A pool heater that turns on and then shuts off is usually responding to a safety condition or losing something it needs to continue operating. Instead of repeatedly resetting the heater, pay attention to exactly what happens before it shuts down. The timing, water flow, display temperature, and error code can help narrow down the problem.

1. Check Water Flow First

Insufficient water flow is one of the most common causes of heater problems. Pool heaters are designed to shut down if there isn't enough water moving through the heat exchanger.

Low flow can be caused by:

• A dirty or clogged pool filter

• A full pump or skimmer basket

• A clogged pump impeller

• A variable-speed pump running at too low of an RPM

• Partially closed valves

• Incorrect valve positioning

• Low pool water level causing the pump to pull air

• A pump that is losing prime or has poor suction

• A malfunctioning pressure or flow switch

What you may notice: The heater starts but shuts down shortly afterward, displays a flow/pressure-related error, or works when the pump speed is increased.

If increasing the pump speed allows the heater to stay on, investigate the circulation system before assuming the heater itself has failed.

2. Check the Filter

A filter can look perfectly normal from the outside while still restricting enough water flow to affect the heater.

Check the filter's pressure gauge against its normal clean pressure. A significant increase usually indicates that the filter needs to be cleaned or backwashed.

Also remember that a pressure gauge only shows pressure at the filter. A dirty filter, plumbing restriction, or valve problem can still reduce the amount of water reaching the heater.

What you may notice: The heater works after cleaning the filter or increasing the pump speed but begins shutting down again as flow decreases.

3. Is the Heater Reading the Correct Water Temperature?

Don't only check what temperature the heater is set to. Compare the temperature shown on the heater display with the actual pool water temperature.

For example, if your pool water is approximately 78°F but the heater says it's 90°F, the heater may believe the pool has already reached its set temperature and shut off.

An incorrect temperature reading can be caused by:

• A failing water-temperature sensor/thermistor

• A loose or damaged sensor connection

• Corroded wiring or connectors

• Poor water circulation through the heater

• A control-board or sensor-circuit problem

Quick check: Compare the heater's displayed water temperature with a reliable pool thermometer. A small difference can be normal, but a large or inconsistent difference should be investigated.

If the heater reaches the set temperature and shuts off normally, that's not a malfunction.

4. Watch Exactly When the Heater Shuts Off

The timing can provide an important clue.

Shuts off almost immediately:
Look for water-flow/pressure problems, ignition failure, flame-sensing problems, or a safety circuit preventing operation.

Runs for a few minutes and shuts off:
Look more closely at restricted water flow, overheating, high-limit conditions, or temperature-sensor problems.

Repeatedly turns on and off:
This is often called short cycling. Possible causes include inadequate flow, an incorrect temperature reading, high-limit activation, thermostat/control problems, or an improperly sized/operating circulation system.

The exact diagnosis depends on the heater model and any error code being displayed.

5. Write Down the Error Code BEFORE Resetting

If an error appears, don't immediately reset the heater.

Write down or take a picture of the exact code first.

Depending on the manufacturer, an error may point toward:

• Low water flow

• Pressure/flow switch problems

• High-limit conditions

• Temperature sensor faults

• Ignition failure

• Flame-sensing problems

• Gas-supply problems

• Airflow or venting faults

• Control-board communication problems

Error-code meanings are manufacturer and model specific, so always check the service documentation for your exact heater.

6. Check for Overheating / High-Limit Shutdown

The heater's high-limit safety system protects it if the water inside the heat exchanger becomes too hot.

This can happen when the heater is producing heat faster than the water can carry it away.

Possible causes include:

• Low water flow

• Dirty filter

• Pump speed set too low

• Partially closed valves

• Internal restriction or scale buildup in the heat exchanger

• Incorrect plumbing/bypass conditions

• A failing high-limit sensor or related circuit

What you may notice: The heater operates normally at first, shuts down after heating for a short period, and may restart after cooling down.

Don't bypass a high-limit switch. If it is opening, determine why.

7. Check the Gas Supply and Ignition

If you have a natural gas or propane heater, it must establish and maintain a proper flame.

Possible causes of ignition-related shutdowns include:

• Gas shutoff valve closed or partially closed

• Low propane supply

• Inadequate gas pressure

• Igniter problems

• Dirty or damaged flame-sensing components

• Gas valve problems

• Burner or combustion problems

• Improper venting or combustion airflow

What you may notice: The heater attempts to ignite several times, you hear clicking without ignition, the flame starts and quickly goes out, or an ignition-related error appears.

Gas pressure, gas valves, burners, and combustion components should be diagnosed by a qualified technician.

8. Check Airflow and the Heater Area

Gas heaters require proper combustion airflow and a clear exhaust path.

Look around the outside of the heater for:

• Leaves and debris

• Vegetation growing against the heater

• Objects blocking ventilation openings

• Evidence of animal or insect nesting

• Obstructions around the exhaust/vent

Indoor or specially vented installations have additional venting requirements and should be inspected by a qualified professional if an airflow or exhaust problem is suspected.

9. Check Your Automation System

If your heater is connected to automation such as Jandy AquaLink, Pentair IntelliCenter/EasyTouch, Hayward OmniLogic, or another controller, the heater may be receiving its temperature command from the automation system rather than directly from the heater's control panel.

Check:

• The temperature set on the automation system

• Whether POOL or SPA mode is selected

• Whether schedules are turning the heater off

• Whether the system is switching between pool and spa mode

• Whether valves are moving to the correct positions

What you may notice: You change the temperature at the heater but it changes back, the heater shuts off at certain times, or the displayed set temperature doesn't match what you entered.

10. Check Pool/Spa Valve Position

On a combination pool and spa system, incorrect valve positioning can create poor circulation through the heater or cause the system to heat a different body of water than expected.

Make sure the suction and return valves are positioned correctly for POOL or SPA operation.

If automated valves aren't moving completely, the problem may be with the actuator, valve, or automation programming.

11. Don't Keep Resetting a Heater That Continues to Shut Down

Resetting can clear a temporary fault, but if the same problem returns, the heater is usually detecting the condition again.

Before calling for service, collect these details:

• Heater manufacturer

• Exact model number

• Error code

• Actual pool temperature

• Temperature shown on the heater

• Heater set temperature

• Pump speed/RPM if using a variable-speed pump

• Filter pressure

• Approximately how long the heater runs before shutting off

Those details can make troubleshooting significantly faster.

When to Call a Professional

If you've confirmed good circulation, a clean filter, correct valve positions, proper temperature settings, and the heater continues shutting down, the problem may be inside the heater.

A qualified technician may need to test the pressure switch, temperature sensors, high-limit circuit, ignition system, gas pressure, flame-sensing system, heat exchanger, wiring, or control board.

Important: Never bypass a pressure switch, flow switch, high-limit switch, gas valve, or other safety device to force a heater to operate. If you smell gas, shut the heater down and have the gas system inspected by a qualified professional.