Pool Heater Is Not Heating
A heater that runs but does not raise water temperature is usually being limited rather than broken: not enough flow, a scaled heat exchanger, a bad sensor, or a unit undersized for the volume and conditions.
Likely Causes
Scaled heat exchanger
Calcium buildup insulates the exchanger so heat never reaches the water. Very common in hard-water areas.
Insufficient flow
A dirty filter, low pump speed or a partly closed valve can leave the heater below its required flow rate.
Faulty temperature sensor
The heater believes the water is already at setpoint and stops calling for heat.
Bypass valve open
Water routing around the heater instead of through it.
Undersized heater or heavy loss
An uncovered pool in cool, windy weather can lose heat as fast as a small heater adds it.
Short cycling
The unit fires briefly then shuts down on a limit, never running long enough to heat.
What You Can Safely Check
- Clean or check the filter and confirm strong return flow
- Verify the heater bypass valve position
- Confirm the setpoint is above current water temperature
- Note whether the burner stays lit or cuts out quickly
When to Call a Professional
Anything involving the burner, gas supply, exchanger or internal sensors is a professional repair. Do not open the cabinet.
Safety: Gas appliances combine fuel, ignition and electricity. Homeowner troubleshooting should stop at the filter, valves and setpoint.