Weil McLain F43

What does Weil McLain F43 mean?

F43 indicates the heat exchanger protection function has activated. The controller detects that the CH supply temperature is rising too rapidly — faster than the heat exchanger protection slope parameter — indicating insufficient water circulation through the heat exchanger. When the supply sensor temperature drops below 113°F, the boiler resumes normal operation automatically.

Symptoms

• Boiler displays F43 and stops firing.
• Supply pipe temperature spikes rapidly when the burner runs.
• Gurgling or sloshing sounds from the heat exchanger area.
• Boiler resumes operation after the supply sensor cools below 113°F.

Common causes

• No circulation through the central heating system — circulator pump not running.
• Air trapped in the system or heat exchanger blocking water flow.
• Zone valves or manual balancing valves closed, preventing flow.
• System strainer or filter blocked, restricting flow to the boiler.

Diagnostic steps

1. 01

   Verify circulator pump operation

   Confirm the circulator pump is energized and running during a call for heat. Listen for the pump hum and check for vibration. A non-running pump requires wiring inspection or pump replacement.

2. 02

   Purge air from the system

   Bleed all air from the heating circuits and the boiler heat exchanger. Air pockets prevent water from absorbing combustion heat, causing rapid temperature spikes.

3. 03

   Check zone valves and manual valves

   Verify all zone valves are powered open and all manual balancing valves are open during testing. A single closed zone on a small system can cause F43 if it's the only active circuit.

4. 04

   Inspect and clean the system strainer

   Locate the inline strainer on the boiler return or system primary loop. Remove and clean the strainer screen. Heavy scale or debris blockage will restrict flow and trigger F43.

5. 05

   Reset and test

   After confirming circulation, press Reset for 1.5 seconds and initiate a heating call. Monitor the supply temperature rise rate to confirm F43 does not reappear.

When to call a professional

Call a licensed technician if F43 continues after confirming circulation, purging air, and cleaning the strainer. Persistent F43 points to incorrect primary loop piping configuration, an undersized or mis-selected circulator pump, or pump head pressure insufficient for the installed system that requires hydraulic analysis.