gas — a serious safety hazard.

6. Tripped Breaker or Electrical Issue

• Check the breaker panel for a tripped breaker (reset by turning fully off then back on)
• Check for a tripped GFCI outlet or disconnect box near the equipment pad
• Inspect wiring connections at the heater for corrosion — always turn power off first
• Electric resistance heaters require a dedicated 240V circuit — confirm the circuit is correct

If the breaker trips repeatedly after resetting, there is an underlying electrical fault. Call a licensed electrician — do not continue resetting a repeatedly tripping breaker.

7. Frozen Evaporator Coil (Heat Pumps)

Heat pumps extract heat from ambient air. When air temperature drops below 50°F (10°C), the evaporator coil can ice over, blocking airflow and shutting the unit down.

• Turn off the heat pump and allow it to defrost naturally (1 to 4 hours)
• Do not pour hot water on the coil — thermal shock can crack components
• Once defrosted, clean the coil fins with a low-pressure garden hose
• If coil freezes regularly in mild temperatures, refrigerant charge may be low — requires a certified HVAC technician

8. Faulty Pressure Switch or Safety Lockout

Pool heaters are equipped with multiple safety devices: high-limit switches, pressure switches, and flow switches. Any of these can trip or fail, locking the heater out even when no real problem exists.

High-limit switch trips when the heater exceeds safe operating temperature — usually from low flow or a stuck bypass. After resolving the flow issue, the switch may reset automatically or require manual reset per your heater’s manual.

Modern digital heaters display specific error codes when a safety device triggers a shutdown. Look up your heater model’s error code list — this is the fastest path to identifying the exact component at fault.

Gas vs. Electric vs. Heat Pump — Issue Comparison

Should You Repair or Replace Your Pool Heater?

• Under 5 years old: Repair in almost all cases — components available and likely under warranty
• 5 to 10 years old: Compare repair cost against replacement. If repair exceeds 40% of replacement cost, consider upgrading
• Over 10 to 12 years old: Replacement often more economical — newer heat pumps offer significantly better efficiency
• Repeated failures: Multiple repairs in a short period signal systemic wear — replacement makes more financial sense

When to Call a Professional

• The issue involves gas lines, gas valves, or the combustion chamber
• You smell gas at or near the heater
• The heater repeatedly trips a breaker or safety switch
• An error code points to a refrigerant issue (heat pumps)
• Repairs require accessing internal components of the heat exchanger or burner assembly

Gas heater repairs involving burners, heat exchangers, or gas valves must be handled by a certified technician. Improper repairs on gas equipment create serious safety hazards.