Pool Heater Not Heating – Causes & Fixes

Why is my pool heater not heating?

If your pool heater is running but not heating, the most common causes are low water flow, blocked filters, faulty pressure switches, incorrect temperature settings, or ignition issues in gas heaters.

• Low water flow or dirty filter
• Blocked plumbing or circulation issues
• Faulty pressure switch or flow sensor
• Incorrect thermostat or temperature settings
• Ignition or burner problems (gas heaters)
• Heat pump performance limitations in cold weather

If your heater is turning on but the water temperature is not increasing, use the troubleshooting steps below to identify the issue.

Most Common Reasons a Pool Heater Is Not Heating

1. Low Water Flow Through the Heater

Low water flow is the most common reason a pool heater will not heat properly. Pool heaters rely on consistent water circulation to transfer heat efficiently.

Common causes of low flow include:

• Dirty or blocked pool filter
• Blocked skimmer or pump basket
• Partially closed valves
• Failing pool pump

Learn how to diagnose water flow issues →

2. Dirty Filter or Blocked Plumbing

A clogged filter or restriction in the plumbing system can reduce flow enough to prevent the heater from operating efficiently.

Fixes include:

• Backwashing or cleaning the filter
• Checking for blockages in pipes or baskets
• Ensuring valves are fully open

3. Faulty Pressure Switch or Flow Sensor

The pressure switch ensures there is enough water flow before allowing the heater to operate. If faulty, the heater may run but not heat properly.

Diagnose pressure switch faults →

4. Incorrect Temperature Settings

Sometimes the issue is simply incorrect thermostat settings or the heater being set below the current water temperature.

• Check the set temperature
• Ensure heating mode is active
• Confirm the thermostat is working correctly

5. Gas Heater Ignition or Burner Issues

If a gas heater fails to ignite or maintain a flame, it may run but not produce heat.

Troubleshoot ignition problems →

6. Heat Pump Not Heating in Cold Weather

Heat pumps lose efficiency in colder temperatures and may struggle to heat the pool effectively.

View heat pump troubleshooting guide →

Pool Heater Running But Not Heating

If your heater is running but not heating, it usually indicates a flow issue, sensor fault, or internal component problem.

Check:

• Water flow rate
• Filter cleanliness
• Error codes on the display
• Heater cycling behaviour

Related Pool Heater Problems

• Pool heater not igniting
• Heater starts then stops
• Pool heater error codes
• Full troubleshooting guide

When to Call a Pool Heater Technician

If your pool heater is not heating despite correct water flow and settings, the issue may involve internal components such as sensors, gas valves, or control boards.

Professional diagnosis is recommended for:

• Persistent heating failure
• Error codes that won’t clear
• Gas supply or ignition issues
• Electrical or control faults

Book a Pool Heater Repair →

This guide is for general troubleshooting only. HeaterFix recommends all repairs be carried out by a licensed technician.