Heater runs but pool water does not reach set temperature; very slow heating performance.

Safety first: Turn off power and gas before opening panels. Water-side work requires shutting off the pump and closing isolation valves.

Step-by-step checks:

• Verify heater capacity: Confirm that the Pahlen heater model kW rating is appropriate for your pool size and desired temperature rise. An undersized heater will always heat slowly.
• Check gas supply: Ensure the gas line size and meter capacity meet Pahlen specifications. If other appliances are running, the heater may be starved for gas. A gas technician can measure operating pressure.
• Inspect bypass settings: If a manual or automatic bypass is installed, adjust it so that adequate flow passes through the heater. Too much bypass will reduce heat transfer.
• Evaluate heat exchanger condition: Look for signs of scale at the inlet/outlet unions or in the filter. If water chemistry has been poor, the internal tubes may be scaled, reducing efficiency. Professional chemical descaling of the Pahlen heat exchanger may be required.

When to stop: If basic hydraulic and gas checks are satisfactory but performance remains poor, have a technician test flue gas temperature and efficiency to determine if the heat exchanger is fouled.