Heater Not Igniting

Why Your Pool or Spa Heater Won’t Start Heating

If your gas heater or heat pump won’t ignite—or repeatedly tries and fails—there are several common causes. Most issues come down to flow, sensors, ignition components, gas supply, or automation settings.

This guide explains the most likely reasons your heater will not ignite and what you can check before requesting service.

⭐ 1. Low Water Flow (Most Common Cause of Ignition Failure)

Heaters require a minimum flow rate to ignite safely. If flow is too low, the heater will NOT turn on.

Signs:

✔ “LO FLO” or “FLO” error

✔ Heater clicks but doesn’t fire

✔ Filter pressure unusually low or high

Fix:

• Clean filter
• Clean pump and skimmer baskets
• Increase variable-speed pump RPM (2,800–3,200 RPM for spa mode)
• Check valves for correct placement
• Ensure there are no suction air leaks

Low flow is the #1 reason heaters won’t start.

⭐ 2. Gas Supply Off or Too Low (Gas Heaters Only)

If your heater can’t get enough gas, it will not ignite.

Check:

✔ Gas valve ON

✔ Propane level not empty

✔ No recent gas line work

✔ No rotten-egg smell (stop immediately if so)

Fix:

• Make sure the gas valve is open
• Confirm propane tank has fuel
• If natural gas, ensure utility hasn’t shut off flow

If you smell gas → turn the system OFF immediately and call a professional.

⭐ 3. Heater in Standby or Locked Out

Heaters and automation systems have safety lockouts.

Fix:

• Make sure heater mode is POOL or SPA
• Ensure temperature is set higher than current water temp
• Power cycle the heater (OFF → wait → ON)
• Check for error messages on the display

If the heater locked out due to a repeated fault, a reset may clear it.

⭐ 4. Air in the System (Pump Not Fully Primed)

Air reduces flow and can keep the heater from igniting.

Signs:

✔ Pump lid showing bubbles

✔ Return jets blowing air

✔ Heater cycles on/off

Fix:

• Raise water level
• Clean & lubricate pump lid O-ring
• Tighten unions
• Remove debris from pump baskets

⭐ 5. Clogged or Dirty Filter

If the filter is full, flow drops to the heater.

Fix:

• Clean cartridge
• Backwash sand/DE filters
• Confirm pressure returns to normal

⭐ 6. Faulty Pressure Switch or Flow Sensor

The pressure switch confirms safe water flow.

If it is weak, dirty, or failing, the heater will not ignite.

Signs:

✔ Heater won’t fire even with strong flow

✔ “LO FLO” persists after cleaning filter

✔ Heater cycles repeatedly

This usually requires service.

⭐ 7. Igniter Failure (Gas Heaters)

The igniter lights the burner.

If it is cracked or worn out, the heater will not ignite.

Signs:

✔ Clicking sound but no flame

✔ Ignition sequence stops after a few seconds

✔ Burned or cracked igniter

This is a replaceable component.

⭐ 8. Faulty Gas Valve or Burners (Gas Heaters)

If gas cannot reach the burner tray:

✔ Heater will attempt ignition multiple times

✔ Then show a fault or shut down

Common causes:

• Dirty burner tray
• Rust
• Debris
• Failed gas valve

Needs professional service.

⭐ 9. Heat Pump Won’t Run in Cold Weather (Heat Pumps Only)

Heat pumps rely on warm outside air.

They will NOT operate below approx. 50–55°F depending on model.

Signs:

✔ “Low Ambient”

✔ “Defrost Mode”

✔ Unit running but not heating

✔ Cold mornings = no heat

Fix:

• Wait for warmer temps
• Use spa mode later in the day
• Supplement with gas heater if available

This limitation is normal.

⭐ 10. Automation System Not Sending Heat Command

Hayward automation may not be passing the heat request.

Check:

✔ Spa mode activated

✔ Correct body of water selected (POOL vs. SPA)

✔ Heater set to “Enabled” in OmniLogic settings

✔ “Heater Lockout” not enabled

Try bypassing automation by turning the heater on manually at the unit.

⭐ When to Contact Professional Pool Company

Call us if:

✔ Heater repeatedly tries but fails to ignite

✔ “LO FLO” persists even after cleaning filter

✔ Pump is primed but heater won’t start

✔ Heater shuts off after a few seconds

✔ Automation and heater don’t communicate

✔ Burner tray or igniter may be damaged

✔ You suspect a gas supply issue

We can diagnose:

• Pressure switch or flow sensor failure
• Igniter issues
• Gas valve problems
• Dirty burners
• Automation communication errors
• Internal heater safety lockouts

⭐ Summary

A heater that won’t ignite is typically caused by:

• Low flow
• Dirty filter
• Air in system
• Gas supply issue
• Igniter or burner malfunction
• Cold weather (heat pumps)
• Automation not calling for heat

Most ignition failures can be traced to flow or gas issues.

⭐ Need Help?

If your heater still won’t ignite after basic checks, contact Professional Pool Company. We can diagnose the heater, plumbing, automation, and gas supply to get your system heating again.

Updated on: 29/11/2025